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Guaranteed Fast Shipping!!!!!! Colors may have slight variations depending on monitor resolution settings, designs on front only.Tyrannosaurus[nb 1] is a genus of coelurosaurian theropod dinosaur. The species Tyrannosaurus rex (rex meaning "king" in Latin), is one of the most well-represented of the large theropods. Tyrannosaurus lived throughout what is now western North America, on what was then an island continent known as Laramidia. Tyrannosaurus had a much wider range than other tyrannosaurids. Fossils are found in a variety of rock formations dating to the Maastrichtian age of the upper Cretaceous Period, 68 to 66 million years ago.[2] It was the last known member of the tyrannosaurids,[3] and among the last non-avian dinosaurs to exist before the Cretaceous–Paleogene extinction.Like other tyrannosaurids, Tyrannosaurus was a bipedal carnivore with a massive skull balanced by a long, heavy tail. Relative to its large and powerful hind limbs, Tyrannosaurus fore limbs were short but unusually powerful for their size and had two clawed digits. The most complete specimen measures up to 12.3 m (40 ft) in length,[4] up to 3.66 meters (12 ft) tall at the hips,[5] and according to most modern estimates 8.4 metric tons (9.3 short tons) to 14 metric tons (15.4 short tons) in weight.[4][6][7] Although other theropods rivaled or exceeded Tyrannosaurus rex in size, it is still among the largest known land predators and is estimated to have exerted the largest bite force among all terrestrial animals.[8][9] By far the largest carnivore in its environment, Tyrannosaurus rex was most likely an apex predator, preying upon hadrosaurs, armoured herbivores like ceratopsians and ankylosaurs, and possibly sauropods.[10] Some experts have suggested the dinosaur was primarily a scavenger. The question of whether Tyrannosaurus was an apex predator or a pure scavenger was among the longest ongoing debates in paleontology.[11] It is accepted now that Tyrannosaurus rex acted as a predator, and opportunistically scavenged as modern mammalian and avian predators do.More than 50 specimens of Tyrannosaurus rex have been identified, some of which are nearly complete skeletons. Soft tissue and proteins have been reported in at least one of these specimens. The abundance of fossil material has allowed significant research into many aspects of its biology, including its life history and biomechanics. The feeding habits, physiology and potential speed of Tyrannosaurus rex are a few subjects of debate. Its taxonomy is also controversial, as some scientists consider Tarbosaurus bataar from Asia to be a second Tyrannosaurus species while others maintain Tarbosaurus is a separate genus. Several other genera of North American tyrannosaurids have also been synonymized with Tyrannosaurus.As the archetypal theropod, Tyrannosaurus is one of the best-known dinosaurs since the 20th century, and has been featured in film, advertising, and postal stamps, as well as many other types of media.Contents  [hide] 1 Description1.1 Skin and possible feathers2 History of research2.1 Earliest finds2.2 Manospondylus2.3 Notable specimens3 Classification4 Paleobiology4.1 Life history4.2 Sexual dimorphism4.3 Posture4.4 Arms4.5 Soft tissue4.6 Thermoregulation4.7 Footprints4.8 Locomotion4.9 Brain and senses4.10 Feeding strategies4.10.1 Cannibalism4.10.2 Pack behavior4.11 Pathology5 Paleoecology6 In popular culture7 See also8 References9 Further reading10 External linksDescriptionSize (in green) compared with selected giant theropodsTyrannosaurus rex was one of the largest land carnivores of all time; the largest complete specimen, located at the Field Museum of Natural History under the name FMNH PR2081 and nicknamed Sue, measured 12.3 meters (40 ft) long,[4] and was 3.66 meters (12 ft) tall at the hips,[5] and according to the most recent studies estimated to have weighed between 8.4 metric tons (9.3 short tons) to 14 metric tons (15.4 short tons) when alive.[4][6][7] Not every adult Tyrannosaurus specimen recovered is as big. Historically average adult mass estimates have varied widely over the years, from as low as 4.5 metric tons (5.0 short tons),[12][13] to more than 7.2 metric tons (7.9 short tons),[14] with most modern estimates ranging between 5.4 metric tons (6.0 short tons) and 8.0 metric tons (8.8 short tons).[4][15][16][17][18] Hutchinson et al. (2011) found that the maximum weight of Sue, the largest complete Tyrannosaurus specimen, was between 9.5 and 18.5 metric tons (9.3–18.2 long tons; 10.5–20.4 short tons), though the authors stated that their upper and lower estimates were based on models with wide error bars and that they "consider [them] to be too skinny, too fat, or too disproportionate" and provided a mean estimate at 14 metric tons (15.4 short tons) for this specimen.[4] Packard et al. (2009) tested dinosaur mass estimation procedures on elephants and concluded that those of dinosaurs are flawed and produce over-estimations; thus, the weight of Tyrannosaurus, as well as other dinosaurs, could have been much less.[19] Other estimations have concluded that the largest known Tyrannosaurus specimens had masses approaching[6] or exceeding 9 tonnes.[4][7]Life restoration of T. rex with feathers, a trait inferred by phylogenetic bracketingDue to the relatively small number of recovered specimens and the large population of individuals present at any given time when Tyrannosaurus was alive, there could have easily been larger specimens than those currently known including "Sue", though discovery of these largest individuals may be forever untenable due to the incomplete nature of the fossil record.[20] Holtz has also suggested that "it is very reasonable to suspect that there were individuals that were 10, 15, or even 20 percent larger than Sue in any T. rex population."[21]The neck of Tyrannosaurus rex formed a natural S-shaped curve like that of other theropods, but was short and muscular to support the massive head. The forelimbs had only two clawed fingers,[22] along with an additional small metacarpal representing the remnant of a third digit.[23] In contrast the hind limbs were among the longest in proportion to body size of any theropod. The tail was heavy and long, sometimes containing over forty vertebrae, in order to balance the massive head and torso. To compensate for the immense bulk of the animal, many bones throughout the skeleton were hollow, reducing its weight without significant loss of strength.[22]Profile view of a skull (AMNH 5027)The largest known Tyrannosaurus rex skull measures up to 1.52 meters (5.0 ft) in length.[5] Large fenestrae (openings) in the skull reduced weight and provided areas for muscle attachment, as in all carnivorous theropods. But in other respects Tyrannosaurus's skull was significantly different from those of large non-tyrannosauroid theropods. It was extremely wide at the rear but had a narrow snout, allowing unusually good binocular vision.[24][25] The skull bones were massive and the nasals and some other bones were fused, preventing movement between them; but many were pneumatized (contained a "honeycomb" of tiny air spaces) which may have made the bones more flexible as well as lighter. These and other skull-strengthening features are part of the tyrannosaurid trend towards an increasingly powerful bite, which easily surpassed that of all non-tyrannosaurids.[8][9][26] The tip of the upper jaw was U-shaped (most non-tyrannosauroid carnivores had V-shaped upper jaws), which increased the amount of tissue and bone a tyrannosaur could rip out with one bite, although it also increased the stresses on the front teeth.[27][28]The teeth of Tyrannosaurus rex displayed marked heterodonty (differences in shape).[22][29] The premaxillary teeth at the front of the upper jaw were closely packed, D-shaped in cross-section, had reinforcing ridges on the rear surface, were incisiform (their tips were chisel-like blades) and curved backwards. The D-shaped cross-section, reinforcing ridges and backwards curve reduced the risk that the teeth would snap when Tyrannosaurus bit and pulled. The remaining teeth were robust, like "lethal bananas" rather than daggers, more widely spaced and also had reinforcing ridges.[30] Those in the upper jaw were larger than those in all but the rear of the lower jaw. The largest found so far is estimated to have been 30.5 centimeters (12 in) long including the root when the animal was alive, making it the largest tooth of any carnivorous dinosaur yet found.[31]Skin and possible feathersMain article: Feathered dinosaurHead model showing "traditional" naked skin and lipless jaws, Natural History Museum of ViennaWhile there is no direct evidence for Tyrannosaurus rex having had feathers, many scientists now consider it likely that T. rex had feathers on at least parts of its body,[32] due to their presence in related species. Mark Norell of the American Museum of Natural History summarized the balance of evidence by stating that: "we have as much evidence that T. rex was feathered, at least during some stage of its life, as we do that australopithecines like Lucy had hair."[33]The first evidence for feathers in tyrannosauroids came from the small species Dilong paradoxus, found in the Yixian Formation of China, and reported in 2004. As with many other theropods discovered in the Yixian, the fossil skeleton was preserved with a coat of filamentous structures which are commonly recognized as the precursors of feathers.[34] Because all known skin impressions from larger tyrannosauroids known at the time showed evidence of scales, the researchers who studied Dilong speculated that feathers may correlate negatively with body size—that juveniles may have been feathered, then shed the feathers and expressed only scales as the animal became larger and no longer needed insulation to stay warm.[34] Subsequent discoveries showed that even some large tyrannosauroids had feathers covering much of their bodies, casting doubt on the hypothesis that they were a size-related feature.[35]Full size model in Poland, depicting Tyrannosaurus with both feathers and scales, as well as lipped jawsWhile skin impressions from a Tyrannosaurus rex specimen nicknamed "Wyrex" (BHI 6230) discovered in Montana in 2002,[36] as well as some other giant tyrannosauroid specimens, show at least small patches of mosaic scales,[37] others, such as Yutyrannus huali (which was up to 9 meters (30 ft) long and weighed about 1,400 kilograms (3,100 lb)), preserve feathers on various sections of the body, strongly suggesting that its whole body was covered in feathers.[35] It is possible that the extent and nature of feather covering in tyrannosauroids may have changed over time in response to body size, a warmer climate, or other factors.[35] In 2017, based on skin impressions from the "Wyrex" (BHI 6230) specimen and other closely related tyrannosaurids, it was suggested that large-bodied tyrannosaurids were scaly and, if partly feathered, these were limited to the dorsum.[38]A study in 2016 proposed that large theropods like Tyrannosaurus had teeth covered in lips like extant lizards instead of bare teeth like crocodilians. This was based on the presence of enamel, which according to the study needs to remain hydrated, an issue not faced by aquatic animals like crocodilians or toothless animals like birds.[39][40]Based on comparisons of bone texture of Daspletosaurus with extant crocodilians, a study in 2017 by Thomas D. Carr et al. found that tyrannosaurs had large, flat scales that left no room for lips. They also found that at the center of these scales were small keratinised patches. In crocodilians, such patches cover bundles of sensory neurons that can detect vibrations and temperature and chemical stimulus.[41] They proposed that the facial scales of tyrannosaurs also covered bundles of sensory neurons and may have used them to identify objects, measure the temperature of their nests and pick-up eggs and hatchlings.[42]History of researchMain article: Timeline of tyrannosaur researchSkeletal restoration by William D. Matthew from 1905, the first reconstruction of this dinosaur ever published[43]Henry Fairfield Osborn, president of the American Museum of Natural History, named Tyrannosaurus rex in 1905. The generic name is derived from the Greek words τυράννος (tyrannos, meaning "tyrant") and σαύρος (sauros, meaning "lizard"). Osborn used the Latin word rex, meaning "king", for the specific name. The full binomial therefore translates to "tyrant lizard the king" or "King Tyrant Lizard",[44] emphasizing the animal's size and perceived dominance over other species of the time.[45]Earliest findsType specimen of Dynamosaurus imperiosusTeeth from what is now documented as a Tyrannosaurus rex were found in 1874 by Arthur Lakes near Golden, Colorado. In the early 1890s, John Bell Hatcher collected postcranial elements in eastern Wyoming. The fossils were believed to be from a large species of Ornithomimus (O. grandis) but are now considered Tyrannosaurus rex remains. Vertebral fragments found by Edward Drinker Cope in western South Dakota in 1892 and assigned to Manospondylus gigas have also been recognized as belonging to Tyrannosaurus rex.[46]Barnum Brown, assistant curator of the American Museum of Natural History, found the first partial skeleton of Tyrannosaurus rex in eastern Wyoming in 1900. H. F. Osborn originally named this skeleton Dynamosaurus imperiosus in a paper in 1905. Brown found another partial skeleton in the Hell Creek Formation in Montana in 1902. Osborn used this holotype to describe Tyrannosaurus rex in the same paper in which D. imperiosus was described.[45] In 1906, Osborn recognized the two as synonyms, and acted as first revisor by selecting Tyrannosaurus as the valid name.[47] The original Dynamosaurus material resides in the collections of the Natural History Museum, London.[48]In total, Brown found five Tyrannosaurus partial skeletons. In 1941, Brown's 1902 find was sold to the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. Brown's fourth and largest find, also from Hell Creek, is on display in the American Museum of Natural History in New York.[49][page needed]ManospondylusIllustration of the type specimen (AMNH 3982) of Manospondylus gigasThe first named fossil specimen which can be attributed to Tyrannosaurus rex consists of two partial vertebrae (one of which has been lost) found by Edward Drinker Cope in 1892. Cope believed that they belonged to an "agathaumid" (ceratopsid) dinosaur, and named them Manospondylus gigas, meaning "giant porous vertebra" in reference to the numerous openings for blood vessels he found in the bone.[46] The M. gigas remains were later identified as those of a theropod rather than a ceratopsid, and H.F. Osborn recognized the similarity between M. gigas and Tyrannosaurus rex as early as 1917. Owing to the fragmentary nature of the Manospondylus vertebrae, Osborn did not synonymize the two genera.[50]In June 2000, the Black Hills Institute located the type locality of M. gigas in South Dakota and unearthed more tyrannosaur bones there. These were judged to represent further remains of the same individual, and to be identical to those of Tyrannosaurus rex.[51] According to the rules of the International Code of Zoological Nomenclature (ICZN), the system that governs the scientific naming of animals, Manospondylus gigas should therefore have priority over Tyrannosaurus rex, because it was named first. The Fourth Edition of the ICZN, which took effect on January 1, 2000, states that "the prevailing usage must be maintained" when "the senior synonym or homonym has not been used as a valid name after 1899" and "the junior synonym or homonym has been used for a particular taxon, as its presumed valid name, in at least 25 works, published by at least 10 authors in the immediately preceding 50 years ..."[52] Tyrannosaurus rex may qualify as the valid name under these conditions and would most likely be considered a nomen protectum ("protected name") under the ICZN if it is ever formally published on, which it has not yet been. Manospondylus gigas could then be deemed a nomen oblitum ("forgotten name").[53]Notable specimensMain article: Specimens of TyrannosaurusSue specimen, Field Museum of Natural History, ChicagoSue Hendrickson, amateur paleontologist, discovered the most complete (approximately 85%) and the largest Tyrannosaurus fossil skeleton known in the Hell Creek Formation near Faith, South Dakota, on August 12, 1990. This Tyrannosaurus, nicknamed Sue in her honor, was the object of a legal battle over its ownership. In 1997 this was settled in favor of Maurice Williams, the original land owner. The fossil collection was purchased by the Field Museum of Natural History at auction for $7.6 million, making it the most expensive dinosaur skeleton to date. From 1998 to 1999 Field Museum of Natural History preparators spent over 25,000 man-hours taking the rock off each of the bones.[54] The bones were then shipped off to New Jersey where the mount was made. The finished mount was then taken apart, and along with the bones, shipped back to Chicago for the final assembly. The mounted skeleton opened to the public on May 17, 2000 in the great hall (Stanley Field Hall) at the Field Museum of Natural History. A study of this specimen's fossilized bones showed that Sue reached full size at age 19 and died at age 28, the longest any tyrannosaur is known to have lived.[55] Early speculation that Sue may have died from a bite to the back of the head was not confirmed. Though subsequent study showed many pathologies in the skeleton, no bite marks were found.[22][56] Damage to the back of the skull may have been caused by post-mortem trampling. Recent speculation indicates that Sue may have died of starvation after contracting a parasitic infection from eating diseased meat; the resulting infection would have caused inflammation in the throat, ultimately leading Sue to starve because she could no longer swallow food. This hypothesis is substantiated by smooth-edged holes in her skull which are similar to those caused in modern-day birds that contract the same parasite.[57]The specimens "Sue", AMNH 5027, "Stan", and "Jane", to scale with a human.Another Tyrannosaurus, nicknamed Stan, in honor of amateur paleontologist Stan Sacrison, was found in the Hell Creek Formation near Buffalo, South Dakota, in the spring of 1987. It was not collected until 1992, as it was mistakenly thought to be a Triceratops skeleton. Stan is 63% complete and is on display in the Black Hills Institute of Geological Research in Hill City, South Dakota, after an extensive world tour during 1995 and 1996.[36] This tyrannosaur, too, was found to have many bone pathologies, including broken and healed ribs, a broken (and healed) neck and a spectacular hole in the back of its head, about the size of a Tyrannosaurus tooth.[58]In the summer of 2000, Jack Horner discovered five Tyrannosaurus skeletons near the Fort Peck Reservoir in Montana. One of the specimens was reported to be perhaps the largest Tyrannosaurus ever found.[59]Skeleton of Bucky and cast of Stan, at the Children's Museum of IndianapolisIn 2001, a 50% complete skeleton of a juvenile Tyrannosaurus was discovered in the Hell Creek Formation in Montana, by a crew from the Burpee Museum of Natural History of Rockford, Illinois. Dubbed Jane, the find was initially considered the first known skeleton of the pygmy tyrannosaurid Nanotyrannus but subsequent research has revealed that it is more likely a juvenile Tyrannosaurus.[60] It is the most complete and best preserved juvenile example known to date. Jane has been examined by Jack Horner, Pete Larson, Robert Bakker, Greg Erickson, and several other renowned paleontologists, because of the uniqueness of her age. Jane is currently on exhibit at the Burpee Museum of Natural History in Rockford, Illinois.[61][62]In a press release on April 7, 2006, Bozeman Campus, Montana State University, US revealed that it possessed the largest Tyrannosaurus skull yet discovered. Discovered in the 1960s and only recently reconstructed, the skull measured 59 inches (150 cm) long compared to the 55.4 inches (141 cm) of Sue's skull, a difference of 6.5%.[63][64]ClassificationTyrannosaurus is the type genus of the superfamily Tyrannosauroidea, the family Tyrannosauridae, and the subfamily Tyrannosaurinae; in other words it is the standard by which paleontologists decide whether to include other species in the same group. Other members of the tyrannosaurine subfamily include the North American Daspletosaurus and the Asian Tarbosaurus,[65][66] both of which have occasionally been synonymized with Tyrannosaurus.[28][page needed] Tyrannosaurids were once commonly thought to be descendants of earlier large theropods such as megalosaurs and carnosaurs, although more recently they were reclassified with the generally smaller coelurosaurs.[27]Diagram showing the differences between a generalized Tarbosaurus (A) and Tyrannosaurus (B) skullIn 1955, Soviet paleontologist Evgeny Maleev named a new species, Tyrannosaurus bataar, from Mongolia.[67] By 1965, this species had been renamed Tarbosaurus bataar.[68] Despite the renaming, many phylogenetic analyses have found Tarbosaurus bataar to be the sister taxon of Tyrannosaurus rex,[66] and it has often been considered an Asian species of Tyrannosaurus.[27][69][70] A recent redescription of the skull of Tarbosaurus bataar has shown that it was much narrower than that of Tyrannosaurus rex and that during a bite, the distribution of stress in the skull would have been very different, closer to that of Alioramus, another Asian tyrannosaur.[71] A related cladistic analysis found that Alioramus, not Tyrannosaurus, was the sister taxon of Tarbosaurus, which, if true, would suggest that Tarbosaurus and Tyrannosaurus should remain separate.[65] The discovery and description of Qianzhousaurus would later disprove this and revealed that Alioramus belonged to the clade Alioramini.[72][73] The discovery of the tyrannosaurid Lythronax further indicates that Tarbosaurus and Tyrannosaurus are closely related, forming a clade with fellow Asian tyrannosaurid Zhuchengtyrannus, with Lythronax being their sister taxon.[74][75] A further study from 2016 by Steve Brusatte, Thomas Carr et al., also indicates Tyrannosaurus may have been an immigrant from Asia, as well as a possible descendent of Tarbosaurus. The study further indicates the possibility that Tyrannosaurus may have driven other tyrannosaurids that were native to North America extinct through competition.[76] Other finds in 2006 indicate giant tyrannosaurs may have been present in North America as early as 75 million years ago. Whether or not this specimen belongs to Tyrannosaurus rex, a new species of Tyrannosaurus, or a new genus entirely is still unknown.[77]Other tyrannosaurid fossils found in the same formations as Tyrannosaurus rex were originally classified as separate taxa, including Aublysodon and Albertosaurus megagracilis,[28] the latter being named Dinotyrannus megagracilis in 1995.[78] These fossils are now universally considered to belong to juvenile Tyrannosaurus rex.[79] A small but nearly complete skull from Montana, 60 centimeters (2.0 ft) long, may be an exception. This skull was originally classified as a species of Gorgosaurus (G. lancensis) by Charles W. Gilmore in 1946,[80] but was later referred to a new genus, Nanotyrannus.[81] Opinions remain divided on the validity of N. lancensis. Many paleontologists consider the skull to belong to a juvenile Tyrannosaurus rex.[82] There are minor differences between the two species, including the higher number of teeth in N. lancensis, which lead some scientists to recommend keeping the two genera separate until further research or discoveries clarify the situation.[66][83]Nanotyrannus lancensis holotype, possibly a juvenile TyrannosaurusBelow is the cladogram of Tyrannosauridae based on the phylogenetic analysis conducted by Loewen et al. in 2013.[74]Tyrannosauridae Albertosaurinae Gorgosaurus libratusGorgosaurus flipped.pngAlbertosaurus sarcophagusTyrannosaurinae Dinosaur Park tyrannosauridDaspletosaurus torosusDaspletosaurus torosus steveoc flipped.jpgTwo Medicine tyrannosauridTeratophoneus currieiBistahieversor sealeyiLythronax argestesLythronax by Tomopteryx flipped.pngTyrannosaurus rexRjpalmer tyrannosaurusrex (white background).jpgTarbosaurus bataarTarbosaurus Steveoc86 flipped.jpgZhuchengtyrannus magnusPaleobiologyLife historyA graph showing the hypothesized growth curve, body mass versus age (drawn in black, with other tyrannosaurids for comparison). Based on Erickson et al. 2004The identification of several specimens as juvenile Tyrannosaurus rex has allowed scientists to document ontogenetic changes in the species, estimate the lifespan, and determine how quickly the animals would have grown. The smallest known individual (LACM 28471, the "Jordan theropod") is estimated to have weighed only 30 kg (66 lb), while the largest, such as FMNH PR2081 (Sue) most likely weighed about 5,650 kg (12,460 lb). Histologic analysis of Tyrannosaurus rex bones showed LACM 28471 had aged only 2 years when it died, while Sue was 28 years old, an age which may have been close to the maximum for the species.[15]Histology has also allowed the age of other specimens to be determined. Growth curves can be developed when the ages of different specimens are plotted on a graph along with their mass. A Tyrannosaurus rex growth curve is S-shaped, with juveniles remaining under 1,800 kg (4,000 lb) until approximately 14 years of age, when body size began to increase dramatically. During this rapid growth phase, a young Tyrannosaurus rex would gain an average of 600 kg (1,300 lb) a year for the next four years. At 18 years of age, the curve plateaus again, indicating that growth slowed dramatically. For example, only 600 kg (1,300 lb) separated the 28-year-old Sue from a 22-year-old Canadian specimen (RTMP 81.12.1).[15] A 2004 histological study performed by different workers corroborates these results, finding that rapid growth began to slow at around 16 years of age.[84]11-year-old juvenile (Jane) specimen, with adult in the background, Burpee Museum of Natural HistoryAnother study corroborated the latter study's results but found the growth rate to be much faster, finding it to be around 1800 kilograms (4000 lbs). Although these results were much higher than previous estimations, the authors noted that these results significantly lowered the great difference between its actual growth rate and the one which would be expected of an animal of its size.[4] The sudden change in growth rate at the end of the growth spurt may indicate physical maturity, a hypothesis which is supported by the discovery of medullary tissue in the femur of a 16 to 20-year-old Tyrannosaurus rex from Montana (MOR 1125, also known as B-rex). Medullary tissue is found only in female birds during ovulation, indicating that B-rex was of reproductive age.[85] Further study indicates an age of 18 for this specimen.[86] In 2016, it was finally confirmed by Mary Higby Schweitzer and Lindsay Zanno et al that the soft tissue within the femur of MOR 1125 was medullary tissue. This also confirmed the identity of the specimen as a female. The discovery of medullary bone tissue within Tyrannosaurus may prove valuable in determining the sex of other dinosaur species in future examinations, as the chemical makeup of medullary tissue is unmistakable.[87] Other tyrannosaurids exhibit extremely similar growth curves, although with lower growth rates corresponding to their lower adult sizes.[88]Over half of the known Tyrannosaurus rex specimens appear to have died within six years of reaching sexual maturity, a pattern which is also seen in other tyrannosaurs and in some large, long-lived birds and mammals today. These species are characterized by high infant mortality rates, followed by relatively low mortality among juveniles. Mortality increases again following sexual maturity, partly due to the stresses of reproduction. One study suggests that the rarity of juvenile Tyrannosaurus rex fossils is due in part to low juvenile mortality rates; the animals were not dying in large numbers at these ages, and so were not often fossilized. This rarity may also be due to the incompleteness of the fossil record or to the bias of fossil collectors towards larger, more spectacular specimens.[88] In a 2013 lecture, Thomas Holtz Jr. suggested that dinosaurs "lived fast and died young" because they reproduced quickly whereas mammals have long life spans because they take longer to reproduce.[89] Gregory S. Paul also writes that Tyrannosaurus reproduced quickly and died young, but attributes their short life spans to the dangerous lives they lived.[90]Sexual dimorphismSkeleton casts mounted in a mating position, Jurassic Museum of AsturiasAs the number of known specimens increased, scientists began to analyze the variation between individuals and discovered what appeared to be two distinct body types, or morphs, similar to some other theropod species. As one of these morphs was more solidly built, it was termed the 'robust' morph while the other was termed 'gracile'. Several morphological differences associated with the two morphs were used to analyze sexual dimorphism in Tyrannosaurus rex, with the 'robust' morph usually suggested to be female. For example, the pelvis of several 'robust' specimens seemed to be wider, perhaps to allow the passage of eggs.[91] It was also thought that the 'robust' morphology correlated with a reduced chevron on the first tail vertebra, also ostensibly to allow eggs to pass out of the reproductive tract, as had been erroneously reported for crocodiles.[92]In recent years, evidence for sexual dimorphism has been weakened. A 2005 study reported that previous claims of sexual dimorphism in crocodile chevron anatomy were in error, casting doubt on the existence of similar dimorphism between Tyrannosaurus rex sexes.[93] A full-sized chevron was discovered on the first tail vertebra of Sue, an extremely robust individual, indicating that this feature could not be used to differentiate the two morphs anyway. As Tyrannosaurus rex specimens have been found from Saskatchewan to New Mexico, differences between individuals may be indicative of geographic variation rather than sexual dimorphism. The differences could also be age-related, with 'robust' individuals being older animals.[22]Only a single Tyrannosaurus rex specimen has been conclusively shown to belong to a specific sex. Examination of B-rex demonstrated the preservation of soft tissue within several bones. Some of this tissue has been identified as a medullary tissue, a specialized tissue grown only in modern birds as a source of calcium for the production of eggshell during ovulation. As only female birds lay eggs, medullary tissue is only found naturally in females, although males are capable of producing it when injected with female reproductive hormones like estrogen. This strongly suggests that B-rex was female, and that she died during ovulation.[85] Recent research has shown that medullary tissue is never found in crocodiles, which are thought to be the closest living relatives of dinosaurs, aside from birds. The shared presence of medullary tissue in birds and theropod dinosaurs is further evidence of the close evolutionary relationship between the two.[94]PostureOutdated reconstruction (by Charles R. Knight), showing upright poseModern representations in museums, art, and film show Tyrannosaurus rex with its body approximately parallel to the ground and tail extended behind the body to balance the head.[28]Like many bipedal dinosaurs, Tyrannosaurus rex was historically depicted as a 'living tripod', with the body at 45 degrees or less from the vertical and the tail dragging along the ground, similar to a kangaroo. This concept dates from Joseph Leidy's 1865 reconstruction of Hadrosaurus, the first to depict a dinosaur in a bipedal posture.[95] In 1915, convinced that the creature stood upright, Henry Fairfield Osborn, former president of the American Museum of Natural History, further reinforced the notion in unveiling the first complete Tyrannosaurus rex skeleton arranged this way. It stood in an upright pose for 77 years, until it was dismantled in 1992.[96]By 1970, scientists realized this pose was incorrect and could not have been maintained by a living animal, as it would have resulted in the dislocation or weakening of several joints, including the hips and the articulation between the head and the spinal column.[97] The inaccurate AMNH mount inspired similar depictions in many films and paintings (such as Rudolph Zallinger's famous mural The Age of Reptiles in Yale University's Peabody Museum of Natural History)[98] until the 1990s, when films such as Jurassic Park introduced a more accurate posture to the general public.[99]ArmsThe forelimbs might have been used to help T. rex rise from a resting pose, as seen in this cast (Bucky specimen)When Tyrannosaurus rex was first discovered, the humerus was the only element of the forelimb known.[45] For the initial mounted skeleton as seen by the public in 1915, Osborn substituted longer, three-fingered forelimbs like those of Allosaurus.[50] A year earlier, Lawrence Lambe described the short, two-fingered forelimbs of the closely related Gorgosaurus.[100] This strongly suggested that Tyrannosaurus rex had similar forelimbs, but this hypothesis was not confirmed until the first complete Tyrannosaurus rex forelimbs were identified in 1989, belonging to MOR 555 (the "Wankel rex").[49] The remains of Sue also include complete forelimbs.[22] Tyrannosaurus rex arms are very small relative to overall body size, measuring only 1 meter (3.3 ft) long, and some scholars have labelled them as vestigial. The bones show large areas for muscle attachment, indicating considerable strength. This was recognized as early as 1906 by Osborn, who speculated that the forelimbs may have been used to grasp a mate during copulation.[47] It has also been suggested that the forelimbs were used to assist the animal in rising from a prone position.[97]Diagram illustrating arm anatomyAnother possibility is that the forelimbs held struggling prey while it was killed by the tyrannosaur's enormous jaws. This hypothesis may be supported by biomechanical analysis. Tyrannosaurus rex forelimb bones exhibit extremely thick cortical bone, which have been interpreted as evidence that they were developed to withstand heavy loads. The biceps brachii muscle of an adult Tyrannosaurus rex was capable of lifting 199 kilograms (439 lb) by itself; other muscles such as the brachialis would work along with the biceps to make elbow flexion even more powerful. The M. biceps muscle of T. rex was 3.5 times as powerful as the human equivalent. A Tyrannosaurus rex forearm had a limited range of motion, with the shoulder and elbow joints allowing only 40 and 45 degrees of motion, respectively. In contrast, the same two joints in Deinonychus allow up to 88 and 130 degrees of motion, respectively, while a human arm can rotate 360 degrees at the shoulder and move through 165 degrees at the elbow. The heavy build of the arm bones, strength of the muscles, and limited range of motion may indicate a system evolved to hold fast despite the stresses of a struggling prey animal. In the first detailed scientific description of Tyrannosaurus forelimbs, paleontologists Kenneth Carpenter and Matt Smith dismissed notions that the forelimbs were useless or that Tyrannosaurus rex was an obligate scavenger.[101]According to paleontologist Steven Stanley from the University of Hawaii, the roughly 1 meter long arms of a Tyrannosaurus rex were used for slashing prey. Especially by juvenile dinosaurs as their arms grow slower in proportion to their bodies and a younger Tyrannosaurus rex would have proportionally much longer arms than an adult one.[102]Soft tissueIn the March 2005 issue of Science, Mary Higby Schweitzer of North Carolina State University and colleagues announced the recovery of soft tissue from the marrow cavity of a fossilized leg bone from a Tyrannosaurus rex. The bone had been intentionally, though reluctantly, broken for shipping and then not preserved in the normal manner, specifically because Schweitzer was hoping to test it for soft tissue.[103] Designated as the Museum of the Rockies specimen 1125, or MOR 1125, the dinosaur was previously excavated from the Hell Creek Formation. Flexible, bifurcating blood vessels and fibrous but elastic bone matrix tissue were recognized. In addition, microstructures resembling blood cells were found inside the matrix and vessels. The structures bear resemblance to ostrich blood cells and vessels. Whether an unknown process, distinct from normal fossilization, preserved the material, or the material is original, the researchers do not know, and they are careful not to make any claims about preservation.[104] If it is found to be original material, any surviving proteins may be used as a means of indirectly guessing some of the DNA content of the dinosaurs involved, because each protein is typically created by a specific gene. The absence of previous finds may be the result of people assuming preserved tissue was impossible, therefore not looking. Since the first, two more tyrannosaurs and a hadrosaur have also been found to have such tissue-like structures.[103] Research on some of the tissues involved has suggested that birds are closer relatives to tyrannosaurs than other modern animals.[105]T. rex femur (MOR 1125) from which demineralized matrix and peptides (insets) were obtainedIn studies reported in Science in April 2007, Asara and colleagues concluded that seven traces of collagen proteins detected in purified Tyrannosaurus rex bone most closely match those reported in chickens, followed by frogs and newts. The discovery of proteins from a creature tens of millions of years old, along with similar traces the team found in a mastodon bone at least 160,000 years old, upends the conventional view of fossils and may shift paleontologists' focus from bone hunting to biochemistry. Until these finds, most scientists presumed that fossilization replaced all living tissue with inert minerals. Paleontologist Hans Larsson of McGill University in Montreal, who was not part of the studies, called the finds "a milestone", and suggested that dinosaurs could "enter the field of molecular biology and really slingshot paleontology into the modern world".[106]Subsequent studies in April 2008 confirmed the close connection of Tyrannosaurus rex to modern birds. Postdoctoral biology researcher Chris Organ at Harvard University announced, "With more data, they would probably be able to place T. rex on the evolutionary tree between alligators and chickens and ostriches." Co-author John M. Asara added, "We also show that it groups better with birds than modern reptiles, such as alligators and green anole lizards."[107]The presumed soft tissue was called into question by Thomas Kaye of the University of Washington and his co-authors in 2008. They contend that what was really inside the tyrannosaur bone was slimy biofilm created by bacteria that coated the voids once occupied by blood vessels and cells.[108] The researchers found that what previously had been identified as remnants of blood cells, because of the presence of iron, were actually framboids, microscopic mineral spheres bearing iron. They found similar spheres in a variety of other fossils from various periods, including an ammonite. In the ammonite they found the spheres in a place where the iron they contain could not have had any relationship to the presence of blood.[109] Schweitzer has strongly criticized Kaye's claims and argues that there is no reported evidence that biofilms can produce branching, hollow tubes like those noted in her study.[110] San Antonio, Schweitzer and colleagues published an analysis in 2011 of what parts of the collagen had been recovered, finding that it was the inner parts of the collagen coil that had been preserved, as would have been expected from a long period of protein degradation.[111] Other research challenges the identification of soft tissue as biofilm and confirms finding "branching, vessel-like structures" from within fossilized bone.[112]ThermoregulationMain article: Physiology of dinosaursLife restoration based on specimen FMNH PR2081 "Sue", an adult T. rexAs of 2014, it is not clear if Tyrannosaurus was endothermic (warm-blooded). Tyrannosaurus, like most dinosaurs, was long thought to have an ectothermic ("cold-blooded") reptilian metabolism. The idea of dinosaur ectothermy was challenged by scientists like Robert T. Bakker and John Ostrom in the early years of the "Dinosaur Renaissance", beginning in the late 1960s.[113][114] Tyrannosaurus rex itself was claimed to have been endothermic ("warm-blooded"), implying a very active lifestyle.[13] Since then, several paleontologists have sought to determine the ability of Tyrannosaurus to regulate its body temperature. Histological evidence of high growth rates in young Tyrannosaurus rex, comparable to those of mammals and birds, may support the hypothesis of a high metabolism. Growth curves indicate that, as in mammals and birds, Tyrannosaurus rex growth was limited mostly to immature animals, rather than the indeterminate growth seen in most other vertebrates.[84]Oxygen isotope ratios in fossilized bone are sometimes used to determine the temperature at which the bone was deposited, as the ratio between certain isotopes correlates with temperature. In one specimen, the isotope ratios in bones from different parts of the body indicated a temperature difference of no more than 4 to 5 °C (7 to 9 °F) between the vertebrae of the torso and the tibia of the lower leg. This small temperature range between the body core and the extremities was claimed by paleontologist Reese Barrick and geochemist William Showers to indicate that Tyrannosaurus rex maintained a constant internal body temperature (homeothermy) and that it enjoyed a metabolism somewhere between ectothermic reptiles and endothermic mammals.[115] Other scientists have pointed out that the ratio of oxygen isotopes in the fossils today does not necessarily represent the same ratio in the distant past, and may have been altered during or after fossilization (diagenesis).[116] Barrick and Showers have defended their conclusions in subsequent papers, finding similar results in another theropod dinosaur from a different continent and tens of millions of years earlier in time (Giganotosaurus).[117] Ornithischian dinosaurs also showed evidence of homeothermy, while varanid lizards from the same formation did not.[118] Even if Tyrannosaurus rex does exhibit evidence of homeothermy, it does not necessarily mean that it was endothermic. Such thermoregulation may also be explained by gigantothermy, as in some living sea turtles.[119][120]FootprintsProbable footprint from New MexicoTwo isolated fossilized footprints have been tentatively assigned to Tyrannosaurus rex. The first was discovered at Philmont Scout Ranch, New Mexico, in 1983 by American geologist Charles Pillmore. Originally thought to belong to a hadrosaurid, examination of the footprint revealed a large 'heel' unknown in ornithopod dinosaur tracks, and traces of what may have been a hallux, the dewclaw-like fourth digit of the tyrannosaur foot. The footprint was published as the ichnogenus Tyrannosauripus pillmorei in 1994, by Martin Lockley and Adrian Hunt. Lockley and Hunt suggested that it was very likely the track was made by a Tyrannosaurus rex, which would make it the first known footprint from this species. The track was made in what was once a vegetated wetland mud flat. It measures 83 centimeters (33 in) long by 71 centimeters (28 in) wide.[121]A second footprint that may have been made by a Tyrannosaurus was first reported in 2007 by British paleontologist Phil Manning, from the Hell Creek Formation of Montana. This second track measures 72 centimeters (28 in) long, shorter than the track described by Lockley and Hunt. Whether or not the track was made by Tyrannosaurus is unclear, though Tyrannosaurus and Nanotyrannus are the only large theropods known to have existed in the Hell Creek Formation.[122][123]A set of footprints in Glenrock, Wyoming dating to the Maastrichtian stage of the late cretaceous and hailing from the Lance Formation were recently described by Scott Persons, Phil Currie et al. in January 2016, and are believed to belong to either a juvenile Tyrannosaurus rex or the dubious tyrannosaurid genus Nanotyrannus lancensis. From measurements and based on the positions of the footprints, the animal was believed to be traveling at a walking speed of around 2.8 to 5 miles per hour and was estimated to have a hip height of 1.56 m (5.1 ft) to 2.06 m (6.8 ft).[124][125][126] A follow-up paper appeared in 2017, increasing the speed estimations by 50-80 %.[127]LocomotionOnly known tyrannosaurid trackway (Bellatoripes fredlundi), from the Wapiti Formation, British ColumbiaThere are two main issues concerning the locomotory abilities of Tyrannosaurus: how well it could turn; and what its maximum straight-line speed was likely to have been. Both are relevant to the debate about whether it was a hunter or a scavenger.Tyrannosaurus may have been slow to turn, possibly taking one to two seconds to turn only 45° — an amount that humans, being vertically oriented and tailless, can spin in a fraction of a second.[128] The cause of the difficulty is rotational inertia, since much of Tyrannosaurus' mass was some distance from its center of gravity, like a human carrying a heavy timber horizontally — although it might have reduced the average distance by arching its back and tail and pulling its head and forelimbs close to its body, rather like the way ice skaters pull their arms closer in order to spin faster.[129]Scientists have produced a wide range of maximum speed estimates, mostly around 11 meters per second (40 km/h; 25 mph), but a few as low as 5–11 meters per second (18–40 km/h; 11–25 mph), and a few as high as 20 meters per second (72 km/h; 45 mph). Researchers have to rely on various estimating techniques because, while there are many tracks of very large theropods walking, so far none have been found of very large theropods running—and this absence may indicate that they did not run.[130] Scientists who think that Tyrannosaurus was able to run point out that hollow bones and other features that would have lightened its body may have kept adult weight to a mere 4.5 metric tons (5.0 short tons) or so, or that other animals like ostriches and horses with long, flexible legs are able to achieve high speeds through slower but longer strides. Some have also argued that Tyrannosaurus had relatively larger leg muscles than any animal alive today, which could have enabled fast running at 40–70 kilometers per hour (25–43 mph).[131]Femur (thigh bone)Tibia (shin bone)Metatarsals (foot bones)DewclawPhalanges (toe bones)Skeletal anatomy of a T. rex right legJack Horner and Don Lessem argued in 1993 that Tyrannosaurus was slow and probably could not run (no airborne phase in mid-stride), because its ratio of femur (thigh bone) to tibia (shin bone) length was greater than 1, as in most large theropods and like a modern elephant.[49] Holtz (1998) noted that tyrannosaurids and some closely related groups had significantly longer distal hindlimb components (shin plus foot plus toes) relative to the femur length than most other theropods, and that tyrannosaurids and their close relatives had a tightly interlocked metatarsus that more effectively transmitted locomotory forces from the foot to the lower leg than in earlier theropods ("metatarsus" means the foot bones, which function as part of the leg in digitigrade animals). He therefore concluded that tyrannosaurids and their close relatives were the fastest large theropods.[132] Thomas Holtz Jr. echoed these sentiments in his 2013 lecture, stating that the giant allosaurs had shorter feet for the same body size than Tyrannosaurus, whereas Tyrannosaurus had longer, skinnier and more interlocked feet for the same body size; attributes of faster moving animals.[89]T. rex foot showing the compressed arctometatarsalian condition of the middle metatarsal, compared to that of AllosaurusA study by Eric Snively and Anthony P. Russel published in 2003 also found that the tyrannosaurid arctometatarsals and elastic ligaments worked together in what he called a 'tensile keystone model' to strengthen the feet of Tyrannosaurus, increase the animal's stability and add greater resistance to dissociation over that of other theropod families; while still allowing resiliency that is otherwise reduced in ratites, horses, giraffids and other animals with metapodia to a single element. The study also pointed out that elastic ligaments in larger vertebrates could store and return relatively more elastic strain energy, which could have improved locomotor efficiency and decrease the strain energy transferred to the bones. The study suggested that this mechanism could have worked efficiently in tyrannosaurids as well. Hence, the study involved identifying the type of ligaments attached to the metatarsals, then how they functioned together and comparing it to those of other theropods and modern day analogs. The scientists found that arctometatarsals may have enabled tyrannosaurid feet to absorb forces such as linear deceleration, lateral acceleration and torsion more effectively than those of other theropods. It is also stated in their study that this may imply, though not demonstrate, that tyrannosaurids such as Tyrannosaurus had greater agility than other large theropods without an arctometatarsus.[133]Christiansen (1998) estimated that the leg bones of Tyrannosaurus were not significantly stronger than those of elephants, which are relatively limited in their top speed and never actually run (there is no airborne phase), and hence proposed that the dinosaur's maximum speed would have been about 11 meters per second (40 km/h; 25 mph), which is about the speed of a human sprinter. But he also noted that such estimates depend on many dubious assumptions.[134]Farlow and colleagues (1995) have argued that a Tyrannosaurus weighing 5.4 metric tons (6.0 short tons) to 7.3 metric tons (8.0 short tons) would have been critically or even fatally injured if it had fallen while moving quickly, since its torso would have slammed into the ground at a deceleration of 6 g (six times the acceleration due to gravity, or about 60 meters/s²) and its tiny arms could not have reduced the impact.[16] Giraffes have been known to gallop at 50 kilometers per hour (31 mph), despite the risk that they might break a leg or worse, which can be fatal even in a safe environment such as a zoo.[135][136] Thus it is possible that Tyrannosaurus also moved fast when necessary and had to accept such risks.[137][138]In another study, Gregory S. Paul pointed out that the flexed kneed and digitigrade adult Tyrannosaurus were much better adapted for running than elephants or humans, pointing out that Tyrannosaurus had a large ilium bone and cnemial crest that would have supported large muscles needed for running. He also mentioned that Alexander's (1989) formula to calculate speed by bone strength was only partly reliable. He suggests that the formula is overly sensitive to bone length; making long bones artificially weak. He also pointed out that the lowered risk of being wounded in combat may have been worth the risk of Tyrannosaurus falling while running.[139]Restoration of a walking T. rexMost recent research on Tyrannosaurus locomotion does not support speeds faster than 40 kilometers per hour (25 mph), i.e. moderate-speed running. For example, a 2002 paper in Nature used a mathematical model (validated by applying it to three living animals, alligators, chickens, and humans; later eight more species including emus and ostriches[130]) to gauge the leg muscle mass needed for fast running (over 40 km/h or 25 mph).[131] They found that proposed top speeds in excess of 40 kilometers per hour (25 mph) were infeasible, because they would require very large leg muscles (more than approximately 40–86% of total body mass). Even moderately fast speeds would have required large leg muscles. This discussion is difficult to resolve, as it is unknown how large the leg muscles actually were in Tyrannosaurus. If they were smaller, only 18 kilometers per hour (11 mph) walking or jogging might have been possible.[131]A study in 2007 used computer models to estimate running speeds, based on data taken directly from fossils, and claimed that Tyrannosaurus rex had a top running speed of 8 meters per second (29 km/h; 18 mph). An average professional football (soccer) player would be slightly slower, while a human sprinter can reach 12 meters per second (43 km/h; 27 mph). These computer models predict a top speed of 17.8 meters per second (64 km/h; 40 mph) for a 3-kilogram (6.6 lb) Compsognathus[140][141] (probably a juvenile individual).[142]Muscle mass reconstruction of M. caudofemoralis longusIn 2010, Scott Persons, a graduate student from the University of Alberta, proposed that Tyrannosaurus's speed may have been enhanced by strong tail muscles.[143] He found that theropods such as T. rex had certain muscle arrangements that are different from modern day birds and mammals but with some similarities to modern reptiles.[144] He concluded that the caudofemoralis muscles which link the tail bones and the upper leg bones could have assisted Tyrannosaurus in leg retraction and enhanced its running ability, agility and balance. The caudofemoralis muscle would have been a key muscle in femoral retraction; pulling back the leg at the femur.[143] The study also found that theropod skeletons such as those of Tyrannosaurus had adaptations (such as elevated transverse processes in the tail vertebrae) to enable the growth of larger tail muscles and that Tyrannosaurus's tail muscle mass may have been underestimated by over 25 percent and perhaps as much as 45 percent. The caudofemoralis muscle was found to comprise 58 percent of the muscle mass in the tail of Tyrannosaurus. Tyrannosaurus also had the largest absolute and relative caudofemoralis muscle mass out of the three extinct organisms in the study. This is because Tyrannosaurus also had additional adaptations to enable large tail muscles; the elongation of its tail's hemal arches. According to Persons, the increase in tail muscle mass would have moved the center of mass closer to the hindquarters and hips which would have lessened the strain on the leg muscles to support its weight; improving its overall balance and agility. This would also have made the animal less front-heavy, thus reducing rotational inertia. Persons also notes that the tail is also rich in tendons and septa which could have been stores of elastic energy, and thereby improved locomotive efficiency. Persons adds that this means non-avian theropods actually had broader tails than previously depicted, as broad or broader laterally than dorsoventrally near the base.[143][144]Heinrich Mallison from Berlin's Museum of Natural History also presented a theory in 2011, suggesting that Tyrannosaurus and many other dinosaurs may have achieved relatively high speeds through short rapid strides instead of the long strides employed by modern birds and mammals when running, likening their movement to power-walking. This, according to Mallison, would have been achievable irrespective of joint strength and lessened the need for additional muscle mass in the legs, particularly at the ankles. To support his theory, Mallison assessed the limbs of various dinosaurs and found that they were different from those of modern mammals and birds; having their stride length greatly limited by their skeletons, but also having relatively large muscles at the hindquarters. He found a few similarities between the muscles in dinosaurs and race-walkers; having less muscle mass in the ankles but more at the hindquarters. John Hutchinson advised caution regarding this theory, suggesting that they must first look into dinosaur muscles to see how frequently they could have contracted.[145]In July 2017, a study by William Sellers et al., published in the journal PeerJ found that an adult Tyrannosaurus was incapable of running due to very high skeletal loads. The study used the latest computing technology to test its findings. The researchers used two different structural mechanical systems to create the computer model. The weight they settled on for their calculations was a conservative estimate of 7 tons. The model showed that speeds above 11 mph (18 km/h) would have probably shattered the leg bones of Tyrannosaurus. The finding may mean that running was also not possible for other giant theropod dinosaurs like Giganotosaurus, Mapusaurus and Acrocanthosaurus.[146]Another study in July 2017 by Miriam Hirt et al., published in the journal Nature Ecology & Evolution found that the top speed of Tyrannosaurus was around 17 mph (27 km/h). Other dinosaurs including Triceratops, Velociraptor and Brachiosaurus were also analyzed in the study, as were many living animals like elephants, cheetahs and rabbits. The speed of Tyrannosaurus was calculated by factoring its weight in conjunction with the medium upon which it travelled (in the case of the theropod, land) and by the assumptions that: one; animals reach their maximum speeds during comparatively short sprints, and two; Newton's laws of motion dictate that mass has to overcome inertia. It found that large animals like Tyrannosaurus exhaust their energy reserves long before they reach their theoretical top speed, resulting in a parabola-like relationship between size and speed. The equation can calculate the top speed of an animal with almost 90% accuracy and can be applied to both living and extinct animals.[147][148]Those who argue that Tyrannosaurus was incapable of running estimate the top speed of Tyrannosaurus at about 17 kilometers per hour (11 mph). This is still faster than its most likely prey species, hadrosaurids and ceratopsians.[131] In addition, some advocates of the idea that Tyrannosaurus was a predator claim that tyrannosaur running speed is not important, since it may have been slow but still faster than its probable prey.[149] Thomas Holtz also noted that Tyrannosaurus had proportionately longer feet than the animals it hunted: duck-billed dinosaurs and horned dinosaurs.[89] Paul and Christiansen (2000) argued that at least the later ceratopsians had upright forelimbs and the larger species may have been as fast as rhinos.[150] Healed Tyrannosaurus bite wounds on ceratopsian fossils are interpreted as evidence of attacks on living ceratopsians (see below). If the ceratopsians that lived alongside Tyrannosaurus were fast, that casts doubt on the argument that Tyrannosaurus did not have to be fast to catch its prey.[138]Brain and sensesThe eye-sockets faced mainly forwards, giving it good binocular vision (Sue specimen).A study conducted by Lawrence Witmer and Ryan Ridgely of Ohio University found that Tyrannosaurus shared the heightened sensory abilities of other coelurosaurs, highlighting relatively rapid and coordinated eye and head movements, as well as an enhanced ability to sense low frequency sounds that would allow tyrannosaurs to track prey movements from long distances and an enhanced sense of smell.[151] A study published by Kent Stevens of the University of Oregon concluded that Tyrannosaurus had keen vision. By applying modified perimetry to facial reconstructions of several dinosaurs including Tyrannosaurus, the study found that Tyrannosaurus had a binocular range of 55 degrees, surpassing that of modern hawks, and had 13 times the visual acuity of a human, thereby surpassing the visual acuity of an eagle which is only 3.6 times that of a person. This would have allowed Tyrannosaurus to discern objects[definition needed] as far as 6 km (3.7 mi) away, which is greater than the 1.6 km (1 mi) that a human can see.[24][25][152][153]Thomas Holtz Jr. would note that high depth perception of Tyrannosaurus may have been due to the prey it had to hunt; noting that it had to hunt horned dinosaurs such as Triceratops, armored dinosaurs such as Ankylosaurus and the duck-billed dinosaurs may have had complex social behaviors. He would suggest that this made precision more crucial for Tyrannosaurus enabling it to, "get in, get that blow in and take it down." In contrast, Acrocanthosaurus had limited depth perception because they hunted large sauropods, which were relatively rare during the time of Tyrannosaurus.[89]Cast of the braincase at the Australian Museum, Sydney.Tyrannosaurus had very large olfactory bulbs and olfactory nerves relative to their brain size, the organs responsible for a heightened sense of smell. This suggests that the sense of smell was highly developed, and implies that tyrannosaurs could detect carcasses by scent alone across great distances. The sense of smell in tyrannosaurs may have been comparable to modern vultures, which use scent to track carcasses for scavenging. Research on the olfactory bulbs has shown that Tyrannosaurus rex had the most highly developed sense of smell of 21 sampled non-avian dinosaur species.[154]Somewhat unusually among theropods, T. rex had a very long cochlea. The length of the cochlea is often related to hearing acuity, or at least the importance of hearing in behavior, implying that hearing was a particularly important sense to tyrannosaurs. Specifically, data suggests that Tyrannosaurus rex heard best in the low-frequency range, and that low-frequency sounds were an important part of tyrannosaur behavior.[151]A study by Grant R. Hurlburt, Ryan C. Ridgely and Lawrence Witmer obtained estimates for Encephalization Quotients (EQs), based on reptiles and birds, as well as estimates for the ratio of cerebrum to brain mass. The study concluded that Tyrannosaurus had the relatively largest brain of all adult non-avian dinosaurs with the exception of certain small maniraptoriforms (Bambiraptor, Troodon and Ornithomimus). The study found that Tyrannosaurus's relative brain size was still within the range of modern reptiles, being at most 2 standard deviations above the mean of non-avian reptile EQs. The estimates for the ratio of cerebrum mass to brain mass would range from 47.5 to 49.53 percent. According to the study, this is more than the lowest estimates for extant birds (44.6 percent), but still close to the typical ratios of the smallest sexually mature alligators which range from 45.9–47.9 percent.[155]Feeding strategiesMain article: Feeding behaviour of TyrannosaurusTyrannosaurus tooth marks on bones of various herbivorous dinosaursA 2012 study by scientists Karl Bates and Peter Falkingham suggested that the bite force of Tyrannosaurus could have been the strongest of any terrestrial animal that has ever lived. The calculations suggested that adult T. rex could have generated from 35,000 to 57,000 Newtons of force in the back teeth.[156][157][158] Even higher estimates were made by professor Mason B. Meers of the University of Tampa in 2003. In his study, Meers estimated a possible bite force of around 183,000 to 235,000 Newtons or 18.3 to 23.5 metric tons (20.2 to 25.9 short tons).[9] Research done by Greg Erikson and Paul Gignac et al and published in the journal Scientific Reports indicates that Tyrannosaurus could bite down with around 8,000 pounds of force when feeding, exerting a pressure of 431,000 pounds per square inch with their teeth. This allowed Tyrannosaurus to drive open cracks present in bone during repetitive, mammal-like biting and produce high-pressure fracture arcades, leading to a catastrophic explosion of some bones and allowing the theropod to fully exploit carcasses of other dinosaurs, giving it access to the mineral salts and marrow within bone that other carnivores in the same environment could not access.[159] Research done by Stephan Lautenschlager et al. of the University of Bristol, also reveals Tyrannosaurus was also capable of a maximum jaw gape of around 63.5 degrees, a necessary adaptation for a wide range of jaw angles in order to power the creature's strong bite.[160][161]The debate about whether Tyrannosaurus was a predator or a pure scavenger is as old as the debate about its locomotion. Lambe (1917) described a good skeleton of Tyrannosaurus close relative Gorgosaurus and concluded that it and therefore also Tyrannosaurus was a pure scavenger, because the Gorgosaurus teeth showed hardly any wear.[162] This argument is no longer taken seriously, because theropods replaced their teeth quite rapidly. Ever since the first discovery of Tyrannosaurus most scientists have speculated that it was a predator; like modern large predators it would readily scavenge or steal another predator's kill if it had the opportunity.[163]Paleontologist Jack Horner has been a major advocate of the idea that Tyrannosaurus was exclusively a scavenger and did not engage in active hunting at all,[49][164][165] though Horner himself has claimed that he never published this idea in the peer-reviewed scientific literature and used it mainly as a tool to teach a popular audience, particularly children, the dangers of making assumptions in science (such as assuming T. rex was a hunter) without using evidence.[166] Nevertheless, Horner presented several arguments in the popular literature to support the pure scavenger hypothesis:Tyrannosaur arms are short when compared to other known predators. Horner argues that the arms were too short to make the necessary gripping force to hold on to prey.[167]Tyrannosaurs had large olfactory bulbs and olfactory nerves (relative to their brain size). These suggest a highly developed sense of smell which could sniff out carcasses over great distances, as modern vultures do. Research on the olfactory bulbs of dinosaurs has shown that Tyrannosaurus had the most highly developed sense of smell of 21 sampled dinosaurs.[168] Opponents of the pure scavenger hypothesis have used the example of vultures in the opposite way, arguing that the scavenger hypothesis is implausible because the only modern pure scavengers are large gliding birds, which use their keen senses and energy-efficient gliding to cover vast areas economically.[169] Researchers from Glasgow concluded that an ecosystem as productive as the current Serengeti would provide sufficient carrion for a large theropod scavenger, although the theropod might have had to be cold-blooded in order to get more calories from carrion than it spent on foraging (see Metabolism of dinosaurs). They also suggested that modern ecosystems like the Serengeti have no large terrestrial scavengers because gliding birds now do the job much more efficiently, while large theropods did not face competition for the scavenger ecological niche from gliding birds.[170]Tyrannosaur teeth could crush bone, and therefore could extract as much food (bone marrow) as possible from carcass remnants, usually the least nutritious parts. Karen Chin and colleagues have found bone fragments in coprolites (fossilized feces) that they attribute to tyrannosaurs, but point out that a tyrannosaur's teeth were not well adapted to systematically chewing bone like hyenas do to extract marrow.[171]Since at least some of Tyrannosaurus's potential prey could move quickly, evidence that it walked instead of ran could indicate that it was a scavenger.[164][172] On the other hand, recent analyses suggest that Tyrannosaurus, while slower than large modern terrestrial predators, may well have been fast enough to prey on large hadrosaurs and ceratopsians.[131][149]Other evidence suggests hunting behavior in Tyrannosaurus. The eye sockets of tyrannosaurs are positioned so that the eyes would point forward, giving them binocular vision slightly better than that of modern hawks. Horner also pointed out that the tyrannosaur lineage had a history of steadily improving binocular vision. It is not obvious why natural selection would have favored this long-term trend if tyrannosaurs had been pure scavengers, which would not have needed the advanced depth perception that stereoscopic vision provides.[24][25] In modern animals, binocular vision is found mainly in predators.The damage to the tail vertebrae of this Edmontosaurus annectens skeleton (on display at the Denver Museum of Nature and Science) indicates that it may have been bitten by a TyrannosaurusA skeleton of the hadrosaurid Edmontosaurus annectens has been described from Montana with healed tyrannosaur-inflicted damage on its tail vertebrae. The fact that the damage seems to have healed suggests that the Edmontosaurus survived a tyrannosaur's attack on a living target, i.e. the tyrannosaur had attempted active predation.[173] There is also evidence for an aggressive interaction between a Triceratops and a Tyrannosaurus in the form of partially healed tyrannosaur tooth marks on a Triceratops brow horn and squamosal (a bone of the neck frill); the bitten horn is also broken, with new bone growth after the break. It is not known what the exact nature of the interaction was, though: either animal could have been the aggressor.[174] Since the Triceratops wounds healed, it is most likely that the Triceratops survived the encounter and managed to overcome the Tyrannosaurus. Paleontologist Peter Dodson estimates that in a battle against a bull Triceratops, the Triceratops had the upper hand and would successfully defend itself by inflicting fatal wounds to the Tyrannosaurus using its sharp horns.[175]When examining Sue, paleontologist Pete Larson found a broken and healed fibula and tail vertebrae, scarred facial bones and a tooth from another Tyrannosaurus embedded in a neck vertebra. If correct, these might be strong evidence for aggressive behavior between tyrannosaurs but whether it would have been competition for food and mates or active cannibalism is unclear.[176] Further recent investigation of these purported wounds has shown that most are infections rather than injuries (or simply damage to the fossil after death) and the few injuries are too general to be indicative of intraspecific conflict.[164] Some researchers argue that if Tyrannosaurus were a scavenger, another dinosaur had to be the top predator in the Amerasian Upper Cretaceous. Top prey were the larger marginocephalians and ornithopods. The other tyrannosaurids share so many characteristics that only small dromaeosaurs and troodontids remain as feasible top predators. In this light, scavenger hypothesis adherents have suggested that the size and power of tyrannosaurs allowed them to steal kills from smaller predators,[172] although they may have had a hard time finding enough meat to scavenge, being outnumbered by smaller theropods.[177] Most paleontologists accept that Tyrannosaurus was both an active predator and a scavenger like most large carnivores.Two teeth from the lower jaw of specimen MOR 1125, "B-rex", showing the variation in tooth size within an individualTyrannosaurus may have had infectious saliva used to kill its prey. This theory was first proposed by William Abler.[178] Abler examined the teeth of tyrannosaurids between each tooth serration; the serrations may have held pieces of carcass with bacteria, giving Tyrannosaurus a deadly, infectious bite much like the Komodo dragon was thought to have. Jack Horner regards Tyrannosaurus tooth serrations as more like cubes in shape than the serrations on a Komodo monitor's teeth, which are rounded.[179] All forms of saliva contain possibly hazardous bacteria, so the prospect of it being used as a method of predation is disputable.Tyrannosaurus, and most other theropods, probably primarily processed carcasses with lateral shakes of the head, like crocodilians. The head was not as maneuverable as the skulls of allosauroids, due to flat joints of the neck vertebrae.[180]CannibalismA study from Currie, Horner, Erickson and Longrich in 2010 has been put forward as evidence of cannibalism in the genus Tyrannosaurus.[181] They studied some Tyrannosaurus specimens with tooth marks in the bones, attributable to the same genus. The tooth marks were identified in the humerus, foot bones and metatarsals, and this was seen as evidence for opportunistic scavenging, rather than wounds caused by intraspecific combat. In a fight, they proposed it would be difficult to reach down to bite in the feet of a rival, making it more likely that the bite marks were made in a carcass. As the bite marks were made in body parts with relatively scanty amounts of flesh, it is suggested that the Tyrannosaurus was feeding on a carcass in which the more fleshy parts had already been consumed. They were also open to the possibility that other tyrannosaurids practiced cannibalism.[181] Other evidence for cannibalism has been unearthed.[182]Pack behaviorMounted skeletons of different age groups, Los Angeles Natural History MuseumPhilip J. Currie of the University of Alberta has suggested that Tyrannosaurus may have been pack animals. Currie compared Tyrannosaurus rex favorably to related species Tarbosaurus bataar and Albertosaurus sarcophagus, fossil evidence from which Currie had previously used to suggest that they lived in packs.[183] Currie pointed out that a find in South Dakota preserved three Tyrannosaurus rex skeletons in close proximity to each other.[184] After using CT scanning, Currie stated that Tyrannosaurus would have been capable of such complex behavior, because its brain size is three times greater than what would be expected for an animal of its size. Currie elaborated that Tyrannosaurus had a larger brain-to-body-size proportion than crocodiles and three times more than plant eating dinosaurs such as Triceratops of the same size. Currie believed Tyrannosaurus to be six times smarter than most dinosaurs and other reptiles.[183][185] Because the available prey, such as Triceratops and Ankylosaurus, were well-armored, and that others were fast-moving, it would have been necessary for Tyrannosaurus to hunt in groups. Currie speculated that juveniles and adults would have hunted together, with the faster juveniles chasing down the prey and the more powerful adults making the kill, by analogy to modern-day pack hunters where each member contributes a skill.[183]Currie's pack-hunting hypothesis has been harshly criticized by other scientists. Brian Switek, writing for The Guardian in 2011,[186] noted that Currie's pack hypothesis has not been presented as research in a peer-reviewed scientific journal, but primarily in relation to a television special and tie-in book called Dino Gangs. Switek also noted that Currie's argument for pack hunting in Tyrannosaurus rex is primarily based on analogy to a different species, Tarbosaurus bataar, and that the supposed evidence for pack hunting in T. bataar itself has not yet been published and subjected to scientific scrutiny. According to Switek and other scientists who have participated in panel discussions about the Dino Gangs television program, the evidence for pack hunting in Tarbosaurus and Albertosaurus is weak, based primarily on the association of several skeletons, for which numerous alternative explanations have been proposed (e.g. drought or floods forcing numerous specimens together to die in one place). In fact, Switek notes that the Albertosaurus bonebed site, on which Currie has based most of the interpretations of supposed pack hunting in related species, preserves geological evidence of just such a flood. Switek said, "bones alone are not enough to reconstruct dinosaur behaviour. The geological context in which those bones are found – the intricate details of ancient environments and the pace of prehistoric time – are essential to investigating the lives and deaths of dinosaurs,"[186] and noted that Currie must first describe the geological evidence from other tyrannosaur bonebed sites before jumping to conclusions about social behavior. Switek described the sensational claims provided in press releases and news stories surrounding the Dino Gangs program as "nauseating hype" and noted that the production company responsible for the program, Atlantic Productions, has a poor record involving exaggerating claims about new fossil discoveries, most notably the controversial claim it published regarding the supposed early human ancestor Darwinius, which soon turned out to be a relative of lemurs instead.[186]Lawrence Witmer pointed out that social behavior can't be determined by brain endocasts and the brains of solitary leopards are identical to those of a cooperatively hunting lion; estimated brain sizes only show that an animal may have hunted in groups. In his opinion, the brains of tyrannosaurs were large enough for what he dubs "communal hunting", a semi-organized behavior that falls between solitary and cooperative hunting. Witmer claims that communal hunting is a step towards the evolution of cooperative hunting. He found it hard to believe that tyrannosaurs wouldn't have exploited the opportunity to join others in making a kill, and thus decrease risk and increase their chances of success.[187]On July 23, 2014, evidence, for the first time, in the form of fossilized trackways in Canada, showed that tyrannosaurs may have hunted in groups.[188][189]PathologyRestoration of an individual (based on MOR 980) with parasite infectionsIn 2001, Bruce Rothschild and others published a study examining evidence for stress fractures and tendon avulsions in theropod dinosaurs and the implications for their behavior. Since stress fractures are caused by repeated trauma rather than singular events they are more likely to be caused by regular behavior than other types of injuries. Of the 81 Tyrannosaurus foot bones examined in the study one was found to have a stress fracture, while none of the 10 hand bones were found to have stress fractures. The researchers found tendon avulsions only among Tyrannosaurus and Allosaurus. An avulsion injury left a divot on the humerus of Sue the T. rex, apparently located at the origin of the deltoid or teres major muscles. The presence of avulsion injuries being limited to the forelimb and shoulder in both Tyrannosaurus and Allosaurus suggests that theropods may have had a musculature more complex than and functionally different from those of birds. The researchers concluded that Sue's tendon avulsion was probably obtained from struggling prey. The presence of stress fractures and tendon avulsions in general provides evidence for a "very active" predation-based diet rather than obligate scavenging.[190]A 2009 study showed that holes in the skulls of several specimens that were previously explained by intraspecific attacks might have been caused by Trichomonas-like parasites that commonly infect avians.[191] Further evidence of intraspecific attack were found by Joseph Peterson and his colleagues in the juvenile Tyrannosaurus nicknamed Jane. Peterson and his team found that Jane's skull showed healed puncture wounds on the upper jaw and snout which they believe came from another juvenile Tyrannosaurus. Subsequent CT scans of Jane's skull would further confirm the team's hypothesis, showing that the puncture wounds came from a traumatic injury and that there was subsequent healing.[192] The team would also state that Jane's injuries were structurally different from the parasite-induced lesions found in Sue and that Jane's injuries were on her face whereas the parasite that infected Sue caused lesions to the lower jaw.[193]PaleoecologyTyrannosaurus and other animals of the Hell Creek FormationTyrannosaurus lived during what is referred to as the Lancian faunal stage (Maastrichtian age) at the end of the Late Cretaceous. Tyrannosaurus ranged from Canada in the north to at least Texas and New Mexico in the south of Laramidia. During this time Triceratops was the major herbivore in the northern portion of its range, while the titanosaurian sauropod Alamosaurus "dominated" its southern range. Tyrannosaurus remains have been discovered in different ecosystems, including inland and coastal subtropical, and semi-arid plains.Several notable Tyrannosaurus remains have been found in the Hell Creek Formation. During the Maastrichtian this area was subtropical, with a warm and humid climate. The flora consisted mostly of angiosperms, but also included trees like dawn redwood (Metasequoia) and Araucaria. Tyrannosaurus shared this ecosystem with Triceratops, related ceratopsians Nedoceratops, Tatankaceratops and Torosaurus, the hadrosaurid Edmontosaurus annectens and possibly a species of Parasaurolophus, the armored dinosaurs Denversaurus, Edmontonia and Ankylosaurus, the dome headed dinosaurs Pachycephalosaurus, Stygimoloch, Sphaerotholus, and Dracorex, the hypsilophodont Thescelosaurus, and the theropods Ornithomimus, Struthiomimus, Orcomimus, Acheroraptor, Dakotaraptor, Richardoestesia, Paronychodon, Pectinodon, and Troodon.[194]Another formation with tyrannosaur remains is the Lance Formation of Wyoming. This has been interpreted as a bayou environment similar to today's Gulf Coast. The fauna was very similar to Hell Creek, but with Struthiomimus replacing its relative Ornithomimus. The small ceratopsian Leptoceratops also lived in the area.[195]In its southern range Tyrannosaurus lived alongside the titanosaur Alamosaurus, the ceratopsians Torosaurus, Bravoceratops and Ojoceratops, hadrosaurs which consisted of a species of Edmontosaurus, Kritosaurus and a possible species of Gryposaurus, the nodosaur Glyptodontopelta, the oviraptorid Ojoraptosaurus, possible species of the theropods Troodon and Richardoestesia, and the pterosaur Quetzalcoatlus.[196] The region is thought to have been dominated by semi-arid inland plains, following the probable retreat of the Western Interior Seaway as global sea levels fell.[197]Tyrannosaurus may have also inhabited Mexico's Lomas Coloradas formation in Sonora. Though skeletal evidence is lacking, six shed and broken teeth from the fossil bed have been thoroughly compared with other theropod genera and appear to be identical to those of Tyrannosaurus. If true, the evidence indicates the range of Tyrannosaurus was possibly more extensive than previously believed.[198] It is possible that tyrannosaurs were originally Asian species, migrating to North America before the end of the Cretaceous period.[199]In popular cultureMain article: Tyrannosaurus in popular cultureSince it was first described in 1905, Tyrannosaurus rex has become the most widely recognized dinosaur species in popular culture. It is the only dinosaur that is commonly known to the general public by its full scientific name (binomial name) (Tyrannosaurus rex), and the scientific abbreviation T. rex has also come into wide usage.[22] Robert T. Bakker notes this in The Dinosaur Heresies and explains that a name like "Tyrannosaurus rex is just irresistible to the tongue."[13]See alsoTimeline of tyrannosaur researchDinosaurs are a diverse group of reptiles[note 1] of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 231 million years ago,[1] although the exact origin and timing of the evolution of dinosaurs is the subject of active research.[2] 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. The fossil record indicates that birds are modern feathered dinosaurs,[3] having evolved from earlier theropods during the late Jurassic Period.[4] As such, birds were the only dinosaur lineage to survive the Cretaceous–Paleogene extinction event 66 million years ago.[5] 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 taxonomic, morphological and ecological standpoints. Birds, at over 10,000 living species,[6] are the most diverse group of vertebrates besides perciform fish.[7] Using fossil evidence, paleontologists have identified over 500 distinct genera[8] and more than 1,000 different species of non-avian dinosaurs.[9] Dinosaurs are represented on every continent by both extant species (birds) and fossil remains.[10] 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.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)[11] and heights of 18 meters (59 feet)[12] 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 the media.Contents  [hide] 1 Etymology2 Definition2.1 General description2.2 Distinguishing anatomical features3 Evolutionary history3.1 Origins and early evolution3.2 Evolution and paleobiogeography4 Classification4.1 Taxonomy5 Biology5.1 Size5.1.1 Largest and smallest5.2 Behavior5.3 Communication5.4 Reproductive biology5.5 Physiology6 Origin of birds6.1 Feathers6.2 Skeleton6.3 Soft anatomy6.4 Behavioral evidence7 Extinction of major groups7.1 Impact event7.2 Deccan Traps7.3 Possible Paleocene survivors8 History of study8.1 "Dinosaur renaissance"8.2 Soft tissue and DNA9 Cultural depictions10 See also11 Notes12 References13 Further reading14 External linksEtymologyThe taxon Dinosauria was formally named in 1842 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.[13] The term is derived from Ancient Greek δεινός (deinos), meaning "terrible, potent or fearfully great", and σαῦρος (sauros), meaning "lizard or reptile".[13][14] 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.[15]Other prehistoric animals, including mosasaurs, ichthyosaurs, pterosaurs, plesiosaurs, and Dimetrodon, while often popularly conceived of as dinosaurs, are not taxonomically classified as dinosaurs.DefinitionTriceratops skeleton, Natural History Museum of Los Angeles CountyUnder phylogenetic nomenclature, dinosaurs are usually defined as the group consisting of Triceratops, Neornithes, their most recent common ancestor (MRCA), and all descendants.[16] 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.[17] 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).[18]Birds are now recognized as being the sole surviving lineage of theropod dinosaurs. In traditional taxonomy, birds were considered a separate class that 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. Birds are classified as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs.[19]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 horridus, Passer domesticus, Diplodocus carnegii, and all of its descendants, to ensure that sauropods and kin remain included as dinosaurs.[20][21]General descriptionmontage of four birdsIn 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.[22] Many prehistoric animal groups are popularly conceived of as dinosaurs, such as ichthyosaurs, mosasaurs, plesiosaurs, pterosaurs, and pelycosaurs (especially Dimetrodon), but are not classified scientifically as dinosaurs, and none had the erect hind limb posture characteristic of true dinosaurs.[23] 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.[24]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.[8] An earlier study predicted that about 3400 dinosaur genera existed, including many that would not have been preserved in the fossil record.[25] By September 17, 2008, 1047 different species of dinosaurs had been named.[9]In 2016, the estimated number of dinosaur species that existed in the Mesozoic era was estimated to be 1,543–2,468.[26][27] 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 Ammosaurus 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.[10] 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.[28]Distinguishing anatomical featuresWhile 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.[29]A detailed assessment of archosaur interrelations by Sterling Nesbitt[30] 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 roofepipophyses, 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 vertebraeapex 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 lengthfourth trochanter (projection where the caudofemoralis muscle attaches on the inner rear shaft) on the femur (thighbone) is a sharp flangefourth trochanter asymmetrical, with distal, lower, margin forming a steeper angle to the shafton 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 elementexoccipitals (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 braincasein 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.[16]Diagram of a typical diapsid skullj: jugal bone, po: postorbital bone, p: parietal bone, sq: squamosal bone, q: quadrate bone, qj: quadratojugal boneA 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.[31] 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 sacrum composed of three or more fused vertebrae (three are found in some other archosaurs, but only two are found in Herrerasaurus);[16] and a perforate acetabulum, or hip socket, with a hole at the center of its inside surface (closed in Saturnalia, for example).[32][33] 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.[34]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.[35] 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.[36] Their erect posture enabled early dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that surpassed those of "sprawling" reptiles.[37] Erect limbs probably also helped support the evolution of large size by reducing bending stresses on limbs.[38] 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.[38]Evolutionary historyLife timelineview • discuss • edit-4500 —–-4000 —–-3500 —–-3000 —–-2500 —–-2000 —–-1500 —–-1000 —–-500 —–0 —waterSingle-celledlifephotosynthesisEukaryotesMulticellularlifeLand lifeDinosaurs    MammalsFlowers ←Earliest Earth (−4540)←Earliest water←Earliest life←LHB meteorites←Earliest oxygen←Atmospheric oxygen←Oxygen crisis←Earliest sexual reproduction←Ediacara biota←Cambrian explosion←Earliest humansPhanerozoicProterozoicArcheanHadeanPongolaHuronianCryogenianAndeanKarooQuaternaryAxis scale: millions of years ago.Orange labels: ice ages.Also see: Human timeline and Nature timelineMain article: Evolution of dinosaursOrigins and early evolutionDinosaurs 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.[39][40] 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.[41] Paleontologists think that Eoraptor resembles the common ancestor of all dinosaurs;[42] if this is true, its traits suggest that the first dinosaurs were small, bipedal predators.[43] 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.[44]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 aetosaurs, ornithosuchids and rauisuchians, which were more successful than the dinosaurs.[45] 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 early Rhaetian, respectively,[46][47] and the exact date of their extinction is uncertain. These losses left behind a land fauna of crocodylomorphs, dinosaurs, mammals, pterosaurians, and turtles.[16] The first few lines of early dinosaurs diversified through the Carnian and Norian stages of the Triassic, possibly by occupying the niches of the groups that became extinct.[18]Evolution and paleobiogeographyDinosaur 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.[48] 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.[49] The general homogeneity of dinosaurian faunas continued into the middle and late Jurassic, where most localities had predators consisting of ceratosaurians, spinosauroids, and carnosaurians, and herbivores consisting of stegosaurian ornithischians and large sauropods. Examples of this include the Morrison Formation of North America and Tendaguru Beds of Tanzania. Dinosaurs in China show some differences, with specialized sinraptorid theropods and unusual, long-necked sauropods like Mamenchisaurus.[48] 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.[49] Another notable evolutionary event of the Jurassic was the appearance of true birds, descended from maniraptoran coelurosaurians.[50]Skeleton of Marasuchus lilloensis, a dinosaur-like ornithodiranBy 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 dromaeosaurids, troodontids, 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.[48] 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.[49] Some sauropods also evolved tooth batteries, best exemplified by the rebbachisaurid Nigersaurus.[51]Full skeleton of an early carnivorous dinosaur, displayed in a glass case in a museumThe early forms Herrerasaurus (large), Eoraptor (small) and a Plateosaurus skullThere 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 Gondwana, abelisaurids were the common theropods, and titanosaurian sauropods the common herbivores. Finally, in Europe, dromaeosaurids, rhabdodontid iguanodontians, nodosaurid ankylosaurians, and titanosaurian sauropods were prevalent.[48] Flowering plants were greatly radiating,[49] with the first grasses appearing by the end of the Cretaceous.[52] 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.[49]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, sebecosuchians, turtles, lizards, snakes, sphenodontians, and choristoderans, also survived the event.[53]The surviving lineages of neornithine birds, including the ancestors of modern ratites, ducks and chickens, and a variety of waterbirds, diversified rapidly at the beginning of the Paleogene period, 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 Gastornis, eogruiids, bathornithids, ratites, geranoidids, mihirungs, 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.[54] Terror birds and bathornithids occupied carnivorous guilds alongside predatory mammals,[55][56] and ratites are still being 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.[57]ClassificationMain article: Dinosaur classificationDinosaurs 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.[29]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.[36] This basic form was modified by rotating the pubis backward to varying degrees in several groups (Herrerasaurus,[58] therizinosauroids,[59] dromaeosaurids,[60] and birds[50]). Saurischia includes the theropods (exclusively bipedal and with a wide variety of diets) and sauropodomorphs (long-necked herbivores which include advanced, quadrupedal groups).[61][62]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".)[29]Saurischian pelvis structure (left side) Tyrannosaurus pelvis (showing saurischian structure – left side) Ornithischian pelvis structure (left side) Edmontosaurus pelvis (showing ornithischian structure – left side)TaxonomyThe 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).[5] A more detailed version can be found at Dinosaur classification. The dagger (†) is used to signify groups with no living members.DinosauriaSaurischia ("lizard-hipped"; includes Theropoda and Sauropodomorpha)Theropoda (all bipedal; most were carnivorous)Artist's impression of six dromaeosaurid theropods: from left to right Microraptor, Velociraptor, Austroraptor, Dromaeosaurus, Utahraptor, and Deinonychus†Herrerasauria (early bipedal carnivores)†Coelophysoidea (small, early theropods; includes Coelophysis and close relatives)†Dilophosauridae (early crested and carnivorous theropods)†Ceratosauria (generally elaborately horned, the dominant southern carnivores of the Cretaceous)Tetanurae ("stiff tails"; includes most theropods)†Megalosauroidea (early group of large carnivores including the semiaquatic spinosaurids)†Carnosauria (Allosaurus and close relatives, like Carcharodontosaurus)Coelurosauria (feathered theropods, with a range of body sizes and niches)[3]†Compsognathidae (common early coelurosaurs with reduced forelimbs)†Tyrannosauridae (Tyrannosaurus and close relatives; had reduced forelimbs)†Ornithomimosauria ("ostrich-mimics"; mostly toothless; carnivores to possible herbivores)†Alvarezsauroidea (small insectivores with reduced forelimbs each bearing one enlarged claw)Maniraptora ("hand snatchers"; had long, slender arms and fingers)†Therizinosauria (bipedal herbivores with large hand claws and small heads)†Oviraptorosauria (mostly toothless; their diet and lifestyle are uncertain)†Archaeopterygidae (small, winged theropods or primitive birds)†Deinonychosauria (small- to medium-sized; bird-like, with a distinctive toe claw)Avialae (modern birds and extinct relatives)†Scansoriopterygidae (small primitive avialans with long third fingers)†Omnivoropterygidae (large, early short-tailed avialans)†Confuciusornithidae (small toothless avialans)†Enantiornithes (primitive tree-dwelling, flying avialans)Euornithes (advanced flying birds)†Yanornithiformes (toothed Cretaceous Chinese birds)†Hesperornithes (specialized aquatic diving birds)Aves (modern, beaked birds and their extinct relatives)Artist's impression of four macronarian sauropods: from left to right Camarasaurus, Brachiosaurus, Giraffatitan, and Euhelopus†Sauropodomorpha (herbivores with small heads, long necks, long tails)†Guaibasauridae (small, primitive, omnivorous sauropodomorphs)†Plateosauridae (primitive, strictly bipedal "prosauropods")†Riojasauridae (small, primitive sauropodomorphs)†Massospondylidae (small, primitive sauropodomorphs)†Sauropoda (very large and heavy, usually over 15 m (49 ft) long; quadrupedal)†Vulcanodontidae (primitive sauropods with pillar-like limbs)†Eusauropoda ("true sauropods")†Cetiosauridae ("whale reptiles")†Turiasauria (European group of Jurassic and Cretaceous sauropods)†Neosauropoda ("new sauropods")†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)†Heterodontosauridae (small basal ornithopod herbivores/omnivores with prominent canine-like teeth)†Thyreophora (armored dinosaurs; mostly quadrupeds)†Ankylosauria (scutes as primary armor; some had club-like tails)†Stegosauria (spikes and plates as primary armor)†Neornithischia ("new ornithischians")†Ornithopoda (various sizes; bipeds and quadrupeds; evolved a method of chewing using skull flexibility and numerous teeth)†Marginocephalia (characterized by a cranial growth)†Pachycephalosauria (bipeds with domed or knobby growth on skulls)†Ceratopsia (quadrupeds with frills; many also had horns)BiologyKnowledge about dinosaurs is derived from a variety of fossil and non-fossil records, including fossilized bones, feces, trackways, gastroliths, feathers, impressions of skin, internal organs and soft tissues.[63][64] Many fields of study contribute to our understanding of dinosaurs, including physics (especially biomechanics), chemistry, biology, and the earth sciences (of which paleontology is a sub-discipline).[65][66] Two topics of particular interest and study have been dinosaur size and behavior.[67]SizeMain article: Dinosaur sizeScale diagram comparing the average human to the largest known dinosaurs in five major clades: Sauropoda (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.[42] 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.[68] The mode of Mesozoic dinosaur body masses is between one and ten metric tonnes.[69] This contrasts sharply with the size of Cenozoic mammals, estimated by the National Museum of Natural History as about 2 to 5 kg (4.4 to 11.0 lb).[70]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.[71] 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 formations interpreted as dry or seasonally dry, and the ability to eat large quantities of low-nutrient browse would have been advantageous in such environments.[12]Largest and smallestScientists 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.[72]Comparative size of Giraffatitan to the average humanThe 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;[73] this mount is 12 meters (39 ft) tall and 21.8–22.5 meters (72–74 ft) long,[74][75] 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.[76]Comparative size of Eoraptor to the average humanThere 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); some of the longest were the 33.5 meters (110 ft) long Diplodocus hallorum[12] (formerly Seismosaurus) and the 33–34 meters (108–112 ft) long Supersaurus;[77] 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).[12] 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.[78] The largest known 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).[79][80] Other large carnivorous theropods included Giganotosaurus, Carcharodontosaurus and Tyrannosaurus.[80] Therizinosaurus and Deinocheirus were among the tallest of the theropods.The smallest dinosaur known is the bee hummingbird,[81] with a length of only 5 cm (2.0 in) and mass of around 1.8 g (0.063 oz).[82] The smallest known non-avialan dinosaurs were about the size of pigeons and were those theropods most closely related to birds.[83] For example, Anchiornis huxleyi is currently the smallest non-avialan dinosaur described from an adult specimen, with an estimated weight of 110 grams[84] and a total skeletal length of 34 cm (1.12 ft).[83][84] The smallest herbivorous non-avialan dinosaurs included Microceratus and Wannanosaurus, at about 60 cm (2.0 ft) long each.[5][85]BehaviorA 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 habitat, computer simulations of their biomechanics, and comparisons with modern animals in similar ecological niches.[65]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 Bernissart, Belgium, after they fell into a deep, flooded sinkhole and drowned.[86] 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,[87] although there is no evidence for specific herd structures.[88] 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.[89] The interpretation of dinosaurs as gregarious has also extended to depicting carnivorous theropods as pack hunters working together to bring down large prey.[90][91] 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 Deinonychus and Allosaurus can also be interpreted as the results of fatal disputes between feeding animals, as is seen in many modern diapsid predators.[92]Artist's rendering of two Centrosaurus apertus engaged in intra-specific combatThe crests and frills of some dinosaurs, like the marginocephalians, theropods 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.[93]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,[94] providing evidence that dinosaurs did indeed attack each other.[95] 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.[95] Cannibalism amongst some species of dinosaurs was confirmed by tooth marks found in Madagascar in 2003, involving the theropod Majungasaurus.[96]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 dromaeosaurids, Juravenator, and Megapnosaurus were likely nocturnal. Large and medium-sized herbivorous and omnivorous dinosaurs such as ceratopsians, sauropodomorphs, hadrosaurids, ornithomimosaurs may have been cathemeral, active during short intervals throughout the day, although the small ornithischian Agilisaurus was inferred to be diurnal.[97]Based on current fossil evidence from dinosaurs such as Oryctodromeus, some ornithischian species seem to have led a partially fossorial (burrowing) lifestyle.[98] Many modern birds are arboreal (tree climbing), and this was also true of many Mesozoic birds, especially the enantiornithines.[99] While some early bird-like species may have already been arboreal as well (including dromaeosaurids such as Microraptor[100]) 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,[101] whether diplodocids could create sonic booms via whip-like tail snapping,[102] and whether sauropods could float.[103]CommunicationArtist's impression of a striking and unusual visual display in a Lambeosaurus magnicristatusModern 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.[104] Reconstruction of the plumage color of Anchiornis huxleyi, suggest the importance of color in visual communication in non-avian dinosaurs.[105] 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.[104]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.[106] 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.[107] The crests of the Lambeosaurini and nasal chambers of ankylosaurids have been suggested to function in vocal resonance,[108][109] 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.[104]Reproductive biologySee also: Dinosaur eggThree bluish eggs with black speckling sit atop a layer of white mollusk shell pieces, surrounded by sandy ground and small bits of bluish stoneNest of a plover (Charadrius)All dinosaurs lay amniotic eggs with hard shells made mostly of calcium carbonate.[110] Eggs are usually laid in a nest. Most species create somewhat elaborate nests, which can be cups, domes, plates, beds scrapes, mounds, or burrows.[111] 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.[112]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.[113]Fossil interpreted as a nesting oviraptorid Citipati at the American Museum of Natural History. Smaller fossil far right showing inside one of the eggs.Another widespread trait among modern birds 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, suggesting this behavior might also have been common to all dinosaurs.[114] There is evidence that other non-theropod dinosaurs, like Patagonian titanosaurian sauropods, also nested in large groups.[115] A specimen of the Mongolian oviraptorid Citipati osmolskae was discovered in a chicken-like brooding position in 1993,[116] which may indicate that they had begun using an insulating layer of feathers to keep the eggs warm.[117] Parental care being a trait common to all dinosaurs is supported by other finds. For example, a dinosaur embryo (pertaining to the prosauropod Massospondylus) was found without teeth, indicating that some parental care was required to feed the young dinosaurs.[118] Trackways have also confirmed parental behavior among ornithopods from the Isle of Skye in northwestern Scotland.[119] Nests and eggs have been found for most major groups of dinosaurs, and it appears likely that all dinosaurs cared for their young to some extent either before or shortly after hatching.[120]PhysiologyMain article: Physiology of dinosaursBecause 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.[121] 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.[122]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.[123]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.[124][125]Comparison between the air sacs of an abelisaur and a birdIn 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.[126] 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.[127]Like other reptiles, dinosaurs are primarily uricotelic, that is, their kidneys extract nitrogenous wastes from their bloodstream and excrete it as uric acid instead 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.[128][129][130] However, at least some modern birds (such as hummingbirds) can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.[131] They also excrete creatine, rather than creatinine like mammals.[132] This material, as well as the output of the intestines, emerges from the cloaca.[133][134] 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.[135]Origin of birdsMain article: Origin of birdsThe possibility that dinosaurs were the ancestors of birds was first suggested in 1868 by Thomas Henry Huxley.[136] 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.[137] However, as later discoveries showed, clavicles (or a single fused wishbone, which derived from separate clavicles) were not actually absent;[50] they had been found as early as 1924 in Oviraptor, but misidentified as an interclavicle.[138] In the 1970s, John Ostrom revived the dinosaur–bird theory,[139] which gained momentum in the coming decades with the advent of cladistic analysis,[140] and a great increase in the discovery of small theropods and early birds.[31] 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.[3][50] Birds share over a hundred distinct anatomical features with theropod dinosaurs, which are now generally accepted to have been their closest ancient relatives.[141] They are most closely allied with maniraptoran coelurosaurs.[50] 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,[142] or that maniraptoran theropods are the ancestors of birds but themselves are not dinosaurs, only convergent with dinosaurs.[143]FeathersMain article: Feathered dinosaursVarious feathered non-avian dinosaurs, including Archaeopteryx, Anchiornis, Microraptor and ZhenyuanlongFeathers 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.[144] Direct fossil evidence of feathers or feather-like structures has been discovered in a diverse array of species in many non-avian dinosaur groups,[3] both among saurischians and ornithischians. Simple, branched, feather-like structures are known from heterodontosaurids, primitive neornithischians[145] and theropods,[146] 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.[50][147] Feather-like structures known as pycnofibres have also been found in pterosaurs,[148] suggesting the possibility that feather-like filaments may have been common in the bird lineage and evolved before the appearance of dinosaurs themselves.[144] 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.[149]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.[150] 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.[151]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,[152][153][154] and that maniraptoran dinosaurs with vaned feathers were not actually dinosaurs, but convergent with dinosaurs.[143][153] 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.[155]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.[3][156][157]SkeletonBecause 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, pubis, wrist (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.[158]Soft anatomyPneumatopores on the left ilium of Aerosteon riocoloradensisLarge 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.[159] 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.[160][161]Behavioral evidenceFossils of the troodonts Mei and Sinornithoides demonstrate that some dinosaurs slept with their heads tucked under their arms.[162] 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.[163] 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.[164]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.[165]Extinction of major groupsMain article: Cretaceous–Paleogene extinction eventThe discovery that birds are a type of dinosaur showed that dinosaurs in general are not, in fact, extinct as is commonly stated.[166] However, all non-avian dinosaurs 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.[167] Many other groups of animals also became extinct at this time, including ammonites (nautilus-like mollusks), mosasaurs, plesiosaurs, pterosaurs, and many groups of mammals.[10] 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.[168][169][170]Impact eventMain article: Chicxulub craterThe 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.[171] 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.[172] 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.[173][174] 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.[175]Deccan TrapsMain article: Deccan TrapsBefore 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.[176][177]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.[177] 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]."[176][177]In the years when the Deccan Traps theory 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.[178]Possible Paleocene survivorsMain article: Paleocene dinosaursNon-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 Paleocene epoch, 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.[179] 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.[180] 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.[181][182] 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.[183] If correct, the presence of a handful of dinosaurs in the early Paleocene would not change the underlying facts of the extinction.[181]History of studyFurther information: History of paleontologyDinosaur fossils have been known for millennia, although their true nature was not recognized. The Chinese, whose modern word for dinosaur is kǒnglóng (恐龍, or "terrible dragon"), 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.[184] Villagers in central China have long unearthed fossilized "dragon bones" for use in traditional medicines, a practice that continues today.[185] In Europe, dinosaur fossils were generally believed to be the remains of giants and other biblical creatures.[186]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,[187] was recovered from a limestone quarry at Cornwell near Chipping Norton, Oxfordshire, 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",[188][189] that had been found in Caswell, near Witney, Oxfordshire.[190]William BucklandBetween 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.[187][191] 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.[192][193]The study of these "great fossil lizards" soon became of great interest to European and American scientists, and in 1842 the English paleontologist Richard Owen coined the term "dinosaur". He recognized that the remains that had been found so far, Iguanodon, Megalosaurus 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.[194]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.[195]Edward Drinker CopeOthniel Charles MarshMarsh's 1896 illustration of the bones of Stegosaurus, a dinosaur he described and named in 1877Dinosaur 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.[196]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,[197] although it was 1994 before an Antarctic species, the theropod Cryolophosaurus ellioti, was formally named and described in a scientific journal.[198]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.[151]"Dinosaur renaissance"Main article: Dinosaur renaissancePaleontologist Robert T. Bakker with 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-blooded. Vertebrate paleontology has become a global science. Major new dinosaur discoveries have been made by paleontologists working in previously unexploited regions, including India, South America, Madagascar, Antarctica, and most significantly China (the amazingly well-preserved feathered dinosaurs[3] 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.[199][show]Timeline of notable dinosaur taxonomic descriptionsSoft tissue and DNAOne 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 coelurosaur, Scipionyx samniticus. The fossil includes portions of the intestines, colon, liver, muscles, and windpipe of this immature dinosaur.[63]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.[64] 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 vessels, bone 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.[64]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 dinosaur, Brachylophosaurus 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 Tyrannosaurus bone. 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.[200]The extraction of ancient DNA from dinosaur fossils has been reported on two separate occasions;[201] upon further inspection and peer review, however, neither of these reports could be confirmed.[202] 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.[203] In addition, several proteins, including hemoglobin,[204] have putatively been detected in dinosaur fossils.[205][206]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.[207][208]Outdated Iguanodon statues created by Benjamin Waterhouse Hawkins for the Crystal Palace Park in 1853The battles that may have occurred between Tyrannosaurus rex and Triceratops are a recurring theme in popular science and dinosaurs' depiction in culture.Cultural depictionsMain article: Cultural depictions of dinosaursBy 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.[209]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.[210] 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.[211]The popular preoccupation with dinosaurs has ensured their appearance in literature, film, and other media. Beginning in 1852 with a passing mention in Charles Dickens' Bleak House,[212] dinosaurs have been featured in large numbers of fictional works. Jules Verne's 1864 novel Journey to the Center of the Earth, Sir 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.[213]See alsoAnimal trackDinosaur diet and feedingEvolutionary history of lifeLists of dinosaur-bearing stratigraphic unitsList of dinosaur generaLiving dinosaurAanteroomarmoryassembly roomatticBbackroomballroombasementbathroombedroomboardroomboiler roomboudoirbreakfast nookbreakfast roomCcabincellcellarchamberchanging roomchapelclassroomclean roomcloakroomcold roomcommon roomconference roomconservatorycontrol roomcourtroomcubbyDdarkroomdendining roomdormitorydrawing roomdressing roomdungeonEemergency roomengine roomentryFfamily roomfitting roomformal dining roomfoyerfront roomGgame roomgaragegarretgreat roomgreen roomgrottoguest roomgymHhallhallwayhomeroomhospital roomhotel roomIinglenookJjail cellKkeepkitchenkitchenetteLladies' roomlarderlaundry roomlibraryliving roomlobbylocker roomloftloungelunchroomMmaid's roommailroommen's roommorning roommotel roommud roomNnewsroomnurseryOofficeoperating roomPpanic roompantryparlorplayroompool roompowder roomprison cellRrec roomrecovery roomrestroom roomrumpus roomSsalesroomsalonschoolroomscreen porchsculleryshowroomsick roomsitting roomsolariumstaff roomstateroomstockroomstoreroomstudiostudysuitesunroomTtack roomUutility roomVvestibulevisitor's roomWwaiting roomwardroomwashroomwater closetweight roomwine cellar

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