Details Of Dinosaurs

Dinosaurs are a diverse group of reptiles[note 1] of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago,[1][2] although the exact origin and timing of the evolution of dinosaurs is the subject of active research.[3] They became the dominant terrestrial vertebrates after the Triassic–Jurassic extinction event 201 million years ago; their dominance continued through the Jurassic and Cretaceous periods. Reverse genetic engineering[4] and the fossil record both demonstrate that birds are modern feathered dinosaurs,[5] having evolved from earlier theropods during the late Jurassic Period.[6] As such, birds were the only dinosaur lineage to survive the Cretaceous–Paleogene extinction event 66 million years ago.[7] Dinosaurs can therefore be divided into avian dinosaurs, or birds; and non-avian dinosaurs, which are all dinosaurs other than birds. This article deals primarily with non-avian dinosaurs.

Dinosaurs are a varied group of animals from taxonomicmorphological and ecological standpoints. Birds, at over 10,000 living species,[8] are the most diverse group of vertebrates besides perciform fish.[9] Using fossil evidence, paleontologists have identified over 500 distinct genera[10] and more than 1,000 different species of non-avian dinosaurs.[11] Dinosaurs are represented on every continent by both extantspecies (birds) and fossil remains.[12] Through the first half of the 20th century, before birds were recognized to be dinosaurs, most of the scientific community believed dinosaurs to have been sluggish and cold-blooded. Most research conducted since the 1970s, however, has indicated that all dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction. Some were herbivorous, others carnivorous. Evidence suggests that egg-laying and nest-building are additional traits shared by all dinosaurs, avian and non-avian alike.

While dinosaurs were ancestrally bipedal, many extinct groups included quadrupedal species, and some were able to shift between these stances. Elaborate display structures such as horns or crests are common to all dinosaur groups, and some extinct groups developed skeletal modifications such as bony armor and spines. While the dinosaurs’ modern-day surviving avian lineage (birds) are generally small due to the constraints of flight, many prehistoric dinosaurs (non-avian and avian) were large-bodied—the largest sauropod dinosaurs are estimated to have reached lengths of 39.7 meters (130 feet)[13] and heights of 18 meters (59 feet)[14] and were the largest land animals of all time. Still, the idea that non-avian dinosaurs were uniformly gigantic is a misconception based in part on preservation bias, as large, sturdy bones are more likely to last until they are fossilized. Many dinosaurs were quite small: Xixianykus, for example, was only about 50 cm (20 in) long.

Since the first dinosaur fossils were recognized in the early 19th century, mounted fossil dinosaur skeletons have been major attractions at museums around the world, and dinosaurs have become an enduring part of world culture. The large sizes of some dinosaur groups, as well as their seemingly monstrous and fantastic nature, have ensured dinosaurs’ regular appearance in best-selling books and films, such as Jurassic Park. Persistent public enthusiasm for the animals has resulted in significant funding for dinosaur science, and new discoveries are regularly covered by media.

Etymology

The taxon ‘Dinosauria’ was formally named in 1841 by paleontologist Sir Richard Owen, who used it to refer to the “distinct tribe or sub-order of Saurian Reptiles” that were then being recognized in England and around the world.[15] The term is derived from Ancient Greek δεινός (deinos), meaning ‘terrible, potent or fearfully great’, and σαῦρος (sauros), meaning ‘lizard or reptile’.[15][16]Though the taxonomic name has often been interpreted as a reference to dinosaurs’ teeth, claws, and other fearsome characteristics, Owen intended it merely to evoke their size and majesty.[17]

Other prehistoric animals, including pterosaursmosasaursichthyosaursplesiosaurs, and Dimetrodon, while often popularly conceived of as dinosaurs, are not taxonomically classified as dinosaurs.[18] Pterosaurs are distantly related to dinosaurs, being members of the clade Ornithodira. The other groups mentioned are, like dinosaurs and pterosaurs, members of Sauropsida (the reptile and bird clade), except Dimetrodon (which is a synapsid).

Definition

Triceratops skeleton, Natural History Museum of Los Angeles County

Under phylogenetic nomenclature, dinosaurs are usually defined as the group consisting of the most recent common ancestor (MRCA) of Triceratops and Neornithes, and all its descendants.[19] It has also been suggested that Dinosauria be defined with respect to the MRCA of Megalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria.[20] Both definitions result in the same set of animals being defined as dinosaurs: “Dinosauria = Ornithischia + Saurischia“, encompassing ankylosaurians(armored herbivorous quadrupeds), stegosaurians (plated herbivorous quadrupeds), ceratopsians (herbivorous quadrupeds with horns and frills), ornithopods (bipedal or quadrupedal herbivores including “duck-bills”), theropods (mostly bipedal carnivores and birds), and sauropodomorphs(mostly large herbivorous quadrupeds with long necks and tails).[21]

Birds are now recognized as being the sole surviving lineage of theropod dinosaurs. In traditional taxonomy, birds were considered a separate classthat had evolved from dinosaurs, a distinct superorder. However, a majority of contemporary paleontologists concerned with dinosaurs reject the traditional style of classification in favor of phylogenetic taxonomy; this approach requires that, for a group to be natural, all descendants of members of the group must be included in the group as well. Birds are thus considered to be dinosaurs and dinosaurs are, therefore, not extinct.[22] Birds are classified as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs.[23]

Research by Matthew Baron, David B. Norman, and Paul M. Barrett in 2017 suggested a radical revision of dinosaurian systematics. Phylogenetic analysis by Baron et al. recovered the Ornithischia as being closer to the Theropoda than the Sauropodomorpha, as opposed to the traditional union of theropods with sauropodomorphs. They resurrected the clade Ornithoscelida to refer to the group containing Ornithischia and Theropoda. Dinosauria itself was re-defined as the last common ancestor of Triceratops horridusPasser domesticusDiplodocus carnegii, and all of its descendants, to ensure that sauropods and kin remain included as dinosaurs.[24][25]

General description

montage of four birds

In phylogenetic taxonomy, birds are included in the group Dinosauria.

Using one of the above definitions, dinosaurs can be generally described as archosaurs with hind limbs held erect beneath the body.[26] Many prehistoric animal groups are popularly conceived of as dinosaurs, such as ichthyosaursmosasaursplesiosaurspterosaurs, and pelycosaurs (especially Dimetrodon), but are not classified scientifically as dinosaurs, and none had the erect hind limb posture characteristic of true dinosaurs.[27] Dinosaurs were the dominant terrestrial vertebrates of the Mesozoic, especially the Jurassic and Cretaceous periods. Other groups of animals were restricted in size and niches; mammals, for example, rarely exceeded the size of a domestic cat, and were generally rodent-sized carnivores of small prey.[28]

Dinosaurs have always been an extremely varied group of animals; according to a 2006 study, over 500 non-avian dinosaur genera have been identified with certainty so far, and the total number of genera preserved in the fossil record has been estimated at around 1850, nearly 75% of which remain to be discovered.[10] An earlier study predicted that about 3,400 dinosaur genera existed, including many that would not have been preserved in the fossil record.[29] By September 17, 2008, 1,047 different species of dinosaurs had been named.[11]

In 2016, the estimated number of dinosaur species that existed in the Mesozoic era was estimated to be 1,543–2,468.[30][31] Some are herbivorous, others carnivorous, including seed-eaters, fish-eaters, insectivores, and omnivores. While dinosaurs were ancestrally bipedal (as are all modern birds), some prehistoric species were quadrupeds, and others, such as Anchisaurus and Iguanodon, could walk just as easily on two or four legs. Cranial modifications like horns and crests are common dinosaurian traits, and some extinct species had bony armor. Although known for large size, many Mesozoic dinosaurs were human-sized or smaller, and modern birds are generally small in size. Dinosaurs today inhabit every continent, and fossils show that they had achieved global distribution by at least the early Jurassic period.[12] Modern birds inhabit most available habitats, from terrestrial to marine, and there is evidence that some non-avian dinosaurs (such as Microraptor) could fly or at least glide, and others, such as spinosaurids, had semiaquatic habits.[32]

Distinguishing anatomical features

While recent discoveries have made it more difficult to present a universally agreed-upon list of dinosaurs’ distinguishing features, nearly all dinosaurs discovered so far share certain modifications to the ancestral archosaurian skeleton, or are clear descendants of older dinosaurs showing these modifications. Although some later groups of dinosaurs featured further modified versions of these traits, they are considered typical for Dinosauria; the earliest dinosaurs had them and passed them on to their descendants. Such modifications, originating in the most recent common ancestor of a certain taxonomic group, are called the synapomorphies of such a group.[33]

A detailed assessment of archosaur interrelations by Sterling Nesbitt[34] confirmed or found the following twelve unambiguous synapomorphies, some previously known:

  • in the skull, a supratemporal fossa (excavation) is present in front of the supratemporal fenestra, the main opening in the rear skull roof
  • epipophyses, obliquely backward pointing processes on the rear top corners, present in the anterior (front) neck vertebrae behind the atlas and axis, the first two neck vertebrae
  • apex of deltopectoral crest (a projection on which the deltopectoral muscles attach) located at or more than 30% down the length of the humerus (upper arm bone)
  • radius, a lower arm bone, shorter than 80% of humerus length
  • fourth trochanter (projection where the caudofemoralis muscle attaches on the inner rear shaft) on the femur (thighbone) is a sharp flange
  • fourth trochanter asymmetrical, with distal, lower, margin forming a steeper angle to the shaft
  • on the astragalus and calcaneum, upper ankle bones, the proximal articular facet, the top connecting surface, for the fibula occupies less than 30% of the transverse width of the element
  • exoccipitals (bones at the back of the skull) do not meet along the midline on the floor of the endocranial cavity, the inner space of the braincase
  • in the pelvis, the proximal articular surfaces of the ischium with the ilium and the pubis are separated by a large concave surface (on the upper side of the ischium a part of the open hip joint is located between the contacts with the pubic bone and the ilium)
  • cnemial crest on the tibia (protruding part of the top surface of the shinbone) arcs anterolaterally (curves to the front and the outer side)
  • distinct proximodistally oriented (vertical) ridge present on the posterior face of the distal end of the tibia (the rear surface of the lower end of the shinbone)
  • concave articular surface for the fibula of the calcaneum (the top surface of the calcaneum, where it touches the fibula, has a hollow profile)

Nesbitt found a number of further potential synapomorphies, and discounted a number of synapomorphies previously suggested. Some of these are also present in silesaurids, which Nesbitt recovered as a sister group to Dinosauria, including a large anterior trochanter, metatarsals II and IV of subequal length, reduced contact between ischium and pubis, the presence of a cnemial crest on the tibia and of an ascending process on the astragalus, and many others.[19]

Diagram of a typical diapsid skull
j: jugal bone, po: postorbital bone, p: parietal bone, sq: squamosal bone, q: quadrate bone, qj: quadratojugal bone

A variety of other skeletal features are shared by dinosaurs. However, because they are either common to other groups of archosaurs or were not present in all early dinosaurs, these features are not considered to be synapomorphies. For example, as diapsids, dinosaurs ancestrally had two pairs of temporal fenestrae (openings in the skull behind the eyes), and as members of the diapsid group Archosauria, had additional openings in the snout and lower jaw.[35] Additionally, several characteristics once thought to be synapomorphies are now known to have appeared before dinosaurs, or were absent in the earliest dinosaurs and independently evolved by different dinosaur groups. These include an elongated scapula, or shoulder blade; a sacrumcomposed of three or more fused vertebrae (three are found in some other archosaurs, but only two are found in Herrerasaurus);[19] and a perforate acetabulum, or hip socket, with a hole at the center of its inside surface (closed in Saturnalia, for example).[36][37] Another difficulty of determining distinctly dinosaurian features is that early dinosaurs and other archosaurs from the late Triassic are often poorly known and were similar in many ways; these animals have sometimes been misidentified in the literature.[38]

Hip joints and hindlimb postures of: (left to right) typical reptiles (sprawling), dinosaurs and mammals (erect), and rauisuchians (erect)

Dinosaurs stand with their hind limbs erect in a manner similar to most modern mammals, but distinct from most other reptiles, whose limbs sprawl out to either side.[39] This posture is due to the development of a laterally facing recess in the pelvis (usually an open socket) and a corresponding inwardly facing distinct head on the femur.[40] Their erect posture enabled early dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that surpassed those of “sprawling” reptiles.[41] Erect limbs probably also helped support the evolution of large size by reducing bending stresses on limbs.[42] Some non-dinosaurian archosaurs, including rauisuchians, also had erect limbs but achieved this by a “pillar erect” configuration of the hip joint, where instead of having a projection from the femur insert on a socket on the hip, the upper pelvic bone was rotated to form an overhanging shelf.[42]

Evolutionary history

Main article: Evolution of dinosaurs

Origins and early evolution

Dinosaurs diverged from their archosaur ancestors during the middle to late Triassic period, roughly 20 million years after the Permian–Triassic extinction event wiped out an estimated 95% of all life on Earth.[43][44] Radiometric dating of the rock formation that contained fossils from the early dinosaur genus Eoraptor at 231.4 million years old establishes its presence in the fossil record at this time.[45] Paleontologists think that Eoraptor resembles the common ancestor of all dinosaurs;[46] if this is true, its traits suggest that the first dinosaurs were small, bipedal predators.[47] The discovery of primitive, dinosaur-like ornithodirans such as Marasuchus and Lagerpeton in Argentinian Middle Triassic strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators. Dinosaurs may have appeared as early as 243 million years ago, as evidenced by remains of the genus Nyasasaurus from that period, though known fossils of these animals are too fragmentary to tell if they are dinosaurs or very close dinosaurian relatives.[48] Recently, it has been determined that Staurikosaurus from the Santa Maria Formation dates to 233.23 Ma, making it older in geologic age than Eoraptor.[1]

When dinosaurs appeared, they were not the dominant terrestrial animals. The terrestrial habitats were occupied by various types of archosauromorphs and therapsids, like cynodonts and rhynchosaurs. Their main competitors were the pseudosuchia, such as aetosaursornithosuchids and rauisuchians, which were more successful than the dinosaurs.[49] Most of these other animals became extinct in the Triassic, in one of two events. First, at about 215 million years ago, a variety of basal archosauromorphs, including the protorosaurs, became extinct. This was followed by the Triassic–Jurassic extinction event (about 200 million years ago), that saw the end of most of the other groups of early archosaurs, like aetosaurs, ornithosuchids, phytosaurs, and rauisuchians. Rhynchosaurs and dicynodonts survived (at least in some areas) at least as late as early-mid Norian and late Norian or earliest Rhaetian, respectively,[50][51] and the exact date of their extinction is uncertain. These losses left behind a land fauna of crocodylomorphs, dinosaurs, mammalspterosaurians, and turtles.[19] The first few lines of early dinosaurs diversifiedthrough the Carnian and Norian stages of the Triassic, possibly by occupying the niches of the groups that became extinct.[21] Also notably, there was a heightened rate of extinction during the Carnian Pluvial Event.[52]

Evolution and paleobiogeography

Dinosaur evolution after the Triassic follows changes in vegetation and the location of continents. In the late Triassic and early Jurassic, the continents were connected as the single landmass Pangaea, and there was a worldwide dinosaur fauna mostly composed of coelophysoid carnivores and early sauropodomorph herbivores.[53] Gymnosperm plants (particularly conifers), a potential food source, radiated in the late Triassic. Early sauropodomorphs did not have sophisticated mechanisms for processing food in the mouth, and so must have employed other means of breaking down food farther along the digestive tract.[54] The general homogeneity of dinosaurian faunas continued into the middle and late Jurassic, where most localities had predators consisting of ceratosauriansspinosauroids, and carnosaurians, and herbivores consisting of stegosaurian ornithischians and large sauropods. Examples of this include the Morrison Formationof North America and Tendaguru Beds of Tanzania. Dinosaurs in China show some differences, with specialized sinraptorid theropods and unusual, long-necked sauropods like Mamenchisaurus.[53] Ankylosaurians and ornithopods were also becoming more common, but prosauropods had become extinct. Conifers and pteridophytes were the most common plants. Sauropods, like the earlier prosauropods, were not oral processors, but ornithischians were evolving various means of dealing with food in the mouth, including potential cheek-like organs to keep food in the mouth, and jaw motions to grind food.[54] Another notable evolutionary event of the Jurassic was the appearance of true birds, descended from maniraptoran coelurosaurians.[55]

Skeleton of Marasuchus lilloensis, a dinosaur-like ornithodiran

By the early Cretaceous and the ongoing breakup of Pangaea, dinosaurs were becoming strongly differentiated by landmass. The earliest part of this time saw the spread of ankylosaurians, iguanodontians, and brachiosaurids through Europe, North America, and northern Africa. These were later supplemented or replaced in Africa by large spinosaurid and carcharodontosaurid theropods, and rebbachisaurid and titanosaurian sauropods, also found in South America. In Asia, maniraptoran coelurosaurians like dromaeosauridstroodontids, and oviraptorosaurians became the common theropods, and ankylosaurids and early ceratopsians like Psittacosaurus became important herbivores. Meanwhile, Australia was home to a fauna of basal ankylosaurians, hypsilophodonts, and iguanodontians.[53] The stegosaurians appear to have gone extinct at some point in the late early Cretaceous or early late Cretaceous. A major change in the early Cretaceous, which would be amplified in the late Cretaceous, was the evolution of flowering plants. At the same time, several groups of dinosaurian herbivores evolved more sophisticated ways to orally process food. Ceratopsians developed a method of slicing with teeth stacked on each other in batteries, and iguanodontians refined a method of grinding with tooth batteries, taken to its extreme in hadrosaurids.[54] Some sauropods also evolved tooth batteries, best exemplified by the rebbachisaurid Nigersaurus.[56]

Full skeleton of an early carnivorous dinosaur, displayed in a glass case in a museum

The early forms Herrerasaurus(large), Eoraptor (small) and a Plateosaurus skull

There were three general dinosaur faunas in the late Cretaceous. In the northern continents of North America and Asia, the major theropods were tyrannosaurids and various types of smaller maniraptoran theropods, with a predominantly ornithischian herbivore assemblage of hadrosaurids, ceratopsians, ankylosaurids, and pachycephalosaurians. In the southern continents that had made up the now-splitting Gondwanaabelisaurids were the common theropods, and titanosaurian sauropods the common herbivores. Finally, in Europe, dromaeosaurids, rhabdodontid iguanodontians, nodosauridankylosaurians, and titanosaurian sauropods were prevalent.[53] Flowering plants were greatly radiating,[54] with the first grasses appearing by the end of the Cretaceous.[57] Grinding hadrosaurids and shearing ceratopsians became extremely diverse across North America and Asia. Theropods were also radiating as herbivores or omnivores, with therizinosaurians and ornithomimosaurians becoming common.[54]

The Cretaceous–Paleogene extinction event, which occurred approximately 66 million years ago at the end of the Cretaceous period, caused the extinction of all dinosaur groups except for the neornithine birds. Some other diapsid groups, such as crocodilians, sebecosuchiansturtleslizardssnakessphenodontians, and choristoderans, also survived the event.[58]

The surviving lineages of neornithine birds, including the ancestors of modern ratitesducks and chickens, and a variety of waterbirds, diversified rapidly at the beginning of the Paleogeneperiod, entering ecological niches left vacant by the extinction of Mesozoic dinosaur groups such as the arboreal enantiornithines, aquatic hesperornithines, and even the larger terrestrial theropods (in the form of Gastorniseogruiidsbathornithidsratitesgeranoididsmihirungs, and “terror birds“). It is often cited that mammals out-competed the neornithines for dominance of most terrestrial niches but many of these groups co-existed with rich mammalian faunas for most of the Cenozoic.[59] Terror birds and bathornithids occupied carnivorous guilds alongside predatory mammals,[60][61] and ratites are still fairly successful as mid-sized herbivores; eogruiids similarly lasted from the Eocene to Pliocene, only becoming extinct very recently after over 20 million years of co-existence with many mammal groups.[62]

Classification

Main article: Dinosaur classification

Dinosaurs belong to a group known as archosaurs, which also includes modern crocodilians. Within the archosaur group, dinosaurs are differentiated most noticeably by their gait. Dinosaur legs extend directly beneath the body, whereas the legs of lizards and crocodilians sprawl out to either side.[33]

Collectively, dinosaurs as a clade are divided into two primary branches, Saurischia and Ornithischia. Saurischia includes those taxa sharing a more recent common ancestor with birds than with Ornithischia, while Ornithischia includes all taxa sharing a more recent common ancestor with Triceratops than with Saurischia. Anatomically, these two groups can be distinguished most noticeably by their pelvic structure. Early saurischians—”lizard-hipped”, from the Greek sauros (σαῦρος) meaning “lizard” and ischion (ἰσχίον) meaning “hip joint”—retained the hip structure of their ancestors, with a pubis bone directed cranially, or forward.[40] This basic form was modified by rotating the pubis backward to varying degrees in several groups (Herrerasaurus,[63]therizinosauroids,[64] dromaeosaurids,[65] and birds[55]). Saurischia includes the theropods (exclusively bipedal and with a wide variety of diets) and sauropodomorphs (long-necked herbivoreswhich include advanced, quadrupedal groups).[66][67]

By contrast, ornithischians—”bird-hipped”, from the Greek ornitheios (ὀρνίθειος) meaning “of a bird” and ischion (ἰσχίον) meaning “hip joint”—had a pelvis that superficially resembled a bird’s pelvis: the pubic bone was oriented caudally (rear-pointing). Unlike birds, the ornithischian pubis also usually had an additional forward-pointing process. Ornithischia includes a variety of species which were primarily herbivores. (NB: the terms “lizard hip” and “bird hip” are misnomers – birds evolved from dinosaurs with “lizard hips”.)[33]

Taxonomy

The following is a simplified classification of dinosaur groups based on their evolutionary relationships, and organized based on the list of Mesozoic dinosaur species provided by Holtz (2007).[7]A more detailed version can be found at Dinosaur classification. The dagger (†) is used to signify groups with no living members.

  • Dinosauria
  • Saurischia (“lizard-hipped”; includes Theropoda and Sauropodomorpha)

Artist’s impression of six dromaeosauridtheropods: from left to right MicroraptorVelociraptorAustroraptorDromaeosaurusUtahraptor, and Deinonychus

Artist’s impression of four macronariansauropods: from left to right CamarasaurusBrachiosaurusGiraffatitan, and Euhelopus

  • Diplodocoidea (skulls and tails elongated; teeth typically narrow and pencil-like)
  • Macronaria (boxy skulls; spoon- or pencil-shaped teeth)
  • Brachiosauridae (long-necked, long-armed macronarians)
  • Titanosauria (diverse; stocky, with wide hips; most common in the late Cretaceous of southern continents)

Restoration of six ornithopods; far left: Camptosaurus, left: Iguanodon, center background: Shantungosaurus, center foreground: Dryosaurus, right: Corythosaurus, far right (large) Tenontosaurus.

  • Ornithischia (“bird-hipped”; diverse bipedal and quadrupedal herbivores)
  • Ornithopoda (various sizes; bipeds and quadrupeds; evolved a method of chewing using skull flexibility and numerous teeth)
  • Marginocephalia (characterized by a cranial growth)

Biology

Knowledge about dinosaurs is derived from a variety of fossil and non-fossil records, including fossilized bonesfecestrackwaysgastrolithsfeathers, impressions of skin, internal organs and soft tissues.[68][69] Many fields of study contribute to our understanding of dinosaurs, including physics (especially biomechanics), chemistrybiology, and the earth sciences (of which paleontology is a sub-discipline).[70][71] Two topics of particular interest and study have been dinosaur size and behavior.[72]

Size

Main article: Dinosaur size

Scale diagram comparing the average human to the largest known dinosaurs in five major cladesSauropoda (Argentinosaurus huinculensis), Ornithopoda (Shantungosaurus giganteus), Theropoda(Spinosaurus aegyptiacus), Thyreophora (Stegosaurus armatus) and Marginocephalia (Triceratops prorsus)

Current evidence suggests that dinosaur average size varied through the Triassic, early Jurassic, late Jurassic and Cretaceous periods.[46] Predatory theropod dinosaurs, which occupied most terrestrial carnivore niches during the Mesozoic, most often fall into the 100 to 1000 kg (220 to 2200 lb) category when sorted by estimated weight into categories based on order of magnitude, whereas recent predatory carnivoran mammals peak in the 10 to 100 kg (22 to 220 lb) category.[73] The mode of Mesozoic dinosaur body masses is between one and ten metric tonnes.[74] This contrasts sharply with the average size of Cenozoic mammals, estimated by the National Museum of Natural History as about 2 to 5 kg (4.4 to 11.0 lb).[75]

The sauropods were the largest and heaviest dinosaurs. For much of the dinosaur era, the smallest sauropods were larger than anything else in their habitat, and the largest were an order of magnitude more massive than anything else that has since walked the Earth. Giant prehistoric mammals such as Paraceratherium (the largest land mammal ever) were dwarfed by the giant sauropods, and only modern whales approach or surpass them in size.[76] There are several proposed advantages for the large size of sauropods, including protection from predation, reduction of energy use, and longevity, but it may be that the most important advantage was dietary. Large animals are more efficient at digestion than small animals, because food spends more time in their digestive systems. This also permits them to subsist on food with lower nutritive value than smaller animals. Sauropod remains are mostly found in rock formationsinterpreted as dry or seasonally dry, and the ability to eat large quantities of low-nutrient browse would have been advantageous in such environments.[14]

Largest and smallest

Scientists will probably never be certain of the largest and smallest dinosaurs to have ever existed. This is because only a tiny percentage of animals ever fossilize, and most of these remain buried in the earth. Few of the specimens that are recovered are complete skeletons, and impressions of skin and other soft tissues are rare. Rebuilding a complete skeleton by comparing the size and morphology of bones to those of similar, better-known species is an inexact art, and reconstructing the muscles and other organs of the living animal is, at best, a process of educated guesswork.[77]

Comparative size of Argentinosaurus to the average human

The tallest and heaviest dinosaur known from good skeletons is Giraffatitan brancai (previously classified as a species of Brachiosaurus). Its remains were discovered in Tanzania between 1907 and 1912. Bones from several similar-sized individuals were incorporated into the skeleton now mounted and on display at the Museum für Naturkunde Berlin;[78] this mount is 12 meters (39 ft) tall and 21.8–22.5 meters (72–74 ft) long,[79][80] and would have belonged to an animal that weighed between 30000 and 60000 kilograms (70000 and 130000 lb). The longest complete dinosaur is the 27 meters (89 feet) long Diplodocus, which was discovered in Wyoming in the United States and displayed in Pittsburgh’s Carnegie Natural History Museum in 1907.[81]The longest dinosaur known from good fossil material is the Patagotitan: the skeleton mount in the American Museum of Natural History is 37 meters (121 ft) long. The Carmen Funes Museum has an Argentinosaurus reconstructed skeleton mount 39.7 metres (130 ft) long.[82]

Comparative size of Eoraptor to the average human

There were larger dinosaurs, but knowledge of them is based entirely on a small number of fragmentary fossils. Most of the largest herbivorous specimens on record were discovered in the 1970s or later, and include the massive Argentinosaurus, which may have weighed 80000 to 100000 kilograms (90 to 110 short tons) and reached length of 30–40 metres (98–131 ft); some of the longest were the 33.5 meters (110 ft) long Diplodocus hallorum[14] (formerly Seismosaurus), the 33–34 meters (108–112 ft) long Supersaurus[83] and 37 metres (121 ft) long Patagotitan; and the tallest, the 18 meters (59 ft) tall Sauroposeidon, which could have reached a sixth-floor window. The heaviest and longest dinosaur may have been Amphicoelias fragillimus, known only from a now lost partial vertebral neural arch described in 1878. Extrapolating from the illustration of this bone, the animal may have been 58 meters (190 ft) long and weighed 122400 kg (270000 lb).[14] However, as no further evidence of sauropods of this size has been found, and the discoverer, Edward Cope, had made typographic errors before, it is likely to have been an extreme overestimation.[84]

The largest carnivorous dinosaur was Spinosaurus, reaching a length of 12.6 to 18 meters (41 to 59 ft), and weighing 7–20.9 tonnes (7.7–23 short tons).[85][86] Other large carnivorous theropods included GiganotosaurusCarcharodontosaurus and Tyrannosaurus.[86] Therizinosaurus and Deinocheirus were among the tallest of the theropods. The largest Ornithischian dinosaur was probably the hadrosaurid Shantungosaurus which measured 16 metres (52 ft) and weighed about 13 tonnes (29,000 lb).

The smallest dinosaur known is the bee hummingbird,[87] with a length of only 5 cm (2.0 in) and mass of around 1.8 g (0.063 oz).[88] The smallest known non-avialan dinosaurs were about the size of pigeons and were those theropods most closely related to birds.[89] For example, Anchiornis huxleyi is currently the smallest non-avialan dinosaur described from an adult specimen, with an estimated weight of 110 grams[90] and a total skeletal length of 34 cm (1.12 ft).[89][90] The smallest herbivorous non-avialan dinosaurs included Microceratus and Wannanosaurus, at about 60 cm (2.0 ft) long each.[7][91]

Behavior

A nesting ground of hadrosaur Maiasaura peeblesorum was discovered in 1978.

Many modern birds are highly social, often found living in flocks. There is general agreement that some behaviors that are common in birds, as well as in crocodiles (birds’ closest living relatives), were also common among extinct dinosaur groups. Interpretations of behavior in fossil species are generally based on the pose of skeletons and their habitatcomputer simulations of their biomechanics, and comparisons with modern animals in similar ecological niches.[70]

The first potential evidence for herding or flocking as a widespread behavior common to many dinosaur groups in addition to birds was the 1878 discovery of 31 Iguanodon bernissartensis, ornithischians that were then thought to have perished together in BernissartBelgium, after they fell into a deep, flooded sinkhole and drowned.[92] Other mass-death sites have been discovered subsequently. Those, along with multiple trackways, suggest that gregarious behavior was common in many early dinosaur species. Trackways of hundreds or even thousands of herbivores indicate that duck-bills(hadrosaurids) may have moved in great herds, like the American bison or the African Springbok. Sauropod tracks document that these animals traveled in groups composed of several different species, at least in Oxfordshire, England,[93] although there is no evidence for specific herd structures.[94]Congregating into herds may have evolved for defense, for migratory purposes, or to provide protection for young. There is evidence that many types of slow-growing dinosaurs, including various theropods, sauropods, ankylosaurians, ornithopods, and ceratopsians, formed aggregations of immature individuals. One example is a site in Inner Mongolia that has yielded the remains of over 20 Sinornithomimus, from one to seven years old. This assemblage is interpreted as a social group that was trapped in mud.[95] The interpretation of dinosaurs as gregarious has also extended to depicting carnivorous theropods as pack hunters working together to bring down large prey.[96][97]However, this lifestyle is uncommon among modern birds, crocodiles, and other reptiles, and the taphonomic evidence suggesting mammal-like pack hunting in such theropods as Deinonychusand Allosaurus can also be interpreted as the results of fatal disputes between feeding animals, as is seen in many modern diapsid predators.[98]

Artist’s rendering of two Centrosaurus apertus engaged in intra-specific combat

The crests and frills of some dinosaurs, like the marginocephalianstheropods and lambeosaurines, may have been too fragile to be used for active defense, and so they were likely used for sexual or aggressive displays, though little is known about dinosaur mating and territorialism. Head wounds from bites suggest that theropods, at least, engaged in active aggressive confrontations.[99]

From a behavioral standpoint, one of the most valuable dinosaur fossils was discovered in the Gobi Desert in 1971. It included a Velociraptor attacking a Protoceratops,[100] providing evidence that dinosaurs did indeed attack each other.[101] Additional evidence for attacking live prey is the partially healed tail of an Edmontosaurus, a hadrosaurid dinosaur; the tail is damaged in such a way that shows the animal was bitten by a tyrannosaur but survived.[101]Cannibalism amongst some species of dinosaurs was confirmed by tooth marks found in Madagascar in 2003, involving the theropod Majungasaurus.[102]

Comparisons between the scleral rings of dinosaurs and modern birds and reptiles have been used to infer daily activity patterns of dinosaurs. Although it has been suggested that most dinosaurs were active during the day, these comparisons have shown that small predatory dinosaurs such as dromaeosauridsJuravenator, and Megapnosauruswere likely nocturnal. Large and medium-sized herbivorous and omnivorous dinosaurs such as ceratopsianssauropodomorphshadrosauridsornithomimosaurs may have been cathemeral, active during short intervals throughout the day, although the small ornithischian Agilisaurus was inferred to be diurnal.[103]

Based on current fossil evidence from dinosaurs such as Oryctodromeus, some ornithischian species seem to have led a partially fossorial (burrowing) lifestyle.[104] Many modern birds are arboreal (tree climbing), and this was also true of many Mesozoic birds, especially the enantiornithines.[105] While some early bird-like species may have already been arboreal as well (including dromaeosaurids such as Microraptor[106]) most non-avialan dinosaurs seem to have relied on land-based locomotion. A good understanding of how dinosaurs moved on the ground is key to models of dinosaur behavior; the science of biomechanics, pioneered by Robert McNeill Alexander, has provided significant insight in this area. For example, studies of the forces exerted by muscles and gravity on dinosaurs’ skeletal structure have investigated how fast dinosaurs could run,[107] whether diplodocids could create sonic booms via whip-like tail snapping,[108] and whether sauropods could float.[109]

Communication

Artist’s impression of a striking and unusual visual display in a Lambeosaurus magnicristatus

Modern birds are known to communicate using visual and auditory signals, and the wide diversity of visual display structures among fossil dinosaur groups, such as horns, frills, crests, sails and feathers, suggests that visual communication has always been important in dinosaur biology.[110]Reconstruction of the plumage color of Anchiornis huxleyi, suggest the importance of color in visual communication in non-avian dinosaurs.[111] The evolution of dinosaur vocalization is less certain. Paleontologist Phil Senter suggests that non-avian dinosaurs relied mostly on visual displays and possibly non-vocal acoustic sounds like hissing, jaw grinding or clapping, splashing and wing beating (possible in winged maniraptoran dinosaurs). He states they were unlikely to have been capable of vocalizing since their closest relatives, crocodilians and birds, use different means to vocalize, the former via the larynx and the latter through the unique syrinx, suggesting they evolved independently and their common ancestor was mute.[110]

The earliest remains of a syrinx, which has enough mineral content for fossilization, was found in a specimen of the duck-like Vegavis iaai dated 69-66 million year ago, and this organ is unlikely to have existed in non-avian dinosaurs. However, in contrast to Senter, the researchers have suggested that dinosaurs could vocalize and that the syrinx-based vocal system of birds evolved from a larynx-based one, rather than the two systems evolving independently.[112] A 2016 study suggests that dinosaurs produced closed mouth vocalizations like cooing, which occur in both crocodilians and birds as well as other reptiles. Such vocalizations evolved independently in extant archosaurs numerous times, following increases in body size.[113] The crests of the Lambeosaurini and nasal chambers of ankylosaurids have been suggested to function in vocal resonance,[114][115] though Senter states that the presence of resonance chambers in some dinosaurs is not necessarily evidence of vocalization as modern snakes have such chambers which intensify their hisses.[110]

Reproductive biology

See also: Dinosaur egg

Three bluish eggs with black speckling sit atop a layer of white mollusk shell pieces, surrounded by sandy ground and small bits of bluish stone

Nest of a plover (Charadrius)

All dinosaurs lay amniotic eggs with hard shells made mostly of calcium carbonate.[116] Eggs are usually laid in a nest. Most species create somewhat elaborate nests, which can be cups, domes, plates, beds scrapes, mounds, or burrows.[117] Some species of modern bird have no nests; the cliff-nesting common guillemot lays its eggs on bare rock, and male emperor penguins keep eggs between their body and feet. Primitive birds and many non-avialan dinosaurs often lay eggs in communal nests, with males primarily incubating the eggs. While modern birds have only one functional oviduct and lay one egg at a time, more primitive birds and dinosaurs had two oviducts, like crocodiles. Some non-avialan dinosaurs, such as Troodon, exhibited iterative laying, where the adult might lay a pair of eggs every one or two days, and then ensured simultaneous hatching by delaying brooding until all eggs were laid.[118]

When laying eggs, females grow a special type of bone between the hard outer bone and the marrow of their limbs. This medullary bone, which is rich in calcium, is used to make eggshells. A discovery of features in a Tyrannosaurus rex skeleton provided evidence of medullary bone in extinct dinosaurs and, for the first time, allowed paleontologists to establish the sex of a fossil dinosaur specimen. Further research has found medullary bone in the carnosaur Allosaurus and the ornithopod Tenontosaurus. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that the production of medullary tissue is a general characteristic of all dinosaurs.[119]

Fossil interpreted as a nesting oviraptoridCitipati at the American Museum of Natural History. Smaller fossil far right showing inside one of the eggs.

Another widespread trait among modern birds (but see below in regards to fossil groups and extant megapodes) is parental care for young after hatching. Jack Horner’s 1978 discovery of a Maiasaura (“good mother lizard”) nesting ground in Montana demonstrated that parental care continued long after birth among ornithopods.[120] A specimen of the Mongolian oviraptorid Citipati osmolskae was discovered in a chicken-like brooding position in 1993,[121]which may indicate that they had begun using an insulating layer of feathers to keep the eggs warm.[122] A dinosaur embryo (pertaining to the prosauropod Massospondylus) was found without teeth, indicating that some parental care was required to feed the young dinosaurs.[123] Trackways have also confirmed parental behavior among ornithopods from the Isle of Skye in northwestern Scotland.[124]

However, there is ample evidence of supreprecociality among many dinosaur species, particularly theropods. For instance, non-ornithuromorph birds have been abundantly demonstrated to have had slow growth rates, megapode-like egg burying behaviour and the ability to fly soon after birth.[125][126][127] Both Tyrannosaurus rex and Troodon formosus display juveniles with clear supreprecociality and likely occupying different ecological niches than the adults.[128] Superprecociality has been inferred for sauropods.[129]

Physiology

Main article: Physiology of dinosaurs

Because both modern crocodilians and birds have four-chambered hearts (albeit modified in crocodilians), it is likely that this is a trait shared by all archosaurs, including all dinosaurs.[130] While all modern birds have high metabolisms and are “warm blooded” (endothermic), a vigorous debate has been ongoing since the 1960s regarding how far back in the dinosaur lineage this trait extends. Scientists disagree as to whether non-avian dinosaurs were endothermic, ectothermic, or some combination of both.[131]

After non-avian dinosaurs were discovered, paleontologists first posited that they were ectothermic. This supposed “cold-bloodedness” was used to imply that the ancient dinosaurs were relatively slow, sluggish organisms, even though many modern reptiles are fast and light-footed despite relying on external sources of heat to regulate their body temperature. The idea of dinosaurs as ectothermic and sluggish remained a prevalent view until Robert T. “Bob” Bakker, an early proponent of dinosaur endothermy, published an influential paper on the topic in 1968.[132]

Modern evidence indicates that even non-avian dinosaurs and birds thrived in cooler temperate climates, and that at least some early species must have regulated their body temperature by internal biological means (aided by the animals’ bulk in large species and feathers or other body coverings in smaller species). Evidence of endothermy in Mesozoic dinosaurs includes the discovery of polar dinosaurs in Australia and Antarctica as well as analysis of blood-vessel structures within fossil bones that are typical of endotherms. Scientific debate continues regarding the specific ways in which dinosaur temperature regulation evolved.[133][134]

Comparison between the air sacs of an abelisaur and a bird

In saurischian dinosaurs, higher metabolisms were supported by the evolution of the avian respiratory system, characterized by an extensive system of air sacs that extended the lungs and invaded many of the bones in the skeleton, making them hollow.[135] Early avian-style respiratory systems with air sacs may have been capable of sustaining higher activity levels than those of mammals of similar size and build. In addition to providing a very efficient supply of oxygen, the rapid airflow would have been an effective cooling mechanism, which is essential for animals that are active but too large to get rid of all the excess heat through their skin.[136]

Like other reptiles, dinosaurs are primarily uricotelic, that is, their kidneys extract nitrogenous wastes from their bloodstream and excrete it as uric acidinstead of urea or ammonia via the ureters into the intestine. In most living species, uric acid is excreted along with feces as a semisolid waste.[137][138][139] However, at least some modern birds (such as hummingbirds) can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.[140] They also excrete creatine, rather than creatinine like mammals.[141] This material, as well as the output of the intestines, emerges from the cloaca.[142][143] In addition, many species regurgitate pellets, and fossil pellets that may have come from dinosaurs are known from as long ago as the Cretaceous period.[144]

Origin of birds

Main article: Origin of birds

The possibility that dinosaurs were the ancestors of birds was first suggested in 1868 by Thomas Henry Huxley.[145] After the work of Gerhard Heilmann in the early 20th century, the theory of birds as dinosaur descendants was abandoned in favor of the idea of their being descendants of generalized thecodonts, with the key piece of evidence being the supposed lack of clavicles in dinosaurs.[146] However, as later discoveries showed, clavicles (or a single fused wishbone, which derived from separate clavicles) were not actually absent;[55] they had been found as early as 1924 in Oviraptor, but misidentified as an interclavicle.[147] In the 1970s, John Ostrom revived the dinosaur–bird theory,[148] which gained momentum in the coming decades with the advent of cladistic analysis,[149] and a great increase in the discovery of small theropods and early birds.[35] Of particular note have been the fossils of the Yixian Formation, where a variety of theropods and early birds have been found, often with feathers of some type.[5][55] Birds share over a hundred distinct anatomical features with theropod dinosaurs, which are now generally accepted to have been their closest ancient relatives.[150] They are most closely allied with maniraptoran coelurosaurs.[55] A minority of scientists, most notably Alan Feduccia and Larry Martin, have proposed other evolutionary paths, including revised versions of Heilmann’s basal archosaur proposal,[151] or that maniraptoran theropods are the ancestors of birds but themselves are not dinosaurs, only convergent with dinosaurs.[152]

Feathers

Main article: Feathered dinosaurs

Various feathered non-avian dinosaurs, including ArchaeopteryxAnchiornisMicroraptor and Zhenyuanlong

Feathers are one of the most recognizable characteristics of modern birds, and a trait that was shared by all other dinosaur groups. Based on the current distribution of fossil evidence, it appears that feathers were an ancestral dinosaurian trait, though one that may have been selectively lost in some species.[153] Direct fossil evidence of feathers or feather-like structures has been discovered in a diverse array of species in many non-avian dinosaur groups,[5] both among saurischians and ornithischians. Simple, branched, feather-like structures are known from heterodontosaurids, primitive neornithischians[154] and theropods,[155] and primitive ceratopsians. Evidence for true, vaned feathers similar to the flight feathers of modern birds has been found only in the theropod subgroup Maniraptora, which includes oviraptorosaurs, troodontids, dromaeosaurids, and birds.[55][156] Feather-like structures known as pycnofibres have also been found in pterosaurs,[157] suggesting the possibility that feather-like filaments may have been common in the bird lineage and evolved before the appearance of dinosaurs themselves.[153] Research into the genetics of American alligators has also revealed that crocodylian scutes do possess feather-keratins during embryonic development, but these keratins are not expressed by the animals before hatching.[158]

Archaeopteryx was the first fossil found that revealed a potential connection between dinosaurs and birds. It is considered a transitional fossil, in that it displays features of both groups. Brought to light just two years after Darwin’s seminal The Origin of Species, its discovery spurred the nascent debate between proponents of evolutionary biology and creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, at least one specimen was mistaken for Compsognathus.[159] Since the 1990s, a number of additional feathered dinosaurs have been found, providing even stronger evidence of the close relationship between dinosaurs and modern birds. Most of these specimens were unearthed in the lagerstätte of the Yixian Formation, Liaoning, northeastern China, which was part of an island continent during the Cretaceous. Though feathers have been found in only a few locations, it is possible that non-avian dinosaurs elsewhere in the world were also feathered. The lack of widespread fossil evidence for feathered non-avian dinosaurs may be because delicate features like skin and feathers are not often preserved by fossilization and thus are absent from the fossil record.[160]

The description of feathered dinosaurs has not been without controversy; perhaps the most vocal critics have been Alan Feduccia and Theagarten Lingham-Soliar, who have proposed that some purported feather-like fossils are the result of the decomposition of collagenous fiber that underlaid the dinosaurs’ skin,[161][162][163] and that maniraptoran dinosaurs with vaned feathers were not actually dinosaurs, but convergent with dinosaurs.[152][162] However, their views have for the most part not been accepted by other researchers, to the point that the scientific nature of Feduccia’s proposals has been questioned.[164]

In 2016, it was reported that a dinosaur tail with feathers had been found enclosed in amber. The fossil is about 99 million years old.[5][165][166]

Skeleton

Because feathers are often associated with birds, feathered dinosaurs are often touted as the missing link between birds and dinosaurs. However, the multiple skeletal features also shared by the two groups represent another important line of evidence for paleontologists. Areas of the skeleton with important similarities include the neck, pubiswrist (semi-lunate carpal), arm and pectoral girdle, furcula (wishbone), and breast bone. Comparison of bird and dinosaur skeletons through cladistic analysis strengthens the case for the link.[167]

Soft anatomy

Pneumatopores on the left ilium of Aerosteon riocoloradensis

Large meat-eating dinosaurs had a complex system of air sacs similar to those found in modern birds, according to a 2005 investigation led by Patrick M. O’Connor. The lungs of theropod dinosaurs (carnivores that walked on two legs and had bird-like feet) likely pumped air into hollow sacs in their skeletons, as is the case in birds. “What was once formally considered unique to birds was present in some form in the ancestors of birds”, O’Connor said.[168] In 2008, scientists described Aerosteon riocoloradensis, the skeleton of which supplies the strongest evidence to date of a dinosaur with a bird-like breathing system. CT-scanning of Aerosteon’s fossil bones revealed evidence for the existence of air sacs within the animal’s body cavity.[169][170]

Behavioral evidence

Fossils of the troodonts Mei and Sinornithoides demonstrate that some dinosaurs slept with their heads tucked under their arms.[171] This behavior, which may have helped to keep the head warm, is also characteristic of modern birds. Several deinonychosaur and oviraptorosaur specimens have also been found preserved on top of their nests, likely brooding in a bird-like manner.[172] The ratio between egg volume and body mass of adults among these dinosaurs suggest that the eggs were primarily brooded by the male, and that the young were highly precocial, similar to many modern ground-dwelling birds.[173]

Some dinosaurs are known to have used gizzard stones like modern birds. These stones are swallowed by animals to aid digestion and break down food and hard fibers once they enter the stomach. When found in association with fossils, gizzard stones are called gastroliths.[174]

Extinction of major groups

Main article: Cretaceous–Paleogene extinction event

The discovery that birds are a type of dinosaur showed that dinosaurs in general are not, in fact, extinct as is commonly stated.[175] However, all non-avian dinosaurs, estimated to have been 628-1078 species,[176] as well as many groups of birds did suddenly become extinct approximately 66 million years ago. It has been suggested that because small mammals, squamata and birds occupied the ecological niches suited for small body size, non-avian dinosaurs never evolved a diverse fauna of small-bodied species, which led to their downfall when large-bodied terrestrial tetrapods were hit by the mass extinction event.[177] Many other groups of animals also became extinct at this time, including ammonites (nautilus-like mollusks), mosasaursplesiosaurspterosaurs, and many groups of mammals.[12] Significantly, the insects suffered no discernible population loss, which left them available as food for other survivors. This mass extinction is known as the Cretaceous–Paleogene extinction event. The nature of the event that caused this mass extinction has been extensively studied since the 1970s; at present, several related theories are supported by paleontologists. Though the consensus is that an impact event was the primary cause of dinosaur extinction, some scientists cite other possible causes, or support the idea that a confluence of several factors was responsible for the sudden disappearance of dinosaurs from the fossil record.[178][179][180]

Impact event

Main article: Chicxulub crater

The Chicxulub Crater at the tip of the Yucatán Peninsula; the impactor that formed this crater may have caused the dinosaur extinction.

The asteroid collision theory, which was brought to wide attention in 1980 by Walter Alvarez and colleagues, links the extinction event at the end of the Cretaceous period to a bolide impact approximately 66 million years ago.[181] Alvarez et al. proposed that a sudden increase in iridium levels, recorded around the world in the period’s rock stratum, was direct evidence of the impact.[182] The bulk of the evidence now suggests that a bolide 5 to 15 kilometers (3.1 to 9.3 miles) wide hit in the vicinity of the Yucatán Peninsula (in southeastern Mexico), creating the approximately 180 km (110 mi) Chicxulub Crater and triggering the mass extinction.[183][184] Scientists are not certain whether dinosaurs were thriving or declining before the impact event. Some scientists propose that the meteorite impact caused a long and unnatural drop in Earth’s atmospheric temperature, while others claim that it would have instead created an unusual heat wave. The consensus among scientists who support this theory is that the impact caused extinctions both directly (by heat from the meteorite impact) and also indirectly (via a worldwide cooling brought about when matter ejected from the impact crater reflected thermal radiation from the sun). Although the speed of extinction cannot be deduced from the fossil record alone, various models suggest that the extinction was extremely rapid, being down to hours rather than years.[185]

Deccan Traps

Main article: Deccan Traps

Before 2000, arguments that the Deccan Traps flood basalts caused the extinction were usually linked to the view that the extinction was gradual, as the flood basalt events were thought to have started around 68 million years ago and lasted for over 2 million years. However, there is evidence that two thirds of the Deccan Traps were created in only 1 million years about 66 million years ago, and so these eruptions would have caused a fairly rapid extinction, possibly over a period of thousands of years, but still longer than would be expected from a single impact event.[186][187]

The Deccan Traps in India could have caused extinction through several mechanisms, including the release into the air of dust and sulfuric aerosols, which might have blocked sunlight and thereby reduced photosynthesis in plants. In addition, Deccan Trap volcanism might have resulted in carbon dioxide emissions, which would have increased the greenhouse effect when the dust and aerosols cleared from the atmosphere.[187] Before the mass extinction of the dinosaurs, the release of volcanic gases during the formation of the Deccan Traps “contributed to an apparently massive global warming. Some data point to an average rise in temperature of 8 °C (14 °F) in the last half million years before the impact [at Chicxulub].”[186][187]

In the years when the Deccan Traps hypothesis was linked to a slower extinction, Luis Alvarez (who died in 1988) replied that paleontologists were being misled by sparse data. While his assertion was not initially well-received, later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely or at least partly due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as a drop in sea level and massive volcanic eruptions that produced the Indian Deccan Traps, and these may have contributed to the extinctions.[188]

Possible Paleocene survivors

Main article: Paleocene dinosaurs

Non-avian dinosaur remains are occasionally found above the Cretaceous–Paleogene boundary. In 2001, paleontologists Zielinski and Budahn reported the discovery of a single hadrosaur leg-bone fossil in the San Juan Basin, New Mexico, and described it as evidence of Paleocene dinosaurs. The formation in which the bone was discovered has been dated to the early Paleoceneepoch, approximately 64.5 million years ago. If the bone was not re-deposited into that stratum by weathering action, it would provide evidence that some dinosaur populations may have survived at least a half million years into the Cenozoic Era.[189] Other evidence includes the finding of dinosaur remains in the Hell Creek Formation up to 1.3 m (51 in) above the Cretaceous–Paleogene boundary, representing 40000 years of elapsed time. Similar reports have come from other parts of the world, including China.[190] Many scientists, however, dismissed the supposed Paleocene dinosaurs as re-worked, that is, washed out of their original locations and then re-buried in much later sediments.[191][192] Direct dating of the bones themselves has supported the later date, with U–Pb dating methods resulting in a precise age of 64.8 ± 0.9 million years ago.[193] If correct, the presence of a handful of dinosaurs in the early Paleocene would not change the underlying facts of the extinction.[191]

History of study

Further information: History of paleontology

Dinosaur fossils have been known for millennia, although their true nature was not recognized. The Chinese considered them to be dragon bones and documented them as such. For example, Hua Yang Guo Zhi, a book written by Chang Qu during the Western Jin Dynasty (265–316), reported the discovery of dragon bones at Wucheng in Sichuan Province.[194] Villagers in central China have long unearthed fossilized “dragon bones” for use in traditional medicines, a practice that continues today.[195] In Europe, dinosaur fossils were generally believed to be the remains of giants and other biblical creatures.[196]

Scholarly descriptions of what would now be recognized as dinosaur bones first appeared in the late 17th century in England. Part of a bone, now known to have been the femur of a Megalosaurus,[197] was recovered from a limestone quarry at Cornwell near Chipping NortonOxfordshire, in 1676. The fragment was sent to Robert Plot, Professor of Chemistry at the University of Oxford and first curator of the Ashmolean Museum, who published a description in his Natural History of Oxfordshire in 1677. He correctly identified the bone as the lower extremity of the femur of a large animal, and recognized that it was too large to belong to any known species. He therefore concluded it to be the thigh bone of a giant human similar to those mentioned in the Bible. In 1699, Edward Lhuyd, a friend of Sir Isaac Newton, was responsible for the first published scientific treatment of what would now be recognized as a dinosaur when he described and named a sauropod tooth, “Rutellum implicatum“,[198][199] that had been found in Caswell, near Witney, Oxfordshire.[200]

William Buckland

Between 1815 and 1824, the Rev William Buckland, a professor of geology at Oxford, collected more fossilized bones of Megalosaurus and became the first person to describe a dinosaur in a scientific journal.[197][201] The second dinosaur genus to be identified, Iguanodon, was discovered in 1822 by Mary Ann Mantell – the wife of English geologist Gideon Mantell. Gideon Mantell recognized similarities between his fossils and the bones of modern iguanas. He published his findings in 1825.[202][203]

The study of these “great fossil lizards” soon became of great interest to European and American scientists, and in 1842 the English paleontologist Richard Owencoined the term “dinosaur”. He recognized that the remains that had been found so far, IguanodonMegalosaurus and Hylaeosaurus, shared a number of distinctive features, and so decided to present them as a distinct taxonomic group. With the backing of Prince Albert, the husband of Queen Victoria, Owen established the Natural History Museum, London, to display the national collection of dinosaur fossils and other biological and geological exhibits.[204]

In 1858, William Parker Foulke discovered the first known American dinosaur, in marl pits in the small town of Haddonfield, New Jersey. (Although fossils had been found before, their nature had not been correctly discerned.) The creature was named Hadrosaurus foulkii. It was an extremely important find: Hadrosaurus was one of the first nearly complete dinosaur skeletons found (the first was in 1834, in Maidstone, England), and it was clearly a bipedal creature. This was a revolutionary discovery as, until that point, most scientists had believed dinosaurs walked on four feet, like other lizards. Foulke’s discoveries sparked a wave of dinosaur mania in the United States.[205]

Edward Drinker Cope

Othniel Charles Marsh

Marsh’s 1896 illustration of the bones of Stegosaurus, a dinosaur he described and named in 1877

Dinosaur mania was exemplified by the fierce rivalry between Edward Drinker Cope and Othniel Charles Marsh, both of whom raced to be the first to find new dinosaurs in what came to be known as the Bone Wars. The feud probably originated when Marsh publicly pointed out that Cope’s reconstruction of an Elasmosaurus skeleton was flawed: Cope had inadvertently placed the plesiosaur‘s head at what should have been the animal’s tail end. The fight between the two scientists lasted for over 30 years, ending in 1897 when Cope died after spending his entire fortune on the dinosaur hunt. Marsh ‘won’ the contest primarily because he was better funded through a relationship with the US Geological Survey. Unfortunately, many valuable dinosaur specimens were damaged or destroyed due to the pair’s rough methods: for example, their diggers often used dynamite to unearth bones (a method modern paleontologists would find appalling). Despite their unrefined methods, the contributions of Cope and Marsh to paleontology were vast: Marsh unearthed 86 new species of dinosaur and Cope discovered 56, a total of 142 new species. Cope’s collection is now at the American Museum of Natural History in New York, while Marsh’s is on display at the Peabody Museum of Natural History at Yale University.[206]

After 1897, the search for dinosaur fossils extended to every continent, including Antarctica. The first Antarctic dinosaur to be discovered, the ankylosaurid Antarctopelta oliveroi, was found on James Ross Island in 1986,[207] although it was 1994 before an Antarctic species, the theropod Cryolophosaurus ellioti, was formally named and described in a scientific journal.[208]

Current dinosaur “hot spots” include southern South America (especially Argentina) and China. China in particular has produced many exceptional feathered dinosaur specimens due to the unique geology of its dinosaur beds, as well as an ancient arid climate particularly conducive to fossilization.[160]

“Dinosaur renaissance”

Main article: Dinosaur renaissance

Paleontologist Robert T. Bakkerwith mounted skeleton of a tyrannosaurid (Gorgosaurus libratus)

The field of dinosaur research has enjoyed a surge in activity that began in the 1970s and is ongoing. This was triggered, in part, by John Ostrom‘s discovery of Deinonychus, an active predator that may have been warm-blooded, in marked contrast to the then-prevailing image of dinosaurs as sluggish and cold-bloodedVertebrate paleontology has become a global science. Major new dinosaur discoveries have been made by paleontologists working in previously unexploited regions, including India, South America, MadagascarAntarctica, and most significantly China (the amazingly well-preserved feathered dinosaurs[5] in China have further consolidated the link between dinosaurs and their living descendants, modern birds). The widespread application of cladistics, which rigorously analyzes the relationships between biological organisms, has also proved tremendously useful in classifying dinosaurs. Cladistic analysis, among other modern techniques, helps to compensate for an often incomplete and fragmentary fossil record.[209]

showTimeline of notable dinosaur taxonomic descriptions

Soft tissue and DNA

Scipionyx from the Natural History Museum of Milan, Italy

One of the best examples of soft-tissue impressions in a fossil dinosaur was discovered in Pietraroia, Italy. The discovery was reported in 1998, and described the specimen of a small, very young coelurosaurScipionyx samniticus. The fossil includes portions of the intestines, colon, liver, muscles, and windpipe of this immature dinosaur.[68]

In the March 2005 issue of Science, the paleontologist Mary Higby Schweitzer and her team announced the discovery of flexible material resembling actual soft tissue inside a 68-million-year-old Tyrannosaurus rex leg bone from the Hell Creek Formation in Montana. After recovery, the tissue was rehydrated by the science team.[69] When the fossilized bone was treated over several weeks to remove mineral content from the fossilized bone-marrow cavity (a process called demineralization), Schweitzer found evidence of intact structures such as blood vesselsbone matrix, and connective tissue (bone fibers). Scrutiny under the microscope further revealed that the putative dinosaur soft tissue had retained fine structures (microstructures) even at the cellular level. The exact nature and composition of this material, and the implications of Schweitzer’s discovery, are not yet clear.[69]

In 2009, a team including Schweitzer announced that, using even more careful methodology, they had duplicated their results by finding similar soft tissue in a duck-billed dinosaurBrachylophosaurus canadensis, found in the Judith River Formation of Montana. This included even more detailed tissue, down to preserved bone cells that seem even to have visible remnants of nuclei and what seem to be red blood cells. Among other materials found in the bone was collagen, as in the Tyrannosaurusbone. The type of collagen an animal has in its bones varies according to its DNA and, in both cases, this collagen was of the same type found in modern chickens and ostriches.[210]

The extraction of ancient DNA from dinosaur fossils has been reported on two separate occasions;[211] upon further inspection and peer review, however, neither of these reports could be confirmed.[212] However, a functional peptide involved in the vision of a theoretical dinosaur has been inferred using analytical phylogenetic reconstruction methods on gene sequences of related modern species such as reptiles and birds.[213] In addition, several proteins, including hemoglobin,[214] have putatively been detected in dinosaur fossils.[215][216]

In 2015, researchers reported finding structures similar to blood cells and collagen fibers, preserved in the bone fossils of six Cretaceous dinosaur specimens, which are approximately 75 million years old.[217][218]

Outdated Iguanodon statues created by Benjamin Waterhouse Hawkins for the Crystal Palace Park in 1853

The battles that may have occurred between Tyrannosaurus rex and Triceratops are a recurring theme in popular science and dinosaurs’ depiction in culture.

Cultural depictions

Main article: Cultural depictions of dinosaurs

By human standards, dinosaurs were creatures of fantastic appearance and often enormous size. As such, they have captured the popular imagination and become an enduring part of human culture. Entry of the word “dinosaur” into the common vernacular reflects the animals’ cultural importance: in English, “dinosaur” is commonly used to describe anything that is impractically large, obsolete, or bound for extinction.[219]

Public enthusiasm for dinosaurs first developed in Victorian England, where in 1854, three decades after the first scientific descriptions of dinosaur remains, a menagerie of lifelike dinosaur sculptures were unveiled in London‘s Crystal Palace Park. The Crystal Palace dinosaurs proved so popular that a strong market in smaller replicas soon developed. In subsequent decades, dinosaur exhibits opened at parks and museums around the world, ensuring that successive generations would be introduced to the animals in an immersive and exciting way.[220] Dinosaurs’ enduring popularity, in its turn, has resulted in significant public funding for dinosaur science, and has frequently spurred new discoveries. In the United States, for example, the competition between museums for public attention led directly to the Bone Wars of the 1880s and 1890s, during which a pair of feuding paleontologists made enormous scientific contributions.[221]

The popular preoccupation with dinosaurs has ensured their appearance in literaturefilm, and other media. Beginning in 1852 with a passing mention in Charles Dickens‘ Bleak House,[222] dinosaurs have been featured in large numbers of fictional works. Jules Verne‘s 1864 novel Journey to the Center of the EarthSir Arthur Conan Doyle‘s 1912 book The Lost World, the iconic 1933 film King Kong, the 1954 Godzilla and its many sequels, the best-selling 1990 novel Jurassic Park by Michael Crichton and its 1993 film adaptation are just a few notable examples of dinosaur appearances in fiction. Authors of general-interest non-fiction works about dinosaurs, including some prominent paleontologists, have often sought to use the animals as a way to educate readers about science in general. Dinosaurs are ubiquitous in advertising; numerous companies have referenced dinosaurs in printed or televised advertisements, either in order to sell their own products or in order to characterize their rivals as slow-moving, dim-witted, or obsolete.[223]

Details About Internet.info

The Internet (contraction of interconnected network) is the global system of interconnected computer networks that use the Internet protocol suite(TCP/IP) to link devices worldwide. It is a network of networks that consists of private, public, academic, business, and government networks of local to global scope, linked by a broad array of electronic, wireless, and optical networking technologies. The Internet carries a vast range of information resources and services, such as the inter-linked hypertext documents and applications of the World Wide Web (WWW), electronic mailtelephony, and file sharing.

The origins of the Internet date back to research commissioned by the federal government of the United States in the 1960s to build robust, fault-tolerant communication with computer networks.[1] The primary precursor network, the ARPANET, initially served as a backbone for interconnection of regional academic and military networks in the 1980s. The funding of the National Science Foundation Network as a new backbone in the 1980s, as well as private funding for other commercial extensions, led to worldwide participation in the development of new networking technologies, and the merger of many networks.[2] The linking of commercial networks and enterprises by the early 1990s marked the beginning of the transition to the modern Internet,[3] and generated a sustained exponential growth as generations of institutional, personal, and mobile computers were connected to the network. Although the Internet was widely used by academia since the 1980s, commercialization incorporated its services and technologies into virtually every aspect of modern life.

Most traditional communications media, including telephony, radio, television, paper mail and newspapers are reshaped, redefined, or even bypassed by the Internet, giving birth to new services such as emailInternet telephonyInternet televisiononline music, digital newspapers, and video streaming websites. Newspaper, book, and other print publishing are adapting to website technology, or are reshaped into bloggingweb feeds and online news aggregators. The Internet has enabled and accelerated new forms of personal interactions through instant messagingInternet forums, and social networkingOnline shopping has grown exponentially both for major retailers and small businesses and entrepreneurs, as it enables firms to extend their “brick and mortar” presence to serve a larger market or even sell goods and services entirely onlineBusiness-to-business and financial services on the Internet affect supply chains across entire industries.

The Internet has no single centralized governance in either technological implementation or policies for access and usage; each constituent network sets its own policies.[4] The overreaching definitions of the two principal name spaces in the Internet, the Internet Protocol address (IP address) space and the Domain Name System (DNS), are directed by a maintainer organization, the Internet Corporation for Assigned Names and Numbers(ICANN). The technical underpinning and standardization of the core protocols is an activity of the Internet Engineering Task Force (IETF), a non-profit organization of loosely affiliated international participants that anyone may associate with by contributing technical expertise.[5] In November 2006, the Internet was included on USA Todays list of New Seven Wonders.[6]

Terminology

The Internet Messenger by Buky Schwartz, located in HolonIsraelSee also: Capitalization of “Internet”

When the term Internet is used to refer to the specific global system of interconnected Internet Protocol (IP) networks, the word is a proper noun[7] that should be written with an initial capital letter. In common use and the media, it is often erroneously not capitalized, viz. the internet. Some guides specify that the word should be capitalized when used as a noun, but not capitalized when used as an adjective.[8] The Internet is also often referred to as the Net, as a short form of network. Historically, as early as 1849, the word internetted was used uncapitalized as an adjective, meaning interconnected or interwoven.[9] The designers of early computer networks used internet both as a noun and as a verb in shorthand form of internetwork or internetworking, meaning interconnecting computer networks.[10]

The terms Internet and World Wide Web are often used interchangeably in everyday speech; it is common to speak of “going on the Internet” when using a web browser to view web pages. However, the World Wide Web or the Web is only one of a large number of Internet services. The Web is a collection of interconnected documents (web pages) and other web resources, linked by hyperlinks and URLs.[11] As another point of comparison, Hypertext Transfer Protocol, or HTTP, is the language used on the Web for information transfer, yet it is just one of many languages or protocols that can be used for communication on the Internet.[12] The term Interweb is a portmanteau of Internet and World Wide Web typically used sarcastically to parody a technically unsavvy user.

History

Main articles: History of the Internet and History of the World Wide Web

Research into packet switching, one of the fundamental Internet technologies, started in the early 1960s in the work of Paul Baran and Donald Davies.[13] Packet-switched networks such as the NPL networkARPANET, the Merit NetworkCYCLADES, and Telenet were developed in the late 1960s and early 1970s.[14] The ARPANET project led to the development of protocols for internetworking, by which multiple separate networks could be joined into a network of networks.[15] ARPANET development began with two network nodes which were interconnected between the Network Measurement Center at the University of California, Los Angeles (UCLA) Henry Samueli School of Engineering and Applied Science directed by Leonard Kleinrock, and the NLS system at SRI International (SRI) by Douglas Engelbart in Menlo Park, California, on 29 October 1969.[16] The third site was the Culler-Fried Interactive Mathematics Center at the University of California, Santa Barbara, followed by the University of Utah Graphics Department. In an early sign of future growth, fifteen sites were connected to the young ARPANET by the end of 1971.[17][18] These early years were documented in the 1972 film Computer Networks: The Heralds of Resource Sharing.

Early international collaborations on the ARPANET were rare. European developers were concerned with developing the X.25 networks.[19] Notable exceptions were the Norwegian Seismic Array (NORSAR) in June 1973, followed in 1973 by Sweden with satellite links to the Tanum Earth Station and Peter T. Kirstein‘s research group in the United Kingdom, initially at the Institute of Computer ScienceUniversity of London and later at University College London.[20][21][22] In December 1974, RFC 675 (Specification of Internet Transmission Control Program), by Vinton Cerf, Yogen Dalal, and Carl Sunshine, used the term internet as a shorthand for internetworking and later RFCs repeated this use.[23] Access to the ARPANET was expanded in 1981 when the National Science Foundation (NSF) funded the Computer Science Network (CSNET). In 1982, the Internet Protocol Suite (TCP/IP) was standardized, which permitted worldwide proliferation of interconnected networks. TCP/IP network access expanded again in 1986 when the National Science Foundation Network (NSFNet) provided access to supercomputer sites in the United States for researchers, first at speeds of 56 kbit/s and later at 1.5 Mbit/s and 45 Mbit/s.[24] Commercial Internet service providers (ISPs) emerged in the late 1980s and early 1990s. The ARPANET was decommissioned in 1990.

T3 NSFNET Backbone, c. 1992.

The Internet rapidly expanded in Europe and Australia in the mid to late 1980s[25][26] and to Asia in the late 1980s and early 1990s.[27] The beginning of dedicated transatlantic communication between the NSFNET and networks in Europe was established with a low-speed satellite relay between Princeton University and Stockholm, Sweden in December 1988.[28] Although other network protocols such as UUCP had global reach well before this time, this marked the beginning of the Internet as an intercontinental network.

Public commercial use of the Internet began in mid-1989 with the connection of MCI Mail and Compuserve‘s email capabilities to the 500,000 users of the Internet.[29] Just months later on 1 January 1990, PSInet launched an alternate Internet backbone for commercial use; one of the networks that would grow into the commercial Internet we know today. In March 1990, the first high-speed T1 (1.5 Mbit/s) link between the NSFNET and Europe was installed between Cornell University and CERN, allowing much more robust communications than were capable with satellites.[30] Six months later Tim Berners-Lee would begin writing WorldWideWeb, the first web browser after two years of lobbying CERN management. By Christmas 1990, Berners-Lee had built all the tools necessary for a working Web: the HyperText Transfer Protocol (HTTP) 0.9,[31] the HyperText Markup Language (HTML), the first Web browser (which was also a HTML editor and could access Usenet newsgroups and FTP files), the first HTTP server software (later known as CERN httpd), the first web server,[32] and the first Web pages that described the project itself. In 1991 the Commercial Internet eXchange was founded, allowing PSInet to communicate with the other commercial networks CERFnet and Alternet. Stanford Federal Credit Union was the first financial institution to offer online internet banking services to all of its members in October 1994.[33] In 1996 OP Financial Group, also a cooperative bank, became the second online bank in the world and the first in Europe.[34] By 1995, the Internet was fully commercialized in the U.S. when the NSFNet was decommissioned, removing the last restrictions on use of the Internet to carry commercial traffic.[35]

200520102017a
World population[36]6.5 billion6.9 billion7.4 billion
Users worldwide16%30%48%
Users in the developing world8%21%41.3%
Users in the developed world51%67%81%
a Estimate.
Source: International Telecommunications Union.[37]

Since 1995 the Internet has tremendously impacted culture and commerce, including the rise of near instant communication by email, instant messaging, telephony (Voice over Internet Protocol or VoIP), two-way interactive video calls, and the World Wide Web[38] with its discussion forums, blogs, social networking, and online shopping sites. Increasing amounts of data are transmitted at higher and higher speeds over fiber optic networks operating at 1-Gbit/s, 10-Gbit/s, or more. The Internet continues to grow, driven by ever greater amounts of online information and knowledge, commerce, entertainment and social networking.[39] During the late 1990s, it was estimated that traffic on the public Internet grew by 100 percent per year, while the mean annual growth in the number of Internet users was thought to be between 20% and 50%.[40] This growth is often attributed to the lack of central administration, which allows organic growth of the network, as well as the non-proprietary nature of the Internet protocols, which encourages vendor interoperability and prevents any one company from exerting too much control over the network.[41] As of 31 March 2011, the estimated total number of Internet users was 2.095 billion (30.2% of world population).[42] It is estimated that in 1993 the Internet carried only 1% of the information flowing through two-way telecommunication, by 2000 this figure had grown to 51%, and by 2007 more than 97% of all telecommunicated information was carried over the Internet.[43]

Governance

Main article: Internet governance

ICANN headquarters in the Playa Vista neighborhood of Los Angeles, California, United States.

The Internet is a global network that comprises many voluntarily interconnected autonomous networks. It operates without a central governing body. The technical underpinning and standardization of the core protocols (IPv4 and IPv6) is an activity of the Internet Engineering Task Force (IETF), a non-profit organization of loosely affiliated international participants that anyone may associate with by contributing technical expertise. To maintain interoperability, the principal name spaces of the Internet are administered by the Internet Corporation for Assigned Names and Numbers (ICANN). ICANN is governed by an international board of directors drawn from across the Internet technical, business, academic, and other non-commercial communities. ICANN coordinates the assignment of unique identifiers for use on the Internet, including domain names, Internet Protocol (IP) addresses, application port numbers in the transport protocols, and many other parameters. Globally unified name spaces are essential for maintaining the global reach of the Internet. This role of ICANN distinguishes it as perhaps the only central coordinating body for the global Internet.[44]

Regional Internet Registries (RIRs) allocate IP addresses:

The National Telecommunications and Information Administration, an agency of the United States Department of Commerce, had final approval over changes to the DNS root zone until the IANA stewardship transition on 1 October 2016.[45][46][47][48] The Internet Society (ISOC) was founded in 1992 with a mission to “assure the open development, evolution and use of the Internet for the benefit of all people throughout the world”.[49] Its members include individuals (anyone may join) as well as corporations, organizations, governments, and universities. Among other activities ISOC provides an administrative home for a number of less formally organized groups that are involved in developing and managing the Internet, including: the Internet Engineering Task Force(IETF), Internet Architecture Board (IAB), Internet Engineering Steering Group (IESG), Internet Research Task Force (IRTF), and Internet Research Steering Group (IRSG). On 16 November 2005, the United Nations-sponsored World Summit on the Information Society in Tunis established the Internet Governance Forum (IGF) to discuss Internet-related issues.

Infrastructure

See also: List of countries by number of Internet users and List of countries by Internet connection speeds

2007 map showing submarine fiberoptic telecommunication cables around the world.

The communications infrastructure of the Internet consists of its hardware components and a system of software layers that control various aspects of the architecture.

Routing and service tiers

Packet routing across the Internet involves several tiers of Internet service providers.

Internet service providers (ISPs) establish the worldwide connectivity between individual networks at various levels of scope. End-users who only access the Internet when needed to perform a function or obtain information, represent the bottom of the routing hierarchy. At the top of the routing hierarchy are the tier 1 networks, large telecommunication companies that exchange traffic directly with each other via very high speed fibre optic cables and governed by peering agreements. Tier 2 and lower level networks buy Internet transit from other providers to reach at least some parties on the global Internet, though they may also engage in peering. An ISP may use a single upstream provider for connectivity, or implement multihoming to achieve redundancy and load balancing. Internet exchange points are major traffic exchanges with physical connections to multiple ISPs. Large organizations, such as academic institutions, large enterprises, and governments, may perform the same function as ISPs, engaging in peering and purchasing transit on behalf of their internal networks. Research networks tend to interconnect with large subnetworks such as GEANTGLORIADInternet2, and the UK’s national research and education networkJANET. Both the Internet IP routing structure and hypertext links of the World Wide Web are examples of scale-free networks.[50][disputed (for: unclear whether citation supports claim empricially)  – discuss] Computers and routers use routing tables in their operating system to direct IP packets to the next-hop router or destination. Routing tables are maintained by manual configuration or automatically by routing protocols. End-nodes typically use a default route that points toward an ISP providing transit, while ISP routers use the Border Gateway Protocol to establish the most efficient routing across the complex connections of the global Internet.

An estimated 70 percent of the world’s Internet traffic passes through AshburnVirginia.[51][52][53][54]

Access

Common methods of Internet access by users include dial-up with a computer modem via telephone circuits, broadband over coaxial cablefiber optics or copper wires, Wi-Fisatellite, and cellular telephone technology (e.g. 3G4G). The Internet may often be accessed from computers in libraries and Internet cafesInternet access points exist in many public places such as airport halls and coffee shops. Various terms are used, such as public Internet kioskpublic access terminal, and Web payphone. Many hotels also have public terminals that are usually fee-based. These terminals are widely accessed for various usages, such as ticket booking, bank deposit, or online payment. Wi-Fi provides wireless access to the Internet via local computer networks. Hotspots providing such access include Wi-Fi cafes, where users need to bring their own wireless devices such as a laptop or PDA. These services may be free to all, free to customers only, or fee-based.

Grassroots efforts have led to wireless community networks. Commercial Wi-Fi services covering large city areas are in many cities, such as New YorkLondonViennaTorontoSan FranciscoPhiladelphiaChicago and Pittsburgh. The Internet can then be accessed from places, such as a park bench.[55] Apart from Wi-Fi, there have been experiments with proprietary mobile wireless networks like Ricochet, various high-speed data services over cellular phone networks, and fixed wireless services. High-end mobile phones such as smartphones in general come with Internet access through the phone network. Web browsers such as Opera are available on these advanced handsets, which can also run a wide variety of other Internet software. Internet usage by mobile and tablet devices exceeded desktop worldwide for the first time in October 2016.[56] An Internet access provider and protocol matrix differentiates the methods used to get online.

Internet and mobile

Number of mobile cellular subscriptions 2012–2016, World Trends in Freedom of Expression and Media Development Global Report 2017/2018

According to the International Telecommunication Union (ITU), by the end of 2017, an estimated 48 per cent of individuals regularly connect to the internet, up from 34 per cent in 2012.[57] Mobile internet connectivity has played an important role in expanding access in recent years especially in Asia and the Pacific and in Africa.[58] The number of unique mobile cellular subscriptions increased from 3.89 billion in 2012 to 4.83 billion in 2016, two-thirds of the world’s population, with more than half of subscriptions located in Asia and the Pacific. The number of subscriptions is predicted to rise to 5.69 billion users in 2020.[59] As of 2016, almost 60 per cent of the world’s population had access to a 4G broadband cellular network, up from almost 50 per cent in 2015 and 11 per cent in 2012[disputed – discuss].[59] The limits that users face on accessing information via mobile applications coincide with a broader process of fragmentation of the internet. Fragmentation restricts access to media content and tends to affect poorest users the most.[58]

Zero-rating, the practice of internet providers allowing users free connectivity to access specific content or applications for free, has offered some opportunities for individuals to surmount economic hurdles, but has also been accused by its critics as creating a ‘two-tiered’ internet. To address the issues with zero-rating, an alternative model has emerged in the concept of ‘equal rating’ and is being tested in experiments by Mozilla and Orange in Africa. Equal rating prevents prioritization of one type of content and zero-rates all content up to a specified data cap. A study published by Chatham House, 15 out of 19 countries researched in Latin America had some kind of hybrid or zero-rated product offered. Some countries in the region had a handful of plans to choose from (across all mobile network operators) while others, such as Colombia, offered as many as 30 pre-paid and 34 post-paid plans.[60]

A study of eight countries in the Global South found that zero-rated data plans exist in every country, although there is a great range in the frequency with which they are offered and actually used in each.[61] Across the 181 plans examined, 13 per cent were offering zero-rated services. Another study, covering GhanaKenyaNigeria and South Africa, found Facebook‘s Free Basics and Wikipedia Zero to be the most commonly zero-rated content.[62]

Protocols

Internet protocol suite
Application layer
BGPDHCPDNSFTPHTTPHTTPSIMAPLDAPMGCPMQTTNNTPNTPPOPONC/RPCRTPRTSPRIPSIPSMTPSNMPSSHTelnetTLS/SSLXMPPmore…
Transport layer
TCPUDPDCCPSCTPRSVPmore…
Internet layer
IP IPv4IPv6ICMPICMPv6ECNIGMPIPsecmore…
Link layer
ARPNDPOSPFTunnels L2TPPPPMAC EthernetWi-FiDSLISDNFDDImore…
vte

While the hardware components in the Internet infrastructure can often be used to support other software systems, it is the design and the standardization process of the software that characterizes the Internet and provides the foundation for its scalability and success. The responsibility for the architectural design of the Internet software systems has been assumed by the Internet Engineering Task Force (IETF).[63] The IETF conducts standard-setting work groups, open to any individual, about the various aspects of Internet architecture. Resulting contributions and standards are published as Request for Comments (RFC) documents on the IETF web site. The principal methods of networking that enable the Internet are contained in specially designated RFCs that constitute the Internet Standards. Other less rigorous documents are simply informative, experimental, or historical, or document the best current practices (BCP) when implementing Internet technologies.

The Internet standards describe a framework known as the Internet protocol suite. This is a model architecture that divides methods into a layered system of protocols, originally documented in RFC 1122 and RFC 1123. The layers correspond to the environment or scope in which their services operate. At the top is the application layer, space for the application-specific networking methods used in software applications. For example, a web browser program uses the client-server application model and a specific protocol of interaction between servers and clients, while many file-sharing systems use a peer-to-peer paradigm. Below this top layer, the transport layer connects applications on different hosts with a logical channel through the network with appropriate data exchange methods.

Underlying these layers are the networking technologies that interconnect networks at their borders and exchange traffic across them. The Internet layer enables applications running on computers (“hosts”) to identify each other via port numbers and Internet Protocol (IP) addresses, and route their traffic to each other via any intermediate (transit) networks. Last, at the bottom of the architecture is the link layer, which provides logical connectivity between hosts on the same network link, such as a local area network (LAN) or a dial-up connection. The model, also known as TCP/IP, is designed to be independent of the underlying hardware used for the physical connections, which the model does not concern itself with in any detail. Other models have been developed, such as the OSI model, that attempt to be comprehensive in every aspect of communications. While many similarities exist between the models, they are not compatible in the details of description or implementation. Yet, TCP/IP protocols are usually included in the discussion of OSI networking.

As user data is processed through the protocol stack, each abstraction layer adds encapsulation information at the sending host. Data is transmitted over the wire at the link level between hosts and routers. Encapsulation is removed by the receiving host. Intermediate relays update link encapsulation at each hop, and inspect the IP layer for routing purposes.

The most prominent component of the Internet model is the Internet Protocol (IP), which provides addressing systems, including IP addresses, for computers on the network. IP enables internetworking and, in essence, establishes the Internet itself. Internet Protocol Version 4 (IPv4) is the initial version used on the first generation of the Internet and is still in dominant use. It was designed to address up to ≈4.3 billion (109) hosts. However, the explosive growth of the Internet has led to IPv4 address exhaustion, which entered its final stage in 2011,[64] when the global address allocation pool was exhausted. A new protocol version, IPv6, was developed in the mid-1990s, which provides vastly larger addressing capabilities and more efficient routing of Internet traffic. IPv6 is currently in growing deployment around the world, since Internet address registries (RIRs) began to urge all resource managers to plan rapid adoption and conversion.[65]

IPv6 is not directly interoperable by design with IPv4. In essence, it establishes a parallel version of the Internet not directly accessible with IPv4 software. Thus, translation facilities must exist for internetworking or nodes must have duplicate networking software for both networks. Essentially all modern computer operating systems support both versions of the Internet Protocol. Network infrastructure, however, has been lagging in this development. Aside from the complex array of physical connections that make up its infrastructure, the Internet is facilitated by bi- or multi-lateral commercial contracts, e.g., peering agreements, and by technical specifications or protocols that describe the exchange of data over the network. Indeed, the Internet is defined by its interconnections and routing policies.

Services

Many people use, erroneously, the terms Internet and World Wide Web, or just the Web, interchangeably, but the two terms are not synonymous. The World Wide Web is a primary application program that billions of people use on the Internet, and it has changed their lives immeasurably.[66][67] However, the Internet provides many network services, most prominently include mobile apps such as social media apps, the World Wide Webelectronic mailmultiplayer online gamesInternet telephony, and file sharing and streaming media services.

World Wide Web

Main article: World Wide Web

This NeXT Computer was used by Tim Berners-Lee at CERN and became the world’s first Web server.

World Wide Web browser software, such as Microsoft‘s Internet Explorer/EdgeMozilla FirefoxOperaApple‘s Safari, and Google Chrome, lets users navigate from one web page to another via hyperlinks embedded in the documents. These documents may also contain any combination of computer data, including graphics, sounds, textvideomultimedia and interactive content that runs while the user is interacting with the page. Client-side softwarecan include animations, gamesoffice applications and scientific demonstrations. Through keyword-driven Internet research using search engines like Yahoo!Bing and Google, users worldwide have easy, instant access to a vast and diverse amount of online information. Compared to printed media, books, encyclopedias and traditional libraries, the World Wide Web has enabled the decentralization of information on a large scale.

The Web is therefore a global set of documentsimages and other resources, logically interrelated by hyperlinks and referenced with Uniform Resource Identifiers (URIs). URIs symbolically identify services, servers, and other databases, and the documents and resources that they can provide. Hypertext Transfer Protocol (HTTP) is the main access protocol of the World Wide Web. Web services also use HTTP to allow software systems to communicate in order to share and exchange business logic and data.

The Web has enabled individuals and organizations to publish ideas and information to a potentially large audience online at greatly reduced expense and time delay. Publishing a web page, a blog, or building a website involves little initial cost and many cost-free services are available. However, publishing and maintaining large, professional web sites with attractive, diverse and up-to-date information is still a difficult and expensive proposition. Many individuals and some companies and groups use web logs or blogs, which are largely used as easily updatable online diaries. Some commercial organizations encourage staff to communicate advice in their areas of specialization in the hope that visitors will be impressed by the expert knowledge and free information, and be attracted to the corporation as a result.

Advertising on popular web pages can be lucrative, and e-commerce, which is the sale of products and services directly via the Web, continues to grow. Online advertising is a form of marketingand advertising which uses the Internet to deliver promotional marketing messages to consumers. It includes email marketing, search engine marketing (SEM), social media marketing, many types of display advertising (including web banner advertising), and mobile advertising. In 2011, Internet advertising revenues in the United States surpassed those of cable television and nearly exceeded those of broadcast television.[68]:19 Many common online advertising practices are controversial and increasingly subject to regulation.

When the Web developed in the 1990s, a typical web page was stored in completed form on a web server, formatted in HTML, complete for transmission to a web browser in response to a request. Over time, the process of creating and serving web pages has become dynamic, creating a flexible design, layout, and content. Websites are often created using content managementsoftware with, initially, very little content. Contributors to these systems, who may be paid staff, members of an organization or the public, fill underlying databases with content using editing pages designed for that purpose while casual visitors view and read this content in HTML form. There may or may not be editorial, approval and security systems built into the process of taking newly entered content and making it available to the target visitors.

Communication

Email is an important communications service available on the Internet. The concept of sending electronic text messages between parties in a way analogous to mailing letters or memos predates the creation of the Internet.[69][70] Pictures, documents, and other files are sent as email attachments. Emails can be cc-ed to multiple email addresses.

Internet telephony is another common communications service made possible by the creation of the Internet. VoIP stands for Voice-over-Internet Protocol, referring to the protocol that underlies all Internet communication. The idea began in the early 1990s with walkie-talkie-like voice applications for personal computers. In recent years many VoIP systems have become as easy to use and as convenient as a normal telephone. The benefit is that, as the Internet carries the voice traffic, VoIP can be free or cost much less than a traditional telephone call, especially over long distances and especially for those with always-on Internet connections such as cable or ADSL and mobile data.[71] VoIP is maturing into a competitive alternative to traditional telephone service. Interoperability between different providers has improved and the ability to call or receive a call from a traditional telephone is available. Simple, inexpensive VoIP network adapters are available that eliminate the need for a personal computer.

Voice quality can still vary from call to call, but is often equal to and can even exceed that of traditional calls. Remaining problems for VoIP include emergency telephone number dialing and reliability. Currently, a few VoIP providers provide an emergency service, but it is not universally available. Older traditional phones with no “extra features” may be line-powered only and operate during a power failure; VoIP can never do so without a backup power source for the phone equipment and the Internet access devices. VoIP has also become increasingly popular for gaming applications, as a form of communication between players. Popular VoIP clients for gaming include Ventrilo and Teamspeak. Modern video game consoles also offer VoIP chat features.

Data transfer

File sharing is an example of transferring large amounts of data across the Internet. A computer file can be emailed to customers, colleagues and friends as an attachment. It can be uploaded to a website or File Transfer Protocol (FTP) server for easy download by others. It can be put into a “shared location” or onto a file server for instant use by colleagues. The load of bulk downloads to many users can be eased by the use of “mirror” servers or peer-to-peer networks. In any of these cases, access to the file may be controlled by user authentication, the transit of the file over the Internet may be obscured by encryption, and money may change hands for access to the file. The price can be paid by the remote charging of funds from, for example, a credit card whose details are also passed – usually fully encrypted – across the Internet. The origin and authenticity of the file received may be checked by digital signatures or by MD5 or other message digests. These simple features of the Internet, over a worldwide basis, are changing the production, sale, and distribution of anything that can be reduced to a computer file for transmission. This includes all manner of print publications, software products, news, music, film, video, photography, graphics and the other arts. This in turn has caused seismic shifts in each of the existing industries that previously controlled the production and distribution of these products.

Streaming media is the real-time delivery of digital media for the immediate consumption or enjoyment by end users. Many radio and television broadcasters provide Internet feeds of their live audio and video productions. They may also allow time-shift viewing or listening such as Preview, Classic Clips and Listen Again features. These providers have been joined by a range of pure Internet “broadcasters” who never had on-air licenses. This means that an Internet-connected device, such as a computer or something more specific, can be used to access on-line media in much the same way as was previously possible only with a television or radio receiver. The range of available types of content is much wider, from specialized technical webcasts to on-demand popular multimedia services. Podcasting is a variation on this theme, where – usually audio – material is downloaded and played back on a computer or shifted to a portable media player to be listened to on the move. These techniques using simple equipment allow anybody, with little censorship or licensing control, to broadcast audio-visual material worldwide.

Digital media streaming increases the demand for network bandwidth. For example, standard image quality needs 1 Mbit/s link speed for SD 480p, HD 720p quality requires 2.5 Mbit/s, and the top-of-the-line HDX quality needs 4.5 Mbit/s for 1080p.[72]

Webcams are a low-cost extension of this phenomenon. While some webcams can give full-frame-rate video, the picture either is usually small or updates slowly. Internet users can watch animals around an African waterhole, ships in the Panama Canal, traffic at a local roundabout or monitor their own premises, live and in real time. Video chat rooms and video conferencing are also popular with many uses being found for personal webcams, with and without two-way sound. YouTube was founded on 15 February 2005 and is now the leading website for free streaming video with a vast number of users. It uses a HTML5 based web player by default to stream and show video files.[73] Registered users may upload an unlimited amount of video and build their own personal profile. YouTube claims that its users watch hundreds of millions, and upload hundreds of thousands of videos daily.

Social impact

The Internet has enabled new forms of social interaction, activities, and social associations. This phenomenon has given rise to the scholarly study of the sociology of the Internet.

Users

See also: Global Internet usage and English in computing

Internet users per 100 inhabitantsSource: International Telecommunications Union.[74][75]

Internet users by language[76]

Website content languages[77]

Internet usage has seen tremendous growth. From 2000 to 2009, the number of Internet users globally rose from 394 million to 1.858 billion.[78] By 2010, 22 percent of the world’s population had access to computers with 1 billion Google searches every day, 300 million Internet users reading blogs, and 2 billion videos viewed daily on YouTube.[79] In 2014 the world’s Internet users surpassed 3 billion or 43.6 percent of world population, but two-thirds of the users came from richest countries, with 78.0 percent of Europe countries population using the Internet, followed by 57.4 percent of the Americas.[80]. However, by 2018, this trend had shifted so tremendously that Asia alone accounted for 51% of all internet users, with 2.2 billion out of the 4.3 billion internet users in the world coming from that region. The number of China’s internet users surpassed a major miletsone in 2018, when the country’s internet regulatory authority, China Internet Network Information Centre, announced that there were 802 million internet users in China[81]. By 2019, China was the world’s leading country in terms of internet users, with more than 800 million users, followed closely by India, with some 700 million users, with USA a distant third with 275 million users. However, in terms of penetration, China has a 38.4% penetration rate compared to India’s 40% and USA’s 80%[82].

The prevalent language for communication on the Internet has been English. This may be a result of the origin of the Internet, as well as the language’s role as a lingua franca. Early computer systems were limited to the characters in the American Standard Code for Information Interchange (ASCII), a subset of the Latin alphabet.

After English (27%), the most requested languages on the World Wide Web are Chinese (25%), Spanish (8%), Japanese (5%), Portuguese and German (4% each), Arabic, French and Russian (3% each), and Korean (2%).[76] By region, 42% of the world’s Internet users are based in Asia, 24% in Europe, 14% in North America, 10% in Latin America and the Caribbean taken together, 6% in Africa, 3% in the Middle East and 1% in Australia/Oceania.[83] The Internet’s technologies have developed enough in recent years, especially in the use of Unicode, that good facilities are available for development and communication in the world’s widely used languages. However, some glitches such as mojibake (incorrect display of some languages’ characters) still remain.

In an American study in 2005, the percentage of men using the Internet was very slightly ahead of the percentage of women, although this difference reversed in those under 30. Men logged on more often, spent more time online, and were more likely to be broadband users, whereas women tended to make more use of opportunities to communicate (such as email). Men were more likely to use the Internet to pay bills, participate in auctions, and for recreation such as downloading music and videos. Men and women were equally likely to use the Internet for shopping and banking.[84] More recent studies indicate that in 2008, women significantly outnumbered men on most social networking sites, such as Facebook and Myspace, although the ratios varied with age.[85] In addition, women watched more streaming content, whereas men downloaded more.[86] In terms of blogs, men were more likely to blog in the first place; among those who blog, men were more likely to have a professional blog, whereas women were more likely to have a personal blog.[87]

Forecasts predict that 44% of the world’s population will be users of the Internet by 2020.[88] Splitting by country, in 2012 Iceland, Norway, Sweden, the Netherlands, and Denmark had the highest Internet penetration by the number of users, with 93% or more of the population with access.[89]

Several neologisms exist that refer to Internet users: Netizen (as in “citizen of the net”)[90] refers to those actively involved in improving online communities, the Internet in general or surrounding political affairs and rights such as free speech,[91][92] Internaut refers to operators or technically highly capable users of the Internet,[93][94] digital citizen refers to a person using the Internet in order to engage in society, politics, and government participation.[95]

Usage

Internet users in 2015 as a percentage of a country’s populationSource: International Telecommunications Union.[89]Main articles: Global digital divide and Digital divide

Fixed broadband Internet subscriptions in 2012
as a percentage of a country’s population
Source: International Telecommunications Union.[96]

Mobile broadband Internet subscriptions in 2012
as a percentage of a country’s population
Source: International Telecommunications Union.[97]

The Internet allows greater flexibility in working hours and location, especially with the spread of unmetered high-speed connections. The Internet can be accessed almost anywhere by numerous means, including through mobile Internet devices. Mobile phones, datacardshandheld game consoles and cellular routers allow users to connect to the Internet wirelessly. Within the limitations imposed by small screens and other limited facilities of such pocket-sized devices, the services of the Internet, including email and the web, may be available. Service providers may restrict the services offered and mobile data charges may be significantly higher than other access methods.

Educational material at all levels from pre-school to post-doctoral is available from websites. Examples range from CBeebies, through school and high-school revision guides and virtual universities, to access to top-end scholarly literature through the likes of Google Scholar. For distance education, help with homework and other assignments, self-guided learning, whiling away spare time, or just looking up more detail on an interesting fact, it has never been easier for people to access educational information at any level from anywhere. The Internet in general and the World Wide Web in particular are important enablers of both formal and informal education. Further, the Internet allows universities, in particular, researchers from the social and behavioral sciences, to conduct research remotely via virtual laboratories, with profound changes in reach and generalizability of findings as well as in communication between scientists and in the publication of results.[98]

The low cost and nearly instantaneous sharing of ideas, knowledge, and skills have made collaborative work dramatically easier, with the help of collaborative software. Not only can a group cheaply communicate and share ideas but the wide reach of the Internet allows such groups more easily to form. An example of this is the free software movement, which has produced, among other things, LinuxMozilla Firefox, and OpenOffice.org (later forked into LibreOffice). Internet chat, whether using an IRC chat room, an instant messaging system, or a social networking website, allows colleagues to stay in touch in a very convenient way while working at their computers during the day. Messages can be exchanged even more quickly and conveniently than via email. These systems may allow files to be exchanged, drawings and images to be shared, or voice and video contact between team members.

Content management systems allow collaborating teams to work on shared sets of documents simultaneously without accidentally destroying each other’s work. Business and project teams can share calendars as well as documents and other information. Such collaboration occurs in a wide variety of areas including scientific research, software development, conference planning, political activism and creative writing. Social and political collaboration is also becoming more widespread as both Internet access and computer literacy spread.

The Internet allows computer users to remotely access other computers and information stores easily from any access point. Access may be with computer security, i.e. authentication and encryption technologies, depending on the requirements. This is encouraging new ways of working from home, collaboration and information sharing in many industries. An accountant sitting at home can audit the books of a company based in another country, on a server situated in a third country that is remotely maintained by IT specialists in a fourth. These accounts could have been created by home-working bookkeepers, in other remote locations, based on information emailed to them from offices all over the world. Some of these things were possible before the widespread use of the Internet, but the cost of private leased lines would have made many of them infeasible in practice. An office worker away from their desk, perhaps on the other side of the world on a business trip or a holiday, can access their emails, access their data using cloud computing, or open a remote desktop session into their office PC using a secure virtual private network (VPN) connection on the Internet. This can give the worker complete access to all of their normal files and data, including email and other applications, while away from the office. It has been referred to among system administrators as the Virtual Private Nightmare,[99] because it extends the secure perimeter of a corporate network into remote locations and its employees’ homes.

Social networking and entertainment

See also: Social networking service § Social impact

Many people use the World Wide Web to access news, weather and sports reports, to plan and book vacations and to pursue their personal interests. People use chat, messaging and email to make and stay in touch with friends worldwide, sometimes in the same way as some previously had pen palsSocial networking websites such as FacebookTwitter, and Myspace have created new ways to socialize and interact. Users of these sites are able to add a wide variety of information to pages, to pursue common interests, and to connect with others. It is also possible to find existing acquaintances, to allow communication among existing groups of people. Sites like LinkedIn foster commercial and business connections. YouTube and Flickr specialize in users’ videos and photographs. While social networking sites were initially for individuals only, today they are widely used by businesses and other organizations to promote their brands, to market to their customers and to encourage posts to “go viral“. “Black hat” social media techniques are also employed by some organizations, such as spam accounts and astroturfing.

A risk for both individuals and organizations writing posts (especially public posts) on social networking websites, is that especially foolish or controversial posts occasionally lead to an unexpected and possibly large-scale backlash on social media from other Internet users. This is also a risk in relation to controversial offline behavior, if it is widely made known. The nature of this backlash can range widely from counter-arguments and public mockery, through insults and hate speech, to, in extreme cases, rape and death threats. The online disinhibition effectdescribes the tendency of many individuals to behave more stridently or offensively online than they would in person. A significant number of feminist women have been the target of various forms of harassment in response to posts they have made on social media, and Twitter in particular has been criticised in the past for not doing enough to aid victims of online abuse.[100]

For organizations, such a backlash can cause overall brand damage, especially if reported by the media. However, this is not always the case, as any brand damage in the eyes of people with an opposing opinion to that presented by the organization could sometimes be outweighed by strengthening the brand in the eyes of others. Furthermore, if an organization or individual gives in to demands that others perceive as wrong-headed, that can then provoke a counter-backlash.

Some websites, such as Reddit, have rules forbidding the posting of personal information of individuals (also known as doxxing), due to concerns about such postings leading to mobs of large numbers of Internet users directing harassment at the specific individuals thereby identified. In particular, the Reddit rule forbidding the posting of personal information is widely understood to imply that all identifying photos and names must be censored in Facebook screenshots posted to Reddit. However, the interpretation of this rule in relation to public Twitter posts is less clear, and in any case, like-minded people online have many other ways they can use to direct each other’s attention to public social media posts they disagree with.

Children also face dangers online such as cyberbullying and approaches by sexual predators, who sometimes pose as children themselves. Children may also encounter material which they may find upsetting, or material which their parents consider to be not age-appropriate. Due to naivety, they may also post personal information about themselves online, which could put them or their families at risk unless warned not to do so. Many parents choose to enable Internet filtering, and/or supervise their children’s online activities, in an attempt to protect their children from inappropriate material on the Internet. The most popular social networking websites, such as Facebook and Twitter, commonly forbid users under the age of 13. However, these policies are typically trivial to circumvent by registering an account with a false birth date, and a significant number of children aged under 13 join such sites anyway. Social networking sites for younger children, which claim to provide better levels of protection for children, also exist.[101]

The Internet has been a major outlet for leisure activity since its inception, with entertaining social experiments such as MUDs and MOOs being conducted on university servers, and humor-related Usenet groups receiving much traffic.[citation needed] Many Internet forums have sections devoted to games and funny videos.[citation needed] The Internet pornography and online gamblingindustries have taken advantage of the World Wide Web, and often provide a significant source of advertising revenue for other websites.[102] Although many governments have attempted to restrict both industries’ use of the Internet, in general, this has failed to stop their widespread popularity.[103]

Another area of leisure activity on the Internet is multiplayer gaming.[104] This form of recreation creates communities, where people of all ages and origins enjoy the fast-paced world of multiplayer games. These range from MMORPG to first-person shooters, from role-playing video games to online gambling. While online gaming has been around since the 1970s, modern modes of online gaming began with subscription services such as GameSpy and MPlayer.[105] Non-subscribers were limited to certain types of game play or certain games. Many people use the Internet to access and download music, movies and other works for their enjoyment and relaxation. Free and fee-based services exist for all of these activities, using centralized servers and distributed peer-to-peer technologies. Some of these sources exercise more care with respect to the original artists’ copyrights than others.

Internet usage has been correlated to users’ loneliness.[106] Lonely people tend to use the Internet as an outlet for their feelings and to share their stories with others, such as in the “I am lonely will anyone speak to me” thread.

Cybersectarianism is a new organizational form which involves: “highly dispersed small groups of practitioners that may remain largely anonymous within the larger social context and operate in relative secrecy, while still linked remotely to a larger network of believers who share a set of practices and texts, and often a common devotion to a particular leader. Overseas supporters provide funding and support; domestic practitioners distribute tracts, participate in acts of resistance, and share information on the internal situation with outsiders. Collectively, members and practitioners of such sects construct viable virtual communities of faith, exchanging personal testimonies and engaging in the collective study via email, on-line chat rooms, and web-based message boards.”[107] In particular, the British government has raised concerns about the prospect of young British Muslims being indoctrinated into Islamic extremism by material on the Internet, being persuaded to join terrorist groups such as the so-called “Islamic State“, and then potentially committing acts of terrorism on returning to Britain after fighting in Syria or Iraq.

Cyberslacking can become a drain on corporate resources; the average UK employee spent 57 minutes a day surfing the Web while at work, according to a 2003 study by Peninsula Business Services.[108] Internet addiction disorder is excessive computer use that interferes with daily life. Nicholas G. Carr believes that Internet use has other effects on individuals, for instance improving skills of scan-reading and interfering with the deep thinking that leads to true creativity.[109]

Electronic business

Electronic business (e-business) encompasses business processes spanning the entire value chain: purchasing, supply chain managementmarketingsalescustomer service, and business relationship. E-commerce seeks to add revenue streams using the Internet to build and enhance relationships with clients and partners. According to International Data Corporation, the size of worldwide e-commerce, when global business-to-business and -consumer transactions are combined, equate to $16 trillion for 2013. A report by Oxford Economics adds those two together to estimate the total size of the digital economy at $20.4 trillion, equivalent to roughly 13.8% of global sales.[110]

While much has been written of the economic advantages of Internet-enabled commerce, there is also evidence that some aspects of the Internet such as maps and location-aware services may serve to reinforce economic inequality and the digital divide.[111] Electronic commerce may be responsible for consolidation and the decline of mom-and-popbrick and mortar businesses resulting in increases in income inequality.[112][113][114]

Author Andrew Keen, a long-time critic of the social transformations caused by the Internet, has recently focused on the economic effects of consolidation from Internet businesses. Keen cites a 2013 Institute for Local Self-Reliance report saying brick-and-mortar retailers employ 47 people for every $10 million in sales while Amazon employs only 14. Similarly, the 700-employee room rental start-up Airbnb was valued at $10 billion in 2014, about half as much as Hilton Worldwide, which employs 152,000 people. At that time, transportation network company Uber employed 1,000 full-time employees and was valued at $18.2 billion, about the same valuation as Avis Rent a Car and The Hertz Corporation combined, which together employed almost 60,000 people.[115]

Telecommuting

Telecommuting is the performance within a traditional worker and employer relationship when it is facilitated by tools such as groupwarevirtual private networksconference callingvideoconferencing, and voice over IP (VOIP) so that work may be performed from any location, most conveniently the worker’s home. It can be efficient and useful for companies as it allows workers to communicate over long distances, saving significant amounts of travel time and cost. As broadband Internet connections become commonplace, more workers have adequate bandwidth at home to use these tools to link their home to their corporate intranet and internal communication networks.

Collaborative publishing

Wikis have also been used in the academic community for sharing and dissemination of information across institutional and international boundaries.[116] In those settings, they have been found useful for collaboration on grant writingstrategic planning, departmental documentation, and committee work.[117] The United States Patent and Trademark Office uses a wiki to allow the public to collaborate on finding prior art relevant to examination of pending patent applications. Queens, New York has used a wiki to allow citizens to collaborate on the design and planning of a local park.[118] The English Wikipedia has the largest user base among wikis on the World Wide Web[119] and ranks in the top 10 among all Web sites in terms of traffic.[120]

Politics and political revolutions

See also: Internet censorshipCulture of fear, and Mass surveillance

Banner in Bangkok during the 2014 Thai coup d’état, informing the Thaipublic that ‘like’ or ‘share’ activities on social media could result in imprisonment (observed June 30, 2014).

The Internet has achieved new relevance as a political tool. The presidential campaign of Howard Dean in 2004 in the United States was notable for its success in soliciting donation via the Internet. Many political groups use the Internet to achieve a new method of organizing for carrying out their mission, having given rise to Internet activism, most notably practiced by rebels in the Arab Spring.[121][122] The New York Times suggested that social mediawebsites, such as Facebook and Twitter, helped people organize the political revolutions in Egypt, by helping activists organize protests, communicate grievances, and disseminate information.[123]

Many have understood the Internet as an extension of the Habermasian notion of the public sphere, observing how network communication technologies provide something like a global civic forum. However, incidents of politically motivated Internet censorship have now been recorded in many countries, including western democracies.[citation needed]

Philanthropy

The spread of low-cost Internet access in developing countries has opened up new possibilities for peer-to-peer charities, which allow individuals to contribute small amounts to charitable projects for other individuals. Websites, such as DonorsChoose and GlobalGiving, allow small-scale donors to direct funds to individual projects of their choice. A popular twist on Internet-based philanthropy is the use of peer-to-peer lending for charitable purposes. Kiva pioneered this concept in 2005, offering the first web-based service to publish individual loan profiles for funding. Kiva raises funds for local intermediary microfinance organizations which post stories and updates on behalf of the borrowers. Lenders can contribute as little as $25 to loans of their choice, and receive their money back as borrowers repay. Kiva falls short of being a pure peer-to-peer charity, in that loans are disbursed before being funded by lenders and borrowers do not communicate with lenders themselves.[124][125]

However, the recent spread of low-cost Internet access in developing countries has made genuine international person-to-person philanthropy increasingly feasible. In 2009, the US-based nonprofit Zidisha tapped into this trend to offer the first person-to-person microfinance platform to link lenders and borrowers across international borders without intermediaries. Members can fund loans for as little as a dollar, which the borrowers then use to develop business activities that improve their families’ incomes while repaying loans to the members with interest. Borrowers access the Internet via public cybercafes, donated laptops in village schools, and even smart phones, then create their own profile pages through which they share photos and information about themselves and their businesses. As they repay their loans, borrowers continue to share updates and dialogue with lenders via their profile pages. This direct web-based connection allows members themselves to take on many of the communication and recording tasks traditionally performed by local organizations, bypassing geographic barriers and dramatically reducing the cost of microfinance services to the entrepreneurs.[126]

Security

Main article: Internet security

Internet resources, hardware, and software components are the target of criminal or malicious attempts to gain unauthorized control to cause interruptions, commit fraud, engage in blackmail or access private information.

Malware

Main article: Malware

Malicious software used and spread on the Internet includes computer viruses which copy with the help of humans, computer worms which copy themselves automatically, software for denial of service attacksransomwarebotnets, and spyware that reports on the activity and typing of users. Usually, these activities constitute cybercrime. Defense theorists have also speculated about the possibilities of cyber warfare using similar methods on a large scale.[citation needed]

Surveillance

Main article: Computer and network surveillanceSee also: Signals intelligence and Mass surveillance

The vast majority of computer surveillance involves the monitoring of data and traffic on the Internet.[127] In the United States for example, under the Communications Assistance For Law Enforcement Act, all phone calls and broadband Internet traffic (emails, web traffic, instant messaging, etc.) are required to be available for unimpeded real-time monitoring by Federal law enforcement agencies.[128][129][130] Packet capture is the monitoring of data traffic on a computer network. Computers communicate over the Internet by breaking up messages (emails, images, videos, web pages, files, etc.) into small chunks called “packets”, which are routed through a network of computers, until they reach their destination, where they are assembled back into a complete “message” again. Packet Capture Appliance intercepts these packets as they are traveling through the network, in order to examine their contents using other programs. A packet capture is an information gathering tool, but not an analysis tool. That is it gathers “messages” but it does not analyze them and figure out what they mean. Other programs are needed to perform traffic analysis and sift through intercepted data looking for important/useful information. Under the Communications Assistance For Law Enforcement Act all U.S. telecommunications providers are required to install packet sniffing technology to allow Federal law enforcement and intelligence agencies to intercept all of their customers’ broadband Internet and voice over Internet protocol (VoIP) traffic.[131]

The large amount of data gathered from packet capturing requires surveillance software that filters and reports relevant information, such as the use of certain words or phrases, the access of certain types of web sites, or communicating via email or chat with certain parties.[132] Agencies, such as the Information Awareness OfficeNSAGCHQ and the FBI, spend billions of dollars per year to develop, purchase, implement, and operate systems for interception and analysis of data.[133] Similar systems are operated by Iranian secret police to identify and suppress dissidents. The required hardware and software was allegedly installed by German Siemens AG and Finnish Nokia.[134]

Censorship

Internet censorship and surveillance by country(2018)[135][136][137][138][139]

  Pervasive  Substantial  Selective  Little or none

  Unclassified / No dataMain articles: Internet censorship and Internet freedomSee also: Culture of fear and Great Firewall

Some governments, such as those of BurmaIranNorth Korea, the Mainland ChinaSaudi Arabia and the United Arab Emirates restrict access to content on the Internet within their territories, especially to political and religious content, with domain name and keyword filters.[140]

In Norway, Denmark, Finland, and Sweden, major Internet service providers have voluntarily agreed to restrict access to sites listed by authorities. While this list of forbidden resources is supposed to contain only known child pornography sites, the content of the list is secret.[141] Many countries, including the United States, have enacted laws against the possession or distribution of certain material, such as child pornography, via the Internet, but do not mandate filter software. Many free or commercially available software programs, called content-control software are available to users to block offensive websites on individual computers or networks, in order to limit access by children to pornographic material or depiction of violence.

Performance

This section needs expansion. You can help by adding to it(July 2014)

As the Internet is a heterogeneous network, the physical characteristics, including for example the data transfer rates of connections, vary widely. It exhibits emergent phenomena that depend on its large-scale organization.[142]

Outages

An Internet blackout or outage can be caused by local signalling interruptions. Disruptions of submarine communications cables may cause blackouts or slowdowns to large areas, such as in the 2008 submarine cable disruption. Less-developed countries are more vulnerable due to a small number of high-capacity links. Land cables are also vulnerable, as in 2011 when a woman digging for scrap metal severed most connectivity for the nation of Armenia.[143] Internet blackouts affecting almost entire countries can be achieved by governments as a form of Internet censorship, as in the blockage of the Internet in Egypt, whereby approximately 93%[144] of networks were without access in 2011 in an attempt to stop mobilization for anti-government protests.[145]

Energy use

In 2011, researchers estimated the energy used by the Internet to be between 170 and 307 GW, less than two percent of the energy used by humanity. This estimate included the energy needed to build, operate, and periodically replace the estimated 750 million laptops, a billion smart phones and 100 million servers worldwide as well as the energy that routers, cell towers, optical switches, Wi-Fi transmitters and cloud storage devices use when transmitting Internet traffic.[146][147]

Details about Hackers

computer hacker is any skilled computer expert that uses their technical knowledge to overcome a problem. While “hacker” can refer to any skilled computer programmer, the term has become associated in popular culture with a “security hacker“, someone who, with their technical knowledge, uses bugs or exploits to break into computer systems.

Definitions

General Definition

Reflecting the two types of hackers, there are two definitions of the word “hacker”:

  1. an adherent of the technology and programming subculture; see hacker culture.
  2. someone who is able to subvert computer security. If doing so for malicious purposes, the person can also be called a cracker.[1]

Today, mainstream usage of “hacker” mostly refers to computer criminals, due to the mass media usage of the word since the 1980s. This includes what hacker slang calls “script kiddies“, people breaking into computers using programs written by others, with very little knowledge about the way they work. This usage has become so predominant that the general public is largely unaware that different meanings exist.[2] While the self-designation of hobbyists as hackers is generally acknowledged and accepted by computer security hackers, people from the programming subculture consider the computer intrusion related usage incorrect, and emphasize the difference between the two by calling security breakers “crackers” (analogous to a safecracker).

The controversy is usually based on the assertion that the term originally meant someone messing about with something in a positive sense, that is, using playful cleverness to achieve a goal. But then, it is supposed, the meaning of the term shifted over the decades and came to refer to computer criminals.[3]

As the security-related usage has spread more widely, the original meaning has become less known. In popular usage and in the media, “computer intruders” or “computer criminals” is the exclusive meaning of the word today. (For example, “An Internet ‘hacker’ broke through state government security systems in March.”) In the computer enthusiast (Hacker Culture) community, the primary meaning is a complimentary description for a particularly brilliant programmer or technical expert. (For example, “Linus Torvalds, the creator of Linux, is considered by some to be a hacker.”) A large segment of the technical community insist the latter is the “correct” usage of the word (see the Jargon File definition below).

Representation of Hackers in Mainstream Media

The mainstream media‘s current usage of the term may be traced back to the early 1980s. When the term was introduced to wider society by the mainstream media in 1983, even those in the computer community referred to computer intrusion as “hacking”, although not as the exclusive definition of the word. In reaction to the increasing media use of the term exclusively with the criminal connotation, the computer community began to differentiate their terminology. Alternative terms such as “cracker” were coined in an effort to maintain the distinction between “hackers” within the legitimate programmer community and those performing computer break-ins. Further terms such as “black hat“, “white hat” and “gray hat” developed when laws against breaking into computers came into effect, to distinguish criminal activities from those activities which were legal.

Representation of Hackers in Network News

However, network news use of the term consistently pertained primarily to the criminal activities, despite the attempt by the technical community to preserve and distinguish the original meaning, so today the mainstream media and general public continue to describe computer criminals, with all levels of technical sophistication, as “hackers” and do not generally make use of the word in any of its non-criminal connotations. Members of the media sometimes seem unaware of the distinction, grouping legitimate “hackers” such as Linus Torvalds and Steve Wozniak along with criminal “crackers”.[4]

As a result, the definition is still the subject of heated controversy. The wider dominance of the pejorative connotation is resented by many who object to the term being taken from their cultural jargon and used negatively,[5] including those who have historically preferred to self-identify as hackers. Many advocate using the more recent and nuanced alternate terms when describing criminals and others who negatively take advantage of security flaws in software and hardware. Others prefer to follow common popular usage, arguing that the positive form is confusing and unlikely to become widespread in the general public. A minority still use the term in both senses despite the controversy, leaving context to clarify (or leave ambiguous) which meaning is intended.

However, because the positive definition of hacker was widely used as the predominant form for many years before the negative definition was popularized, “hacker” can therefore be seen as a shibboleth, identifying those who use the technically-oriented sense (as opposed to the exclusively intrusion-oriented sense) as members of the computing community. On the other hand, due to the variety of industries software designers may find themselves in, many prefer not to be referred to as hackers because the word holds a negative denotation in many of those industries.

A possible middle ground position has been suggested, based on the observation that “hacking” describes a collection of skills and tools which are used by hackers of both descriptions for differing reasons. The analogy is made to locksmithing, specifically picking locks, which is a skill which can be used for good or evil. The primary weakness of this analogy is the inclusion of script kiddies in the popular usage of “hacker,” despite their lack of an underlying skill and knowledge base.

Sometimes, “hacker” is simply used synonymously with “geek”: “A true hacker is not a group person. He’s a person who loves to stay up all night, he and the machine in a love-hate relationship… They’re kids who tended to be brilliant but not very interested in conventional goals[…] It’s a term of derision and also the ultimate compliment.”[6]

Fred Shapiro thinks that “the common theory that ‘hacker’ originally was a benign term and the malicious connotations of the word were a later perversion is untrue.” He found that the malicious connotations were already present at MIT in 1963 (quoting The Tech, an MIT student newspaper), and at that time referred to unauthorized users of the telephone network,[7][8] that is, the phreaker movement that developed into the computer security hacker subculture of today.

Types

Hacker culture

Main article: Hacker culture

Hacker culture is an idea derived from a community of enthusiast computer programmers and systems designers in the 1960s around the Massachusetts Institute of Technology‘s (MIT’s) Tech Model Railroad Club (TMRC)[9] and the MIT Artificial Intelligence Laboratory.[10] The concept expanded to the hobbyist home computing community, focusing on hardware in the late 1970s (e.g. the Homebrew Computer Club)[11] and on software (video games,[12] software cracking, the demoscene) in the 1980s/1990s. Later, this would go on to encompass many new definitions such as art, and Life hacking.

Security related hacking

Main article: Security hacker

Security hackers are people involved with circumvention of computer security. Among security hackers, there are several types, including:

White hats are hackers who work to keep data safe from other hackers by finding system vulnerabilities that can be mitigated. White hats are usually employed by the target system’s owner and are typically paid (sometimes quite well) for their work. Their work is not illegal because it is done with the system owner’s consent.

Black hats or crackers are hackers with malicious intentions. They often steal, exploit, and sell data, and are usually motivated by personal gain. Their work is usually illegal. A cracker is like a black hat hacker,[13] but is specifically someone who is very skilled and tries via hacking to make profits or to benefit, not just to vandalize. Crackers find exploits for system vulnerabilities and often use them to their advantage by either selling the fix to the system owner or selling the exploit to other black hat hackers, who in turn use it to steal information or gain royalties.

Grey hats include those who hack for fun or to troll. They may both fix and exploit vulnerabilities, but usually not for financial gain. Even if not malicious, their work can still be illegal, if done without the target system owner’s consent, and grey hats are usually associated with black hat hackers.

Motives

Four primary motives have been proposed as possibilities for why hackers attempt to break into computers and networks. First, there is a criminal financial gain to be had when hacking systems with the specific purpose of stealing credit card numbers or manipulating banking systems. Second, many hackers thrive off of increasing their reputation within the hacker subculture and will leave their handles on websites they defaced or leave some other evidence as proof that they were involved in a specific hack. Third, corporate espionage allows companies to acquire information on products or services that can be stolen or used as leverage within the marketplace. And fourth, state-sponsored attacks provide nation states with both wartime and intelligence collection options conducted on, in, or through cyberspace.[14]

Overlaps and differences

The main basic difference between programmer subculture and computer security hacker is their mostly separate historical origin and development. However, the Jargon File reports that considerable overlap existed for the early phreaking at the beginning of the 1970s. An article from MIT’s student paper The Tech used the term hacker in this context already in 1963 in its pejorative meaning for someone messing with the phone system.[7] The overlap quickly started to break when people joined in the activity who did it in a less responsible way.[15] This was the case after the publication of an article exposing the activities of Draper and Engressia.

According to Raymond, hackers from the programmer subculture usually work openly and use their real name, while computer security hackers prefer secretive groups and identity-concealing aliases.[16] Also, their activities in practice are largely distinct. The former focus on creating new and improving existing infrastructure (especially the software environment they work with), while the latter primarily and strongly emphasize the general act of circumvention of security measures, with the effective use of the knowledge (which can be to report and help fixing the security bugs, or exploitation reasons) being only rather secondary. The most visible difference in these views was in the design of the MIT hackers’ Incompatible Timesharing System, which deliberately did not have any security measures.

There are some subtle overlaps, however, since basic knowledge about computer security is also common within the programmer subculture of hackers. For example, Ken Thompson noted during his 1983 Turing Award lecture that it is possible to add code to the UNIX “login” command that would accept either the intended encrypted password or a particular known password, allowing a backdoor into the system with the latter password. He named his invention the “Trojan horse“. Furthermore, Thompson argued, the C compiler itself could be modified to automatically generate the rogue code, to make detecting the modification even harder. Because the compiler is itself a program generated from a compiler, the Trojan horse could also be automatically installed in a new compiler program, without any detectable modification to the source of the new compiler. However, Thompson disassociated himself strictly from the computer security hackers: “I would like to criticize the press in its handling of the ‘hackers,’ the 414 gang, the Dalton gang, etc. The acts performed by these kids are vandalism at best and probably trespass and theft at worst. … I have watched kids testifying before Congress. It is clear that they are completely unaware of the seriousness of their acts.”[17]

The programmer subculture of hackers sees secondary circumvention of security mechanisms as legitimate if it is done to get practical barriers out of the way for doing actual work. In special forms, that can even be an expression of playful cleverness.[18] However, the systematic and primary engagement in such activities is not one of the actual interests of the programmer subculture of hackers and it does not have significance in its actual activities, either.[16] A further difference is that, historically, members of the programmer subculture of hackers were working at academic institutions and used the computing environment there. In contrast, the prototypical computer security hacker had access exclusively to a home computer and a modem. However, since the mid-1990s, with home computers that could run Unix-like operating systems and with inexpensive internet home access being available for the first time, many people from outside of the academic world started to take part in the programmer subculture of hacking.

Since the mid-1980s, there are some overlaps in ideas and members with the computer security hacking community. The most prominent case is Robert T. Morris, who was a user of MIT-AI, yet wrote the Morris worm. The Jargon File hence calls him “a true hacker who blundered”.[19] Nevertheless, members of the programmer subculture have a tendency to look down on and disassociate from these overlaps. They commonly refer disparagingly to people in the computer security subculture as crackers and refuse to accept any definition of hacker that encompasses such activities. The computer security hacking subculture, on the other hand, tends not to distinguish between the two subcultures as harshly, acknowledging that they have much in common including many members, political and social goals, and a love of learning about technology. They restrict the use of the term cracker to their categories of script kiddies and black hat hackers instead.

All three subcultures have relations to hardware modifications. In the early days of network hacking, phreaks were building blue boxes and various variants. The programmer subculture of hackers has stories about several hardware hacks in its folklore, such as a mysterious ‘magic’ switch attached to a PDP-10 computer in MIT’s AI lab, that when turned off, crashed the computer.[20] The early hobbyist hackers built their home computers themselves, from construction kits. However, all these activities have died out during the 1980s, when the phone network switched to digitally controlled switchboards, causing network hacking to shift to dialing remote computers with modems, when pre-assembled inexpensive home computers were available, and when academic institutions started to give individual mass-produced workstation computers to scientists instead of using a central timesharing system. The only kind of widespread hardware modification nowadays is case modding.

An encounter of the programmer and the computer security hacker subculture occurred at the end of the 1980s, when a group of computer security hackers, sympathizing with the Chaos Computer Club (which disclaimed any knowledge in these activities), broke into computers of American military organizations and academic institutions. They sold data from these machines to the Soviet secret service, one of them in order to fund his drug addiction. The case was solved when Clifford Stoll, a scientist working as a system administrator, found ways to log the attacks and to trace them back (with the help of many others). 23, a German film adaption with fictional elements, shows the events from the attackers’ perspective. Stoll described the case in his book The Cuckoo’s Egg and in the TV documentary The KGB, the Computer, and Me from the other perspective. According to Eric S. Raymond, it “nicely illustrates the difference between ‘hacker’ and ‘cracker’. Stoll’s portrait of himself, his lady Martha, and his friends at Berkeley and on the Internet paints a marvelously vivid picture of how hackers and the people around them like to live and how they think.”[21]

Total Details About Business

Business is the activity of making one’s living or making money by producing or buying and selling products (such as goods and services).[1][2][need quotation to verify][3][4] Simply put, it is “any activity or enterprise entered into for profit. It does not mean it is a company, a corporation, partnership, or have any such formal organization, but it can range from a street peddler to General Motors.”[5]

Having a business name does not separate the business entity from the owner, which means that the owner of the business is responsible and liable for debts incurred by the business. If the business acquires debts, the creditors can go after the owner’s personal possessions. A business structure does not allow for corporate tax rates. The proprietor is personally taxed on all income from the business.

The term is also often used colloquially (but not by lawyers or by public officials) to refer to a company. A company, on the other hand, is a separate legal entity and provides for limited liability, as well as corporate tax rates. A company structure is more complicated and expensive to set up, but offers more protection and benefits for the owner.

Forms[edit]

Main article: List of business entities

Forms of business ownership vary by jurisdiction, but several common entities exist:

  • Sole proprietorship: A sole proprietorship, also known as a sole trader, is owned by one person and operates for their benefit. The owner operates the business alone and may hire employees. A sole proprietor has unlimited liability for all obligations incurred by the business, whether from operating costs or judgments against the business. All assets of the business belong to a sole proprietor, including, for example, a computer infrastructure, any inventorymanufacturing equipment, or retail fixtures, as well as any real property owned by the sole proprietor.
  • Partnership: A partnership is a business owned by two or more people. In most forms of partnerships, each partner has unlimited liability for the debts incurred by the business. The three most prevalent types of for-profit partnerships are general partnershipslimited partnerships, and limited liability partnerships.[6]
  • Corporation: The owners of a corporation have limited liability and the business has a separate legal personality from its owners. Corporations can be either government-owned or privately owned, and they can organize either for profit or as nonprofit organizations. A privately owned, for-profit corporation is owned by its shareholders, who elect a board of directors to direct the corporation and hire its managerial staff. A privately owned, for-profit corporation can be either privately held by a small group of individuals, or publicly held, with publicly traded shares listed on a stock exchange.
  • Cooperative: Often referred to as a “co-op”, a cooperative is a limited-liability business that can organize as for-profit or not-for-profit. A cooperative differs from a corporation in that it has members, not shareholders, and they share decision-making authority. Cooperatives are typically classified as either consumer cooperatives or worker cooperatives. Cooperatives are fundamental to the ideology of economic democracy.
  • Limited liability companies (LLC), limited liability partnerships, and other specific types of business organization protect their owners or shareholders from business failure by doing business under a separate legal entity with certain legal protections. In contrast, unincorporated businesses or persons working on their own are usually not as protected.[7][8]
  • Franchises: A franchise is a system in which entrepreneurs purchase the rights to open and run a business from a larger corporation.[9] Franchising in the United States is widespread and is a major economic powerhouse. One out of twelve retail businesses in the United States are franchised and 8 million people are employed in a franchised business.[10]
  • company limited by guarantee: Commonly used where companies are formed for non-commercial purposes, such as clubs or charities. The members guarantee the payment of certain (usually nominal) amounts if the company goes into insolvent liquidation, but otherwise, they have no economic rights in relation to the company. This type of company is common in England. A company limited by guarantee may be with or without having share capital.
  • company limited by shares: The most common form of the company used for business ventures. Specifically, a limited company is a “company in which the liability of each shareholder is limited to the amount individually invested” with corporations being “the most common example of a limited company.”[11] This type of company is common in England and many English-speaking countries. A company limited by shares may be a
  • A company limited by guarantee with a share capital: A hybrid entity, usually used where the company is formed for non-commercial purposes, but the activities of the company are partly funded by investors who expect a return. This type of company may no longer be formed in the UK, although provisions still exist in law for them to exist.[12]
  • limited liability company: “A company—statutorily authorized in certain states—that is characterized by limited liability, management by members or managers, and limitations on ownership transfer”, i.e., L.L.C.[11] LLC structure has been called “hybrid” in that it “combines the characteristics of a corporation and of a partnership or sole proprietorship”. Like a corporation, it has limited liability for members of the company, and like a partnership, it has “flow-through taxation to the members” and must be “dissolved upon the death or bankruptcy of a member”.[13]
  • An unlimited company with or without a share capital: A hybrid entity, a company where the liability of members or shareholders for the debts (if any) of the company are not limited. In this case, the doctrine of a veil of incorporation does not apply.

Less common types of companies are:

  • Companies formed by letters patent: Most corporations by letters patent are corporations sole and not companies as the term is commonly understood today.
  • Charter corporations: Before the passing of modern companies legislation, these were the only types of companies. Now they are relatively rare, except for very old companies that still survive (of which there are still many, particularly many British banks), or modern societies that fulfill a quasi-regulatory function (for example, the Bank of England is a corporation formed by a modern charter).
  • Statutory companies: Relatively rare today, certain companies have been formed by a private statute passed in the relevant jurisdiction.

Note that “Ltd after the company’s name signifies limited company, and PLC (public limited company) indicates that its shares are widely held.”[14]

In legal parlance, the owners of a company are normally referred to as the “members”. In a company limited or unlimited by shares (formed or incorporated with a share capital), this will be the shareholders. In a company limited by guarantee, this will be the guarantors. Some offshore jurisdictions have created special forms of offshore company in a bid to attract business for their jurisdictions. Examples include “segregated portfolio companies” and restricted purpose companies.

There are, however, many, many sub-categories of types of company that can be formed in various jurisdictions in the world.

Companies are also sometimes distinguished into public companies and private companies for legal and regulatory purposes. Public companies are companies whose shares can be publicly traded, often (although not always) on a stock exchange which imposes listing requirements/Listing Rules as to the issued shares, the trading of shares and a future issue of shares to help bolster the reputation of the exchange or particular market of exchange. Private companies do not have publicly traded shares, and often contain restrictions on transfers of shares. In some jurisdictions, private companies have maximum numbers of shareholders.

parent company is a company that owns enough voting stock in another firm to control management and operations by influencing or electing its board of directors; the second company being deemed as a subsidiary of the parent company. The definition of a parent company differs by jurisdiction, with the definition normally being defined by way of laws dealing with companies in that jurisdiction.

Classifications[edit]

Main article: Industry classification

Activities[edit]

Accounting[edit]

Main article: Accounting

Accounting is the measurement, processing, and communication of financial information about economic entities[15][16] such as businesses and corporations. The modern field was established by the Italian mathematician Luca Pacioli in 1494.[17] Accounting, which has been called the “language of business”,[18] measures the results of an organization’s economic activities and conveys this information to a variety of users, including investorscreditorsmanagement, and regulators.[19] Practitioners of accounting are known as accountants. The terms “accounting” and “financial reporting” are often used as synonyms.

Finance[edit]

Main article: Finance

Finance is a field that deals with the study of investments. It includes the dynamics of assets and liabilities over time under conditions of different degrees of uncertainty and risk. Finance can also be defined as the science of money management. Finance aims to price assets based on their risk level and their expected rate of return. Finance can be broken into three different sub categories: public financecorporate finance, and personal finance.

Manufacturing[edit]

Main article: Manufacturing

Manufacturing is the production of merchandise for use or sale using labour and machinestools, chemical and biological processing, or formulation. The term may refer to a range of human activity, from handicraft to high tech, but is most commonly applied to industrial production, in which raw materials are transformed into finished goods on a large scale.

Marketing[edit]

Main article: Marketing

Marketing is defined by the American Marketing Association as “the activity, set of institutions, and processes for creating, communicating, delivering, and exchanging offerings that have value for customers, clients, partners, and society at large.”[20] The term developed from the original meaning which referred literally to going to a market to buy or sell goods or services. Marketing tactics include advertising as well as determining product pricing.

With the rise in technology, marketing is further divided into a class called digital marketing. It is marketing products and services using digital technologies.

Research and development[edit]

Main article: Research and development

Research and development refer to activities in connection with corporate or government innovation. Research and development constitute the first stage of development of a potential new service or product. Research and development are very difficult to manage since the defining feature of the research is that the researchers do not know in advance exactly how to accomplish the desired result.[citation needed]

Safety[edit]

Main article: Safety

Safety is a key business concept that is defined by Merriam-Webster as “the condition of being safe from undergoing or causing hurt, injury, or loss”.[21] Injuries cost businesses billions of dollars annually.[22] Studies have shown how company acceptance and implementation of comprehensive safety and health management systems reduce incidents, insurance costs, and workers’ compensation claims.[23] New technologies, like wearable safety devices[24] and available online safety training, continue to be developed to encourage employers to invest in protection beyond the “canary in the coal mine” and reduce the cost to businesses of protecting their employees.

Sales[edit]

Main article: Sales

Sales are activity related to selling or the number of goods or services sold in a given time period. Sales are often integrated with all lines of business and are key to a companies’ success.[25]

Management[edit]

Main article: ManagementSee also: Outline of business management

The efficient and effective operation of a business, and study of this subject, is called management. The major branches of management are financial managementmarketing management, human resource managementstrategic managementproduction managementoperations managementservice management, and information technology management.[citation needed]

Owners may manage their businesses themselves, or employ managers to do so for them. Whether they are owners or employees, managers administer three primary components of the business’ value: financial resources, capital (tangible resources), and human resources. These resources are administered in at least six functional areas: legal contracting, manufacturing or service production, marketing, accounting, financing, and human resources.[citation needed]

Restructuring state enterprises[edit]

In recent decades, states modeled some of their assets and enterprises after business enterprises. In 2003, for example, the People’s Republic of China modeled 80% of its state-owned enterprises on a company-type management system.[26] Many state institutions and enterprises in China and Russia have transformed into joint-stock companies, with part of their shares being listed on public stock markets.

Business process management (BPM) is a holistic management approach focused on aligning all aspects of an organization with the wants and needs of clients. BPM attempts to improve processes continuously. It can, therefore, be described as a “process optimization process”. It is argued that BPM enables organizations to be more efficient, effective and capable of change than a functionally focused, traditional hierarchical management approach.[who?]

Organization and regulation[edit]

See also: Theory of the firm

Most legal jurisdictions specify the forms of ownership that a business can take, creating a body of commercial law for each type.

The major factors affecting how a business is organized are usually:

  • The size and scope of the business firm and its structure, management, and ownership, broadly analyzed in the theory of the firm. Generally, a smaller business is more flexible, while larger businesses, or those with wider ownership or more formal structures, will usually tend to be organized as corporations or (less often) partnerships. In addition, a business that wishes to raise money on a stock market or to be owned by a wide range of people will often be required to adopt a specific legal form to do so.
  • The sector and country. Private profit-making businesses are different from government-owned bodies. In some countries, certain businesses are legally obliged to be organized in certain ways.
  • Tax advantages. Different structures are treated differently in tax law and may have advantages for this reason.
  • Disclosure and compliance requirements. Different business structures may be required to make less or more information public (or report it to relevant authorities) and may be bound to comply with different rules and regulations.

Many businesses are operated through a separate entity such as a corporation or a partnership (either formed with or without limited liability). Most legal jurisdictions allow people to organize such an entity by filing certain charter documents with the relevant Secretary of State or equivalent and complying with certain other ongoing obligations. The relationships and legal rights of shareholders, limited partners, or members are governed partly by the charter documents and partly by the law of the jurisdiction where the entity is organized. Generally speaking, shareholders in a corporation, limited partners in a limited partnership, and members in a limited liability company are shielded from personal liability for the debts and obligations of the entity, which is legally treated as a separate “person”. This means that unless there is misconduct, the owner’s own possessions are strongly protected in law if the business does not succeed.

Where two or more individuals own a business together but have failed to organize a more specialized form of vehicle, they will be treated as a general partnership. The terms of a partnership are partly governed by a partnership agreement if one is created, and partly by the law of the jurisdiction where the partnership is located. No paperwork or filing is necessary to create a partnership, and without an agreement, the relationships and legal rights of the partners will be entirely governed by the law of the jurisdiction where the partnership is located. A single person who owns and runs a business is commonly known as a sole proprietor, whether that person owns it directly or through a formally organized entity. Depending on the business needs, an adviser can decide what kind is proprietorship will be most suitable.

A few relevant factors to consider in deciding how to operate a business include:

  1. General partners in a partnership (other than a limited liability partnership), plus anyone who personally owns and operates a business without creating a separate legal entity, are personally liable for the debts and obligations of the business.
  2. Generally, corporations are required to pay tax just like “real” people. In some tax systems, this can give rise to so-called double taxation, because first the corporation pays tax on the profit, and then when the corporation distributes its profits to its owners, individuals have to include dividends in their income when they complete their personal tax returns, at which point a second layer of income tax is imposed.
  3. In most countries, there are laws which treat small corporations differently from large ones. They may be exempt from certain legal filing requirements or labor laws, have simplified procedures in specialized areas, and have simplified, advantageous, or slightly different tax treatment.
  4. “Going public” through a process known as an initial public offering (IPO) means that part of the business will be owned by members of the public. This requires the organization as a distinct entity, to disclose information to the public, and adhering to a tighter set of laws and procedures. Most public entities are corporations that have sold shares, but increasingly there are also public LLC’s that sell units (sometimes also called shares), and other more exotic entities as well, such as, for example, real estate investment trusts in the US, and unit trusts in the UK. A general partnership cannot “go public”.

Commercial law[edit]

Main article: Corporate law

Offices in the Los AngelesDowntown Financial District

A very detailed and well-established body of rules that evolved over a very long period of time applies to commercial transactions. The need to regulate trade and commerce and resolve business disputes helped shape the creation of law and courts. The Code of Hammurabi dates back to about 1772 BC for example and contains provisions that relate, among other matters, to shipping costs and dealings between merchants and brokers.[27] The word “corporation” derives from the Latin corpus, meaning body, and the Maurya Empire in Iron-Age India accorded legal rights to business entities.[28]

In many countries, it is difficult to compile all the laws that can affect a business into a single reference source. Laws can govern the treatment of labour and employee relations, worker protection and safety, discrimination on the basis of age, gender, disability, race, and in some jurisdictions, sexual orientation, and the minimum wage, as well as unions, worker compensation, and working hours and leave.

Some specialized businesses may also require licenses, either due to laws governing entry into certain trades, occupations or professions, that require special education or to raise revenue for local governments. Professions that require special licenses include law, medicine, piloting aircraft, selling liquor, radio broadcasting, selling investment securities, selling used cars, and roofing. Local jurisdictions may also require special licenses and taxes just to operate a business.

Some businesses are subject to ongoing special regulation, for example, public utilities, investment securities, banking, insurance, broadcastingaviation, and health care providers. Environmental regulations are also very complex and can affect many businesses.

Capital[edit]

Mexican Stock Exchange in Paseo de la Reforma, Mexico City

When businesses need to raise money (called capital), they sometimes offer securities for sale.

Capital may be raised through private means, by an initial public offering or IPO on a stock exchange, or in other ways.

Major stock exchanges include the Shanghai Stock ExchangeSingapore ExchangeHong Kong Stock ExchangeNew York Stock Exchange and NASDAQ (the USA), the London Stock Exchange (UK), the Tokyo Stock Exchange (Japan), and Bombay Stock Exchange (India). Most countries with capital markets have at least one.

Businesses that have gone public are subject to regulations concerning their internal governance, such as how executive officers’ compensation is determined, and when and how information is disclosed to shareholders and to the public. In the United States, these regulations are primarily implemented and enforced by the United States Securities and Exchange Commission (SEC). Other western nations have comparable regulatory bodies. The regulations are implemented and enforced by the China Securities Regulation Commission (CSRC) in China. In Singapore, the regulatory authority is the Monetary Authority of Singapore (MAS), and in Hong Kong, it is the Securities and Futures Commission (SFC).

The proliferation and increasing complexity of the laws governing business have forced increasing specialization in corporate law. It is not unheard of for certain kinds of corporate transactions to require a team of five to ten attorneys due to sprawling regulation. Commercial law spans general corporate law, employment and labor law, health-care law, securities law, mergers and acquisitions, tax law, employee benefit plans, food and drug regulation, intellectual property law on copyrights, patents, trademarks, telecommunications law, and financing.

Other types of capital sourcing include crowdsourcing on the Internet, venture capital, bank loans, and debentures.

Intellectual property[edit]

Main article: Intellectual property

Businesses often have important “intellectual property” that needs protection from competitors for the company to stay profitable. This could require patentscopyrightstrademarks, or preservation of trade secrets. Most businesses have names, logos, and similar branding techniques that could benefit from trademarking. Patents and copyrights in the United States are largely governed by federal law, while trade secrets and trademarking are mostly a matter of state law. Because of the nature of intellectual property, a business needs protection in every jurisdiction in which they are concerned about competitors. Many countries are signatories to international treaties concerning intellectual property, and thus companies registered in these countries are subject to national laws bound by these treaties. In order to protect trade secrets, companies may require employees to sign noncompete clauses which will impose limitations on an employee’s interactions with stakeholders, and competitors.

Trade union[edit]

Main article: Trade union

A trade union (or labor union) is an organization of workers who have come together to achieve common goals such as protecting the integrity of its trade, improving safety standards, achieving higher pay and benefits such as health care and retirement, increasing the number of employees an employer assigns to complete the work, and better working conditions. The trade union, through its leadership, bargains with the employer on behalf of union members (rank and file members) and negotiates labor contracts (collective bargaining) with employers. The most common purpose of these associations or unions is “maintaining or improving the conditions of their employment“.[29] This may include the negotiation of wages, work rules, complaint procedures, rules governing hiring, firing, and promotion of workers, benefits, workplace safety and policies.

Total Information about Sharks

Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the sister group to the rays. However, the term “shark” has also been used for extinct members of the subclass Elasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthus, as well as other Chondrichthyes such as the holocephalid eugenedontidans.

Under this broader definition, the earliest known sharks date back to more than 420 million years ago.[2] Acanthodians are often referred to as “spiny sharks”; though they are not part of Chondrichthyes proper, they are a paraphyletic assemblage leading to cartilaginous fish as a whole. Since then, sharks have diversified into over 500 species. They range in size from the small dwarf lanternshark (Etmopterus perryi), a deep sea species of only 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (40 ft) in length.[3] Sharks are found in all seas and are common to depths of 2,000 metres (6,600 ft). They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can be found in both seawater and freshwater.[4] Sharks have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. They have numerous sets of replaceable teeth.[5]

Well-known species such as the great white sharktiger sharkblue sharkmako sharkthresher shark, and hammerhead shark are apex predators—organisms at the top of their underwater food chain. Many shark populations are threatened by human activities.

Etymology

Until the 16th century,[6] sharks were known to mariners as “sea dogs”.[7] This is still evidential in several species termed “dogfish,” or the porbeagle.

The etymology of the word “shark” is uncertain, the most likely etymology states that the original sense of the word was that of “predator, one who preys on others” from the Dutch schurk, meaning “villain, scoundrel” (cf. card sharkloan shark, etc.), which was later applied to the fish due to its predatory behaviour.[8]

A now disproven theory is that it derives from the Yucatec Maya word xok (pronounced ‘shok’), meaning “fish”.[9] Evidence for this etymology came from the Oxford English Dictionary, which notes shark first came into use after Sir John Hawkins‘ sailors exhibited one in London in 1569 and posted “sharke” to refer to the large sharks of the Caribbean Sea. However, the Middle English Dictionary records an isolated occurrence of the word shark (referring to a sea fish) in a letter written by Thomas Beckington in 1442, which rules out a New World etymology.[10]

Evolution

Photo of dozens of yellowish fossilized teeth, the teeth are of various sizes and are spread out randomly on a flat black surface.

A collection of Cretaceousshark teethSee also: Evolution of fish

Evidence for the existence of sharks dates from the Ordovician period, 450–420 million years ago, before land vertebrates existed and before a variety of plants had colonized the continents.[2] Only scales have been recovered from the first sharks and not all paleontologists agree that these are from true sharks, suspecting that these scales are actually those of thelodont agnathans.[11] The oldest generally accepted shark scales are from about 420 million years ago, in the Silurian period.[11] The first sharks looked very different from modern sharks.[12] At this time the most common shark tooth is the cladodont, a style of thin tooth with three tines like a trident, apparently to help catch fish. The majority of modern sharks can be traced back to around 100 million years ago.[13] Most fossils are of teeth, often in large numbers. Partial skeletons and even complete fossilized remains have been discovered. Estimates suggest that sharks grow tens of thousands of teeth over a lifetime, which explains the abundant fossils. The teeth consist of easily fossilized calcium phosphate, an apatite. When a shark dies, the decomposing skeleton breaks up, scattering the apatite prisms. Preservation requires rapid burial in bottom sediments.

Among the most ancient and primitive sharks is Cladoselache, from about 370 million years ago,[12] which has been found within Paleozoic strata in Ohio, Kentucky, and Tennessee. At that point in Earth’s history these rocks made up the soft bottom sediments of a large, shallow ocean, which stretched across much of North America. Cladoselache was only about 1 metre (3.3 ft) long with stiff triangular fins and slender jaws.[12] Its teeth had several pointed cusps, which wore down from use. From the small number of teeth found together, it is most likely that Cladoselache did not replace its teeth as regularly as modern sharks. Its caudal fins had a similar shape to the great white sharks and the pelagic shortfin and longfin makos. The presence of whole fish arranged tail-first in their stomachs suggest that they were fast swimmers with great agility.

Most fossil sharks from about 300 to 150 million years ago can be assigned to one of two groups. The Xenacanthida was almost exclusive to freshwater environments.[14][15] By the time this group became extinct about 220 million years ago, they had spread worldwide. The other group, the hybodonts, appeared about 320 million years ago and lived mostly in the oceans, but also in freshwater.[citation needed] The results of a 2014 study of the gill structure of an unusually well preserved 325-million-year-old fossil suggested that sharks are not “living fossils”, but rather have evolved more extensively than previously thought over the hundreds of millions of years they have been around.[16]

Drawing comparing sizes of Megalodon, great white shark and a man, Megalodon is 18m long and great white 6m.

Megalodon (top two, estimated maximum and conservative sizes) with the whale shark, great white shark, and a human for scale

Modern sharks began to appear about 100 million years ago.[13] Fossil mackerel shark teeth date to the Early Cretaceous. One of the most recently evolved families is the hammerhead shark (family Sphyrnidae), which emerged in the Eocene.[17] The oldest white shark teeth date from 60 to 66 million years ago, around the time of the extinction of the dinosaurs. In early white shark evolution there are at least two lineages: one lineage is of white sharks with coarsely serrated teeth and it probably gave rise to the modern great white shark, and another lineage is of white sharks with finely serrated teeth. These sharks attained gigantic proportions and include the extinct megatoothed shark, C. megalodon. Like most extinct sharks, C. megalodon is also primarily known from its fossil teeth and vertebrae. This giant shark reached a total length (TL) of more than 16 metres (52 ft).[18][19] C. megalodon may have approached a maxima of 20.3 metres (67 ft) in total length and 103 metric tons (114 short tons) in mass.[20] Paleontological evidence suggests that this shark was an active predator of large cetaceans.[20]

Taxonomy

Branching diagram listing distinguishing characteristics, including mouth, snout, fin spines, etc.

Sharks belong to the superorder Selachimorpha in the subclass Elasmobranchii in the class Chondrichthyes. The Elasmobranchii also include rays and skates; the Chondrichthyes also include Chimaeras. It was thought that the sharks form a polyphyletic group: some sharks are more closely related to rays than they are to some other sharks,[21] but current molecular studies support monophyly of both groups of sharks and batoids.[22][23]

The superorder Selachimorpha is divided into Galea (or Galeomorphii), and Squalea (or Squalomorphii). The Galeans are the HeterodontiformesOrectolobiformesLamniformes, and Carcharhiniformes. Lamnoids and Carcharhinoids are usually placed in one clade, but recent studies show the Lamnoids and Orectoloboids are a clade. Some scientists now think that Heterodontoids may be Squalean. The Squaleans are divided into Hexanchiformes and Squalomorpha. The former includes cow shark and frilled shark, though some authors propose both families to be moved to separate orders. The Squalomorpha contains the Squaliformes and the Hypnosqualea. The Hypnosqualea may be invalid. It includes the Squatiniformes, and the Pristorajea, which may also be invalid, but includes the Pristiophoriformes and the Batoidea.[21][24]

There are more than 470 species of sharks split across twelve orders, including four orders of sharks that have gone extinct:[24]

Anatomy

Drawing of a shark labeling major anatomical features, including mouth, snout, nostril, eye, spiracle, dorsal fin spine, caudal keel, clasper, labial furrows, gill openings, precaudal pit and fins: first and second dorsal, anal, pectoral, caudal and pelvic

General anatomical features of sharksMain article: Shark anatomy

Teeth

Main article: Shark tooth

The serrated teeth of a tiger shark, used for sawing through flesh

The teeth of tiger sharks are oblique and serrated to saw through flesh

Shark teeth are embedded in the gums rather than directly affixed to the jaw, and are constantly replaced throughout life. Multiple rows of replacement teeth grow in a groove on the inside of the jaw and steadily move forward in comparison to a conveyor belt; some sharks lose 30,000 or more teeth in their lifetime. The rate of tooth replacement varies from once every 8 to 10 days to several months. In most species, teeth are replaced one at a time as opposed to the simultaneous replacement of an entire row, which is observed in the cookiecutter shark.[25]

Tooth shape depends on the shark’s diet: those that feed on mollusks and crustaceans have dense and flattened teeth used for crushing, those that feed on fish have needle-like teeth for gripping, and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting. The teeth of plankton-feeders such as the basking shark are small and non-functional.[26]

Skeleton

Shark skeletons are very different from those of bony fish and terrestrial vertebrates. Sharks and other cartilaginous fish (skates and rays) have skeletons made of cartilage and connective tissue. Cartilage is flexible and durable, yet is about half the normal density of bone. This reduces the skeleton’s weight, saving energy.[27] Because sharks do not have rib cages, they can easily be crushed under their own weight on land.[28]

Jaw

Jaws of sharks, like those of rays and skates, are not attached to the cranium. The jaw’s surface (in comparison to the shark’s vertebrae and gill arches) needs extra support due to its heavy exposure to physical stress and its need for strength. It has a layer of tiny hexagonal plates called “tesserae“, which are crystal blocks of calcium salts arranged as a mosaic.[29] This gives these areas much of the same strength found in the bony tissue found in other animals.

Generally sharks have only one layer of tesserae, but the jaws of large specimens, such as the bull shark, tiger shark, and the great white shark, have two to three layers or more, depending on body size. The jaws of a large great white shark may have up to five layers.[27] In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts.

Fins

Fin skeletons are elongated and supported with soft and unsegmented rays named ceratotrichia, filaments of elastic protein resembling the horny keratin in hair and feathers.[30] Most sharks have eight fins. Sharks can only drift away from objects directly in front of them because their fins do not allow them to move in the tail-first direction.[28]

Dermal denticles

Main article: Dermal denticle

The dermal denticles of a lemon shark

The dermal denticles of a lemon shark, viewed through a scanning electron microscope

Unlike bony fish, sharks have a complex dermal corset made of flexible collagenous fibers and arranged as a helical network surrounding their body. This works as an outer skeleton, providing attachment for their swimming muscles and thus saving energy.[31] Their dermal teeth give them hydrodynamicadvantages as they reduce turbulence when swimming.[32]

Tails

Tails provide thrust, making speed and acceleration dependent on tail shape. Caudal fin shapes vary considerably between shark species, due to their evolution in separate environments. Sharks possess a heterocercal caudal fin in which the dorsal portion is usually noticeably larger than the ventralportion. This is because the shark’s vertebral column extends into that dorsal portion, providing a greater surface area for muscle attachment. This allows more efficient locomotion among these negatively buoyant cartilaginous fish. By contrast, most bony fish possess a homocercal caudal fin.[33]

Tiger sharks have a large upper lobe, which allows for slow cruising and sudden bursts of speed. The tiger shark must be able to twist and turn in the water easily when hunting to support its varied diet, whereas the porbeagle shark, which hunts schooling fish such as mackerel and herring, has a large lower lobe to help it keep pace with its fast-swimming prey.[34] Other tail adaptations help sharks catch prey more directly, such as the thresher shark’s usage of its powerful, elongated upper lobe to stun fish and squid.

Physiology

Buoyancy

Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. Instead, sharks rely on a large liver filled with oil that contains squalene, and their cartilage, which is about half the normal density of bone.[31] Their liver constitutes up to 30% of their total body mass.[35] The liver’s effectiveness is limited, so sharks employ dynamic lift to maintain depth while swimming. Sand tiger sharks store air in their stomachs, using it as a form of swim bladder. Bottom-dwelling sharks, like the nurse shark, have negative buoyancy, allowing them to rest on the ocean floor.

Some sharks, if inverted or stroked on the nose, enter a natural state of tonic immobility. Researchers use this condition to handle sharks safely.[36]

Respiration

Like other fish, sharks extract oxygen from seawater as it passes over their gills. Unlike other fish, shark gill slits are not covered, but lie in a row behind the head. A modified slit called a spiraclelies just behind the eye, which assists the shark with taking in water during respiration and plays a major role in bottom–dwelling sharks. Spiracles are reduced or missing in active pelagicsharks.[26] While the shark is moving, water passes through the mouth and over the gills in a process known as “ram ventilation”. While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small number of species have lost the ability to pump water through their gills and must swim without rest. These species are obligate ram ventilatorsand would presumably asphyxiate if unable to move. Obligate ram ventilation is also true of some pelagic bony fish species.[37][38]

The respiration and circulation process begins when deoxygenated blood travels to the shark’s two-chambered heart. Here the shark pumps blood to its gills via the ventral aorta artery where it branches into afferent brachial arteries. Reoxygenation takes place in the gills and the reoxygenated blood flows into the efferent brachial arteries, which come together to form the dorsal aorta. The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses. From there blood enters the heart ventricle and the cycle repeats.[39]

Thermoregulation

Most sharks are “cold-blooded” or, more precisely, poikilothermic, meaning that their internal body temperature matches that of their ambient environment. Members of the family Lamnidae (such as the shortfin mako shark and the great white shark) are homeothermic and maintain a higher body temperature than the surrounding water. In these sharks, a strip of aerobic red muscle located near the center of the body generates the heat, which the body retains via a countercurrent exchange mechanism by a system of blood vessels called the rete mirabile (“miraculous net”). The common thresher and bigeye thresher sharks have a similar mechanism for maintaining an elevated body temperature.[40]

Osmoregulation

In contrast to bony fish, with the exception of the coelacanth,[41] the blood and other tissue of sharks and Chondrichthyes is generally isotonic to their marine environments because of the high concentration of urea (up to 2.5%[42]) and trimethylamine N-oxide (TMAO), allowing them to be in osmotic balance with the seawater. This adaptation prevents most sharks from surviving in freshwater, and they are therefore confined to marine environments. A few exceptions exist, such as the bull shark, which has developed a way to change its kidney function to excrete large amounts of urea.[35] When a shark dies, the urea is broken down to ammonia by bacteria, causing the dead body to gradually smell strongly of ammonia.[43][44]

Digestion

Digestion can take a long time. The food moves from the mouth to a J-shaped stomach, where it is stored and initial digestion occurs.[45] Unwanted items may never get past the stomach, and instead the shark either vomits or turns its stomachs inside out and ejects unwanted items from its mouth.[46]

One of the biggest differences between the digestive systems of sharks and mammals is that sharks have much shorter intestines. This short length is achieved by the spiral valve with multiple turns within a single short section instead of a long tube-like intestine. The valve provides a long surface area, requiring food to circulate inside the short gut until fully digested, when remaining waste products pass into the cloaca.[45]

Senses

Smell

Eyelevel photo of hammerhead from the front

The shape of the hammerhead shark‘s head may enhance olfaction by spacing the nostrils further apart.

Sharks have keen olfactory senses, located in the short duct (which is not fused, unlike bony fish) between the anterior and posterior nasal openings, with some species able to detect as little as one part per million of blood in seawater.[47]

Sharks have the ability to determine the direction of a given scent based on the timing of scent detection in each nostril.[48] This is similar to the method mammals use to determine direction of sound.

They are more attracted to the chemicals found in the intestines of many species, and as a result often linger near or in sewage outfalls. Some species, such as nurse sharks, have external barbels that greatly increase their ability to sense prey.

Sight

Eye of a Bigeyed sixgill shark(Hexanchus nakamurai)

Shark eyes are similar to the eyes of other vertebrates, including similar lensescorneas and retinas, though their eyesight is well adapted to the marine environment with the help of a tissue called tapetum lucidum. This tissue is behind the retina and reflects light back to it, thereby increasing visibility in the dark waters. The effectiveness of the tissue varies, with some sharks having stronger nocturnal adaptations. Many sharks can contract and dilate their pupils, like humans, something no teleost fish can do. Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To protect their eyes some species have nictitating membranes. This membrane covers the eyes while hunting and when the shark is being attacked. However, some species, including the great white shark (Carcharodon carcharias), do not have this membrane, but instead roll their eyes backwards to protect them when striking prey. The importance of sight in shark hunting behavior is debated. Some believe that electro- and chemoreception are more significant, while others point to the nictating membrane as evidence that sight is important. Presumably, the shark would not protect its eyes were they unimportant. The use of sight probably varies with species and water conditions. The shark’s field of vision can swap between monocular and stereoscopic at any time.[49] A micro-spectrophotometry study of 17 species of shark found 10 had only rod photoreceptors and no cone cells in their retinas giving them good night vision while making them colorblind. The remaining seven species had in addition to rods a single type of cone photoreceptor sensitive to green and, seeing only in shades of grey and green, are believed to be effectively colorblind. The study indicates that an object’s contrast against the background, rather than colour, may be more important for object detection.[50] [51][52]

Hearing

Although it is hard to test the hearing of sharks, they may have a sharp sense of hearing and can possibly hear prey from many miles away.[53] A small opening on each side of their heads (not the spiracle) leads directly into the inner ear through a thin channel. The lateral line shows a similar arrangement, and is open to the environment via a series of openings called lateral line pores. This is a reminder of the common origin of these two vibration- and sound-detecting organs that are grouped together as the acoustico-lateralis system. In bony fish and tetrapods the external opening into the inner ear has been lost.

Drawing of shark head.

Electromagnetic field receptors (ampullae of Lorenzini) and motion detecting canals in the head of a shark

Electroreception

Main article: Electroreception

The ampullae of Lorenzini are the electroreceptor organs. They number in the hundreds to thousands. Sharks use the ampullae of Lorenzini to detect the electromagnetic fields that all living things produce.[54] This helps sharks (particularly the hammerhead shark) find prey. The shark has the greatest electrical sensitivity of any animal. Sharks find prey hidden in sand by detecting the electric fields they produce. Ocean currents moving in the magnetic field of the Earth also generate electric fields that sharks can use for orientation and possibly navigation.[55]

Lateral line

Main article: Lateral line

This system is found in most fish, including sharks. It is a tactile sensory system which allows the organism to detect water speed and pressure changes near by.[56] The main component of the system is the neuromast, a cell similar to hair cells present in the vertebrate ear that interact with the surrounding aquatic environment. This helps sharks distinguish between the currents around them, obstacles off on their periphery, and struggling prey out of visual view. The shark can sense frequencies in the range of 25 to 50 Hz.[57]

Life history

Photo showing claspers of bottom-resting shark.

The claspers of male spotted wobbegong

Shark egg

Photo of 12 centimetres (4.7 in) egg case adjacent to ruler, the egg case is a brown ovalish shape, with a spiral band running around it from top to bottom.

The spiral egg case of a Port Jackson shark

Shark lifespans vary by species. Most live 20 to 30 years. The spiny dogfish has one of the longest lifespans at more than 100 years.[58] Whale sharks(Rhincodon typus) may also live over 100 years.[59] Earlier estimates suggested the Greenland shark (Somniosus microcephalus) could reach about 200 years, but a recent study found that a 5.02-metre-long (16.5 ft) specimen was 392 ± 120 years old (i.e., at least 272 years old), making it the longest-livedvertebrate known.[60][61]

Reproduction

Unlike most bony fish, sharks are K-selected reproducers, meaning that they produce a small number of well-developed young as opposed to a large number of poorly developed young. Fecundity in sharks ranges from 2 to over 100 young per reproductive cycle.[62] Sharks mature slowly relative to many other fish. For example, lemon sharks reach sexual maturity at around age 13–15.[63]

Sexual

Sharks practice internal fertilization.[64] The posterior part of a male shark’s pelvic fins are modified into a pair of intromittent organs called claspers, analogous to a mammalian penis, of which one is used to deliver sperm into the female.[65]

Mating has rarely been observed in sharks.[66] The smaller catsharks often mate with the male curling around the female. In less flexible species the two sharks swim parallel to each other while the male inserts a clasper into the female’s oviduct. Females in many of the larger species have bite marks that appear to be a result of a male grasping them to maintain position during mating. The bite marks may also come from courtship behavior: the male may bite the female to show his interest. In some species, females have evolved thicker skin to withstand these bites.[65]

Asexual

There have been a number of documented cases in which a female shark who has not been in contact with a male has conceived a pup on her own through parthenogenesis.[67][68] The details of this process are not well understood, but genetic fingerprinting showed that the pups had no paternal genetic contribution, ruling out sperm storage. The extent of this behavior in the wild is unknown. Mammals are now the only major vertebrate group in which asexual reproduction has not been observed.

Scientists say that asexual reproduction in the wild is rare, and probably a last-ditch effort to reproduce when a mate is not present. Asexual reproduction diminishes genetic diversity, which helps build defenses against threats to the species. Species that rely solely on it risk extinction. Asexual reproduction may have contributed to the blue shark‘s decline off the Irish coast.[69]

Brooding

Sharks display three ways to bear their young, varying by species, oviparityviviparity and ovoviviparity.[70][71]

Ovoviviparity

Most sharks are ovoviviparous, meaning that the eggs hatch in the oviduct within the mother’s body and that the egg’s yolk and fluids secreted by glands in the walls of the oviduct nourishes the embryos. The young continue to be nourished by the remnants of the yolk and the oviduct’s fluids. As in viviparity, the young are born alive and fully functional. Lamniforme sharks practice oophagy, where the first embryos to hatch eat the remaining eggs. Taking this a step further, sand tiger shark pups cannibalistically consume neighboring embryos. The survival strategy for ovoviviparous species is to brood the young to a comparatively large size before birth. The whale shark is now classified as ovoviviparous rather than oviparous, because extrauterine eggs are now thought to have been aborted. Most ovoviviparous sharks give birth in sheltered areas, including bays, river mouths and shallow reefs. They choose such areas for protection from predators (mainly other sharks) and the abundance of food. Dogfish have the longest known gestation period of any shark, at 18 to 24 months. Basking sharks and frilled sharks appear to have even longer gestation periods, but accurate data are lacking.[70]

Oviparity

Some species are oviparous, laying their fertilized eggs in the water. In most oviparous shark species, an egg case with the consistency of leather protects the developing embryo(s). These cases may be corkscrewed into crevices for protection. The egg case is commonly called a mermaid’s purse. Oviparous sharks include the horn sharkcatsharkPort Jackson shark, and swellshark.[70][72]

Viviparity

Viviparity is the gestation of young without the use of a traditional egg, and results in live birth.[73] Viviparity in sharks can be placental or aplacental.[73] Young are born fully formed and self-sufficient.[73] Hammerheads, the requiem sharks (such as the bull and blue sharks), and smoothhounds are viviparous.[62][70]

Behavior

The classic view describes a solitary hunter, ranging the oceans in search of food. However, this applies to only a few species. Most live far more social, sedentary, benthic lives, and appear likely to have their own distinct personalities.[74] Even solitary sharks meet for breeding or at rich hunting grounds, which may lead them to cover thousands of miles in a year.[75] Shark migration patterns may be even more complex than in birds, with many sharks covering entire ocean basins.

Sharks can be highly social, remaining in large schools. Sometimes more than 100 scalloped hammerheads congregate around seamounts and islands, e.g., in the Gulf of California.[35] Cross-species social hierarchies exist. For example, oceanic whitetip sharks dominate silky sharks of comparable size during feeding.[62]

When approached too closely some sharks perform a threat display. This usually consists of exaggerated swimming movements, and can vary in intensity according to the threat level.[76]

Speed

In general, sharks swim (“cruise”) at an average speed of 8 kilometres per hour (5.0 mph), but when feeding or attacking, the average shark can reach speeds upwards of 19 kilometres per hour (12 mph). The shortfin mako shark, the fastest shark and one of the fastest fish, can burst at speeds up to 50 kilometres per hour (31 mph).[77] The great white shark is also capable of speed bursts. These exceptions may be due to the warm-blooded, or homeothermic, nature of these sharks’ physiology. Sharks can travel 70 to 80 km in a day.[78]

Intelligence

Sharks possess brain-to-body mass ratios that are similar to mammals and birds,[79] and have exhibited apparent curiosity and behavior resembling play in the wild.[80][81]

There is evidence that juvenile lemon sharks can use observational learning in their investigation of novel objects in their environment.[82]

Sleep

All sharks need to keep water flowing over their gills in order for them to breathe; however, not all species need to be moving to do this. Those that are able to breathe while not swimming do so by using their spiracles to force water over their gills, thereby allowing them to extract oxygen from the water. It has been recorded that their eyes remain open while in this state and actively follow the movements of divers swimming around them[83] and as such they are not truly asleep.

Species that do need to swim continuously to breathe go through a process known as sleep swimming, in which the shark is essentially unconscious. It is known from experiments conducted on the spiny dogfish that its spinal cord, rather than its brain, coordinates swimming, so spiny dogfish can continue to swim while sleeping, and this also may be the case in larger shark species.[83]

Ecology

Feeding

This section is about shark feeding. For the sport of shark feeding, see Shark baiting.

Most sharks are carnivorous.[84] Basking sharkswhale sharks, and megamouth sharks have independently evolved different strategies for filter feeding plankton: basking sharks practice ram feeding, whale sharks use suction to take in plankton and small fishes, and megamouth sharks make suction feeding more efficient by using the luminescent tissue inside of their mouths to attract prey in the deep ocean. This type of feeding requires gill rakers—long, slender filaments that form a very efficient sieve—analogous to the baleen plates of the great whales. The shark traps the plankton in these filaments and swallows from time to time in huge mouthfuls. Teeth in these species are comparatively small because they are not needed for feeding.[84]

Photo of great white on surface with open jaws revealing meal.

Unlike many other sharks, the great white shark is not actually an apex predator in all of its natural environments, as it is sometimes hunted by orcas

Other highly specialized feeders include cookiecutter sharks, which feed on flesh sliced out of other larger fish and marine mammals. Cookiecutter teeth are enormous compared to the animal’s size. The lower teeth are particularly sharp. Although they have never been observed feeding, they are believed to latch onto their prey and use their thick lips to make a seal, twisting their bodies to rip off flesh.[35]

Some seabed–dwelling species are highly effective ambush predators. Angel sharks and wobbegongs use camouflage to lie in wait and suck prey into their mouths.[85] Many benthic sharks feed solely on crustaceans which they crush with their flat molariform teeth.

Other sharks feed on squid or fish, which they swallow whole. The viper dogfish has teeth it can point outwards to strike and capture prey that it then swallows intact. The great white and other large predators either swallow small prey whole or take huge bites out of large animals. Thresher sharks use their long tails to stun shoaling fishes, and sawsharks either stir prey from the seabed or slash at swimming prey with their tooth-studded rostra.

Many sharks, including the whitetip reef shark are cooperative feeders and hunt in packs to herd and capture elusive prey. These social sharks are often migratory, traveling huge distances around ocean basins in large schools. These migrations may be partly necessary to find new food sources.[86]

Range and habitat

Sharks are found in all seas. They generally do not live in fresh water, with a few exceptions such as the bull shark and the river shark which can swim both in seawater and freshwater.[87] Sharks are common down to depths of 2,000 metres (7,000 ft), and some live even deeper, but they are almost entirely absent below 3,000 metres (10,000 ft). The deepest confirmed report of a shark is a Portuguese dogfish at 3,700 metres (12,100 ft).[88]

Relationship with humans

Attacks

Photo of sign.

A sign warning about the presence of sharks in Salt RockSouth Africa

Photo of snorkeler with shark in shallow water.

Snorkeler swims near blacktip reef shark. In rare circumstances involving poor visibility, blacktips may bite a human, mistaking it for prey. Under normal conditions they are harmless and shy.Main article: Shark attack

In 2006 the International Shark Attack File (ISAF) undertook an investigation into 96 alleged shark attacks, confirming 62 of them as unprovoked attacks and 16 as provoked attacks. The average number of fatalities worldwide per year between 2001 and 2006 from unprovoked shark attacks is 4.3.[89]

Contrary to popular belief, only a few sharks are dangerous to humans. Out of more than 470 species, only four have been involved in a significant number of fatal, unprovoked attacks on humans: the great whiteoceanic whitetiptiger, and bull sharks.[90][91] These sharks are large, powerful predators, and may sometimes attack and kill people. Despite being responsible for attacks on humans they have all been filmed without using a protective cage.[92]

The perception of sharks as dangerous animals has been popularized by publicity given to a few isolated unprovoked attacks, such as the Jersey Shore shark attacks of 1916, and through popular fictional works about shark attacks, such as the Jaws film series. Jaws author Peter Benchley, as well as Jaws director Steven Spielberg, later attempted to dispel the image of sharks as man-eating monsters.[93]

To help avoid an unprovoked attack, humans should not wear jewelry or metal that is shiny and refrain from splashing around too much.[94]

In captivity

Main article: Sharks in captivity

Until recently, only a few benthic species of shark, such as hornsharksleopard sharks and catsharks, had survived in aquarium conditions for a year or more. This gave rise to the belief that sharks, as well as being difficult to capture and transport, were difficult to care for. More knowledge has led to more species (including the large pelagic sharks) living far longer in captivity, along with safer transportation techniques that have enabled long distance transportation.[95] For a long time[when?], the great white shark had never been successfully held in captivity for long periods of time, but in September 2004, the Monterey Bay Aquarium successfully kept a young female for 198 days before releasing her.

Photo showing visitors in shadow watching whale shark in front of many other fish.

whale shark in Georgia Aquarium

Most species are not suitable for home aquaria, and not every species sold by pet stores are appropriate. Some species can flourish in home saltwater aquaria.[96] Uninformed or unscrupulous dealers sometimes sell juvenile sharks like the nurse shark, which upon reaching adulthood is far too large for typical home aquaria.[96] Public aquaria generally do not accept donated specimens that have outgrown their housing. Some owners have been tempted to release them.[96] Species appropriate to home aquaria represent considerable spatial and financial investments as they generally approach adult lengths of 3 feet (90 cm) and can live up to 25 years.[96]

In culture

In Hawaii

Sharks figure prominently in Hawaiian mythology. Stories tell of men with shark jaws on their back who could change between shark and human form. A common theme was that a shark-man would warn beach-goers of sharks in the waters. The beach-goers would laugh and ignore the warnings and get eaten by the shark-man who warned them. Hawaiian mythology also includes many shark gods. Among a fishing people, the most popular of all aumakua, or deified ancestor guardians, are shark aumakua. Kamaku describes in detail how to offer a corpse to become a shark. The body transforms gradually until the kahuna can point the awe-struck family to the markings on the shark’s body that correspond to the clothing in which the beloved’s body had been wrapped. Such a shark aumakua becomes the family pet, receiving food, and driving fish into the family net and warding off danger. Like all aumakua it had evil uses such as helping kill enemies. The ruling chiefs typically forbade such sorcery. Many Native Hawaiian families claim such an aumakua, who is known by name to the whole community.[97]

Kamohoali’i is the best known and revered of the shark gods, he was the older and favored brother of Pele,[98] and helped and journeyed with her to Hawaii. He was able to assume all human and fish forms. A summit cliff on the crater of Kilauea is one of his most sacred spots. At one point he had a heiau (temple or shrine) dedicated to him on every piece of land that jutted into the ocean on the island of Molokai. Kamohoali’i was an ancestral god, not a human who became a shark and banned the eating of humans after eating one herself.[99][100] In Fijian mythology, Dakuwaqa was a shark god who was the eater of lost souls.

In American Samoa

On the island of Tutuila in American Samoa (a U.S. territory), there is a location called Turtle and Shark (Laumei ma Malie) which is important in Samoan culture — the location is the site of a legend called O Le Tala I Le Laumei Ma Le Malie, in which two humans are said to have transformed into a turtle and a shark.[101][102][103] According to the U.S. National Park Service, “Villagers from nearby Vaitogi continue to reenact an important aspect of the legend at Turtle and Shark by performing a ritual song intended to summon the legendary animals to the ocean surface, and visitors are frequently amazed to see one or both of these creatures emerge from the sea in apparent response to this call.”[101]

In popular culture

Main article: Sharks in popular culture

In contrast to the complex portrayals by Hawaiians and other Pacific Islanders, the European and Western view of sharks has historically been mostly of fear and malevolence.[104] Sharks are used in popular culture commonly as eating machines, notably in the Jaws novel and the film of the same name, along with its sequels.[105] Sharks are threats in other films such as Deep Blue SeaThe Reef, and others, although they are sometimes used for comedic effect such as in Finding Nemo and the Austin Powers series. These comedic effects can sometimes be unintentional, as seen in Batman: The Movie and various Syfy channel films like Dinoshark and Sharktopus.

Sharks tend to be seen quite often in cartoons whenever a scene involves the ocean. Such examples include the Tom and Jerry cartoons, Jabberjaw, and other shows produced by Hanna-Barbera. They also are used commonly as a clichéd means of killing off a character that is held up by a rope or some similar object as the sharks swim right below them, or the character may be standing on a plank above shark infested waters.

Popular misconceptions

A popular myth is that sharks are immune to disease and cancer, but this is not scientifically supported. Sharks have been known to get cancer.[106][107] Both diseases and parasites affect sharks. The evidence that sharks are at least resistant to cancer and disease is mostly anecdotal and there have been few, if any, scientific or statistical studies that show sharks to have heightened immunity to disease.[108] Other apparently false claims are that fins prevent cancer[109] and treat osteoarthritis.[110] No scientific proof supports these claims; at least one study has shown shark cartilage of no value in cancer treatment.[111]

Threats to sharks

Further information: List of threatened sharks and Shark sanctuary

Photo of shark fin soup in bowl with Chinese spoon

The value of shark fins for shark fin soup has led to an increase in shark catches. Usually only the fins are taken, while the rest of the shark is discarded, usually into the sea.

Graph of shark catch from 1950, linear growth from less than 200,000 tons per year in 1950 to about 500,000 in 2011

The annual shark catch has increased rapidly over the last 60 years.

Photo of suspended tiger shark next to four men.

A 14-foot (4.3 m), 1,200-pound (540 kg) tiger sharkcaught in Kāne’ohe BayOahu in 1966

Fishery

It is estimated that 100 million sharks are killed by people every year, due to commercial and recreational fishing.[112][113] Shark finning yields are estimated at 1.44 million metric tons for 2000, and 1.41 million tons for 2010. Based on an analysis of average shark weights, this translates into a total annual mortality estimate of about 100 million sharks in 2000, and about 97 million sharks in 2010, with a total range of possible values between 63 and 273 million sharks per year.[114][115] Sharks are a common seafood in many places, including Japan and Australia. In the Australian state of Victoria, shark is the most commonly used fish in fish and chips,[citation needed] in which fillets are battered and deep-fried or crumbed and grilled. In fish and chip shops, shark is called flake. In India, small sharks or baby sharks (called sora in Tamil languageTelugu language) are sold in local markets. Since the flesh is not developed, cooking the flesh breaks it into powder, which is then fried in oil and spices (called sora puttu/sora poratu). The soft bones can be easily chewed. They are considered a delicacy in coastal Tamil NaduIcelanders ferment Greenland sharks to produce a delicacy called hákarl.[116]During a four-year period from 1996 to 2000, an estimated 26 to 73 million sharks were killed and traded annually in commercial markets.[117]

Sharks are often killed for shark fin soup. Fishermen capture live sharks, fin them, and dump the finless animal back into the water. Shark finning involves removing the fin with a hot metal blade.[113] The resulting immobile shark soon dies from suffocation or predators.[118] Shark fin has become a major trade within black markets all over the world. Fins sell for about $300/lb in 2009.[119] Poachers illegally fin millions each year. Few governments enforce laws that protect them.[115] In 2010 Hawaii became the first U.S. state to prohibit the possession, sale, trade or distribution of shark fins.[120]From 1996 to 2000, an estimated 38 million sharks had been killed per year for harvesting shark fins.[117] It is estimated by TRAFFIC that over 14,000 tonnes of shark fins were exported into Singapore between 2005–2007 and 2012–2014.[121]

Shark fin soup is a status symbol in Asian countries, and is erroneously considered healthy and full of nutrients. Sharks are also killed for meat. European diners consume dogfishessmoothhoundscatsharks, makos, porbeagle and also skates and rays.[122] However, the U.S. FDA lists sharks as one of four fish (with swordfishking mackerel, and tilefish) whose high mercury content is hazardous to children and pregnant women.

Sharks generally reach sexual maturity only after many years and produce few offspring in comparison to other harvested fish. Harvesting sharks before they reproduce severely impacts future populations. Capture induced premature birth and abortion (collectively called capture-induced parturition) occurs frequently in sharks/rays when fished.[64] Capture-induced parturition is rarely considered in fisheries management despite being shown to occur in at least 12% of live bearing sharks and rays (88 species to date).[64]

The majority of shark fisheries have little monitoring or management. The rise in demand for shark products increases pressure on fisheries.[36] Major declines in shark stocks have been recorded—some species have been depleted by over 90% over the past 20–30 years with population declines of 70% not unusual.[123] A study by the International Union for Conservation of Nature suggests that one quarter of all known species of sharks and rays are threatened by extinction and 25 species were classified as critically endangered.[124][125]

Shark culling

Main article: Shark culling

In 2014, a shark cull in Western Australia killed dozens of sharks (mostly tiger sharks) using drum lines,[126] until it was cancelled after public protests and a decision by the Western Australia EPA; from 2014 to 2017, there was an “imminent threat” policy in Western Australia in which sharks that “threatened” humans in the ocean were shot and killed.[127] This “imminent threat” policy was criticized by senator Rachel Siewart for killing endangered sharks.[128] The “imminent threat” policy was cancelled in March 2017.[129] In August 2018, the Western Australia government announced a plan to re-introduce drum lines (though, this time the drum lines are “SMART” drum lines).[130]

From 1962 to the present,[131] the government of Queensland has targeted and killed sharks in large numbers by using drum lines, under a “shark control” program—this program has also inadvertently killed large numbers of other animals such as dolphins; it has also killed endangered hammerhead sharks.[132][133][134][135] Queensland’s drum line program has been called “outdated, cruel and ineffective”.[135] From 2001 to 2018, a total of 10,480 sharks were killed on lethal drum lines in Queensland, including in the Great Barrier Reef.[136] From 1962 to 2018, roughly 50,000 sharks were killed by Queensland authorities.[137]

The government of New South Wales has a program that deliberately kills sharks using nets.[134][138] The current net program in New South Wales has been described as being “extremely destructive” to marine life, including sharks.[139] Between 1950 and 2008, 352 tiger sharks and 577 great white sharks were killed in the nets in New South Wales — also during this period, a total of 15,135 marine animals were killed in the nets, including dolphins, whales, turtles, dugongs, and critically endangered grey nurse sharks.[140] There has been a very large decrease in the number of sharks in eastern Australia, and the shark-killing programs in Queensland and New South Wales are partly responsible for this decrease.[137]

Kwazulu-Natal, an area of South Africa, has a shark-killing program using nets and drum lines—these nets and drum lines have killed turtles and dolphins, and have been criticized for killing wildlife.[141] During a 30-year period, more than 33,000 sharks have been killed in KwaZulu-Natal’s shark-killing program — during the same 30-year period, 2,211 turtles, 8,448 rays, and 2,310 dolphins were killed in KwaZulu-Natal.[141] Authorities on the French island of Réunion kill about 100 sharks per year.[142]

Killing sharks negatively affects the marine ecosystem.[143][144] Jessica Morris of Humane Society International calls shark culling a “knee-jerk reaction” and says, “sharks are top order predators that play an important role in the functioning of marine ecosystems. We need them for healthy oceans.”[145]

George H. Burgess, the former[146] director of the International Shark Attack File, “describes [shark] culling as a form of revenge, satisfying a public demand for blood and little else”;[147] he also said shark culling is a “retro-type move reminiscent of what people would have done in the 1940s and 50s, back when we didn’t have an ecological conscience and before we knew the consequences of our actions.”[147] Jane Williamson, an associate professor in marine ecology at Macquarie University, says “There is no scientific support for the concept that culling sharks in a particular area will lead to a decrease in shark attacks and increase ocean safety.”[148]

Other threats

Other threats include habitat alteration, damage and loss from coastal development, pollution and the impact of fisheries on the seabed and prey species.[149] The 2007 documentary Sharkwaterexposed how sharks are being hunted to extinction.[150]

Conservation

Further information: List of threatened sharks and Shark sanctuary

In 1991, South Africa was the first country in the world to declare Great White sharks a legally protected species[151] (however, the KwaZulu-Natal Sharks Board is allowed to kill great white sharks in its “shark control” program in eastern South Africa).[141]

Intending to ban the practice of shark finning while at sea, the United States Congress passed the Shark Finning Prohibition Act in 2000.[152] Two years later the Act saw its first legal challenge in United States v. Approximately 64,695 Pounds of Shark Fins. In 2008 a Federal Appeals Court ruled that a loophole in the law allowed non-fishing vessels to purchase shark fins from fishing vessels while on the high seas.[153] Seeking to close the loophole, the Shark Conservation Act was passed by Congress in December 2010, and it was signed into law in January 2011.[154][155]

In 2003, the European Union introduced a general shark finning ban for all vessels of all nationalities in Union waters and for all vessels flying a flag of one of its member states.[156] This prohibition was amended in June 2013 to close remaining loopholes.[157]

In 2009, the International Union for Conservation of Nature‘s IUCN Red List of Endangered Species named 64 species, one-third of all oceanic shark species, as being at risk of extinction due to fishing and shark finning.[158][159]

In 2010, the Convention on International Trade in Endangered Species (CITES) rejected proposals from the United States and Palau that would have required countries to strictly regulate trade in several species of scalloped hammerheadoceanic whitetip and spiny dogfish sharks. The majority, but not the required two-thirds of voting delegates, approved the proposal. China, by far the world’s largest shark market, and Japan, which battles all attempts to extend the convention to marine species, led the opposition.[160][161] In March 2013, three endangered commercially valuable sharks, the hammerheads, the oceanic whitetip and porbeagle were added to Appendix 2 of CITES, bringing shark fishing and commerce of these species under licensing and regulation.[162]

In 2010, Greenpeace International added the school sharkshortfin mako sharkmackerel sharktiger shark and spiny dogfish to its seafood red list, a list of common supermarket fish that are often sourced from unsustainable fisheries.[163] Advocacy group Shark Trust campaigns to limit shark fishing. Advocacy group Seafood Watch directs American consumers to not eat sharks.[164]

Under the auspices of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), also known as the Bonn Convention, the Memorandum of Understanding on the Conservation of Migratory Sharks was concluded and came into effect in March 2010. It was the first global instrument concluded under CMS and aims at facilitating international coordination for the protection, conservation and management of migratory sharks, through multilateral, intergovernmental discussion and scientific research.

In July 2013, New York state, a major market and entry point for shark fins, banned the shark fin trade joining seven other states of the United States and the three Pacific U.S territories in providing legal protection to sharks.[165]

As of September 2018, 12 U.S. states (CaliforniaDelawareHawaiiIllinoisMarylandMassachusettsNevadaNew YorkOregonRhode IslandTexas, and Washington) and 3 U.S. territories(American SamoaGuam and the Northern Mariana Islands) have banned the sale or possession of shark fins.[166]

Several regions now have shark sanctuaries or have banned shark fishing — these regions include American Samoa, the Bahamas, the Cook IslandsFrench PolynesiaGuam, the Maldives, the Marshall IslandsMicronesia, the Northern Mariana Islands, and Palau.[167][168]

Cell:The Functional and Structural Unit

As we know that our body is made up billions and trillions of cells.Hence, we can say that cell is the structural and functional unit of all living things.

Cell Theory:In 1665 a scientist named Robert Hooke discovered the first cell in the dead bark of the cork. Cell means a small room.

All living beings can be differed into 2 types according to their number of cell.They are unicellular and multicellular.Unicellular organisms are of single cell . But multicellular organisms are of many cell.For example: amoeba is a unicellular organism and human beings are multicellular organism.

Each cell is of different shape.Each cell is of different shape and size due to their function and location. For example : nerve cell is the longest cell in human body because it has to transport blood to different parts of body. Red Blood Cell is the smallest cell in human body.

PPLO is the smallest cell which is about 0.1 micron. It’s full form is pleuro pneumonia like organism. Ostrich’s egg is the biggest cell.

Cell consist of cytoplasm, cell organelles, nucleus and cell membrane.

Cytoplasm is clear and it consist of the all the organelles outside the nucleus.Its doesn’t consist of nucleus. Cell is also having protoplasm which consist of the living part of the cell.It include the nucleus.

Cell membrane is outside the layer of the cell.It is selectively permiable in nature as it allows the entry and exit of all the cell oraganelles.

Festival

Image result for festival
festival is an event ordinarily celebrated by a community and centering on some characteristic aspect of that community and its religion or cultures. It is often marked as a local or national holidaymela, or eid. Next to religion and folklore, a significant origin is agricultural. Food is such a vital resource that many festivals are associated with harvest time. Religious commemoration and thanksgiving for good harvests are blended in events that take place in autumn, such as Halloween in the northern hemisphere and Easter in the southern.

Festivals often serve to fulfill specific communal purposes, especially in regard to commemoration or thanking to the gods and goddesses. Celebrations offer a sense of belonging for religious, social, or geographical groups, contributing to group cohesiveness. They may also provide entertainment, which was particularly important to local communities before the advent of mass-produced entertainment. Festivals that focus on cultural or ethnic topics also seek to inform community members of their traditions; the involvement of elders sharing stories and experience provides a means for unity among families.

In Ancient Greece and Rome, festivals such as the Saturnalia were closely associated with social organisation and political processes as well as religion.[1][2][3] In modern times, festivals may be attended by strangers such as tourists, who are attracted to some of the more eccentric or historical ones. The Philippines is one example of a modern society with a large number of festivals, as each day of the year has at least one specific celebration. There are more than 42,000 known major and minor festivals in the country, the majority of which are specific to the barangay (village) level.[4]

Etymology[edit]

A Festival at Antwerp, 17th century

Country Festival in Swabia

The word “festival” was originally used as an adjective from the late fourteenth century, deriving from Latin via Old French.[5] In Middle English, a “festival dai” was a religious holiday.[6] Its first recorded used as a noun was in 1589 (as “Festifall”).[5] Feast first came into usage as a noun circa 1200,[7] and its first recorded use as a verb was circa 1300.[8] The term “feast” is also used in common secular parlance as a synonym for any large or elaborate meal. When used as in the meaning of a festival, most often refers to a religious festival rather than a film or art festival. In the Philippines and many other former Spanish colonies, the Spanish word fiesta is used to denote a communal religious feast to honor a patron saint.[citation needed]

Traditions[edit]

Many festivals have religious origins and entwine cultural and religious significance in traditional activities. The most important religious festivals such as ChristmasRosh HashanahDiwali, and Eid al-Adha serve to mark out the year. Others, such as harvest festivals, celebrate seasonal change. Events of historical significance, such as important military victories or other nation-building events also provide the impetus for a festival. An early example is the festival established by Ancient Egyptian Pharaoh Ramesses III celebrating his victory over the Libyans.[9] In many countries, royal holidays commemorate dynastic events just as agricultural holidays are about harvests. Festivals are often commemorated annually.

There are numerous types of festivals in the world and most countries celebrate important events or traditions with traditional cultural events and activities. Most culminate in the consumption of specially prepared food (showing the connection to “feasting”) and they bring people together. Festivals are also strongly associated with national holidays. Lists of national festivals are published to make participation easier.[10]

Types of festivals[edit]

Religious festivals[edit]

Main article: Religious festival

Among many religions, a feast is a set of celebrations in honour of Gods or God.[11] A feast and a festival are historically interchangeable. Most religions have festivals that recur annually and some, such as Passover, Easter and Eid al-Adha are moveable feasts – that is, those that are determined either by lunar or agricultural cycles or the calendar in use at the time. The Sed festival, for example, celebrated the thirtieth year of an Egyptian pharaoh‘s rule and then every three (or four in one case) years after that.[12]

In the Christian liturgical calendar, there are two principal feasts, properly known as the Feast of the Nativity of our Lord (Christmas) and the Feast of the Resurrection, (Easter). In the CatholicEastern Orthodox, and Anglicanliturgical calendars there are a great number of lesser feasts throughout the year commemorating saints, sacred events or doctrines. In the Philippines, each day of the year has at least one specific religious festival, either from Catholic, Islamic, or indigenous origins.[citation needed]

Buddhist religious festivals, such as Esala Perahera are held in Sri Lanka and Thailand.[13] Hindu festivals, such as Holi are very ancient. The Sikh community celebrates the Vaisakhi festival marking the new year and birth of the Khalsa.[14]Religious festivals

Arts festivals[edit]

Main article: Arts festival

Among the many offspring of general arts festivals are also more specific types of festivals, including ones that showcase intellectual or creative achievement such as science festivalsliterary festivals and music festivals.[15] Sub-categories include comedy festivalsrock festivalsjazz festivals and buskers festivalspoetry festivals,[16] theatre festivals, and storytelling festivals; and re-enactment festivals such as Renaissance fairs. In the Philippines, aside from numerous art festivals scattered throughout the year, February is known as national arts month, the culmination of all art festivals in the entire archipelago.[17]

Film festivals involve the screenings of several different films, and are usually held annually. Some of the most significant film festivals include the Berlin International Film Festival, the Venice Film Festival and the Cannes Film Festival.Arts festivals

Food and drink festivals[edit]

Main article: Food festival

A food festival is an event celebrating food or drink. These often highlight the output of producers from a certain region. Some food festivals are focused on a particular item of food, such as the National Peanut Festival in the United States, or the Galway International Oyster Festival in Ireland. There are also specific beverage festivals, such as the famous Oktoberfest in Germany for beer. Many countries hold festivals to celebrate wine. One example is the global celebration of the arrival of Beaujolais nouveau, which involves shipping the new wine around the world for its release date on the third Thursday of November each year.[18][19] Both Beaujolais nouveau and the Japanese rice wine sake are associated with harvest time. In the Philippines, there are at least two hundred festivals dedicated to food and drinks.[citation needed]Food and drink festivals

  • Soweto Wine Festival, South Africa (2009)
  • Holi Nepal (2011)
  • La Tomatina, Spain (2010)
  • Beer horse cart from the Hofbräuhaus brewery at Oktoberfest Germany (2013)

Seasonal and harvest festivals[edit]

Seasonal festivals, such as Beltane, are determined by the solar and the lunar calendars and by the cycle of the seasons, especially because of its effect on food supply, as a result of which there is a wide range of ancient and modern harvest festivals. Ancient Egyptians relied upon the seasonal inundation caused by the Nile River, a form of irrigation, which provided fertile land for crops.[20] In the Alps, in autumn the return of the cattle from the mountain pastures to the stables in the valley is celebrated as Almabtrieb. A recognized winter festival, the Chinese New Year, is set by the lunar calendar, and celebrated from the day of the second new moon after the winter solsticeDree Festival of the Apatanis living in Lower Subansiri District of Arunachal Pradesh is celebrated every year from July 4 to 7 by praying for a bumper crop harvest.[21]

Midsummer or St John’s Day, is an example of a seasonal festival, related to the feast day of a Christian saint as well as a celebration of the time of the summer solstice in the northern hemisphere, where it is particularly important in Sweden. Winter carnivals also provide the opportunity to utilise to celebrate creative or sporting activities requiring snow and ice. In the Philippines, each day of the year has at least one festival dedicated to harvesting of crops, fishes, crustaceans, milk, and other local goods.[citation needed]Seasonal and harvest festivals

Study of festivals[edit]