Tyrannosaurus

Tyrannosaurus is a genus of tyrannosaurid dinosaur from 77-65 million years ago (ma/mya) of the Late Cretaceous. It was native to North America and Asia. Tyrannosaurus varied in size between the four species (T. bataar, T. lancensis, T. rex, T. torosus) from the 17 ft (5.1 m) Tyrannosaurus lancensis to the 41 ft (12.4 m) famous Tyrannosaurus rex. Tyrannosaurus is the largest carnivorous dinosaur to roam Western North America and Asia. Tyrannosaurus is defined by puny arms only two feet (0.6 m) in length (some such as in T. lancensis were even smaller because of dwarfism) and giant jaws (mandibles) with gargantuan teeth. Tyrannosaurus is one of the most famous of the dinosaurs because when it was described in 1905 it was the largest known theropod discovered. Not until the late 1990's and early 2000's Tyrannosaurus rex reigned as the largest carnivorous dinosaur (the largest theropod is actually Spinosaurus marocannus at 59 ft (17.8 m) in length).

In contrast to common belief, Tyrannosaurus Rex wasn't the only Tyrannosaurus around:

(measurements given are based on found fossils and estimates)

Tyrannosaurus torosus (Russel, 1970)

Skull length : 1.1 m.

Total length : 9 m.

Hip height : 2.5 m.

Weight : 2.3 tonnes

Tyrannosaurus torosus, commonly called Daspletosaurus (pronounced /dæsˌpliːtɵˈsɔrəs/ das-PLEET-o-SAWR-əs, meaning 'frightful lizard') is a genus of tyrannosaurid theropod dinosaur that lived in western North America between 77 and 74 million years ago, during the Late Cretaceous Period. Fossils of the only named species (D. torosus) were found in Alberta, although other possible species from Alberta, Montana and New Mexico await description. Including these undescribed species, Daspletosaurus is the most species-rich genus of tyrannosaur.

Daspletosaurus is closely related to the much larger and more recent Tyrannosaurus. Like most known tyrannosaurids, it was a multi-ton bipedal predator equipped with dozens of large, sharp teeth. Daspletosaurus had the small forelimbs typical of tyrannosaurids, although they were proportionately longer than in other genera.

As an apex predator, Daspletosaurus was at the top of the food chain, probably preying on large dinosaurs like the ceratopsid Centrosaurus and the hadrosaur Hypacrosaurus. In some areas, Daspletosaurus coexisted with another tyrannosaurid, Gorgosaurus, though there is some evidence of niche differentiation between the two. While Daspletosaurus fossils are rarer than other tyrannosaurids, the available specimens allow some analysis of the biology of these animals, including social behavior, diet and life history. {| class="toc" id="toc"

Contents
[hide]*1 Description
 * 2 Classification and systematics
 * 3 Discovery and naming
 * 3.1 Unnamed species
 * 4 Paleobiology
 * 4.1 Coexistence with Gorgosaurus
 * 4.2 Social behavior
 * 4.3 Life history
 * 5 Paleoecology
 * 6 In popular culture
 * 7 References
 * 8 External links
 * }

Description
While very large by the standard of modern predators, Daspletosaurus was not the largest tyrannosaurid. Adults could reach a length of 8–9 meters (26–30 ft) from snout to tail.[1] Mass estimates have centered around 2.5 tonnes (2.75 short tons)[1] [2] [3] but have ranged between 1.8 tonnes (2 tons)[4] and 3.8 tonnes (4.1 tons).[5] Daspletosaurus had a massive skull that could reach more than 1 meter (3.3 ft) in length.[1] The bones were heavily constructed and some, including the nasal bones on top of the snout, were fused for strength. Large fenestrae (openings) in the skull reduced its weight. An adult Daspletosaurus was armed with about six dozen teeth that were very long but oval in cross section rather than blade-like. Unlike its other teeth, those in the premaxilla at the end of the upper jaw had a D-shaped cross section, an example of heterodonty always seen in tyrannosaurids. Unique skull features included the rough outer surface of the maxilla (upper jaw bone) and the pronounced crests around the eyes on the lacrimal, postorbital, and jugal bones. The orbit (eye socket) was a tall oval, somewhere in between the circular shape seen in Gorgosaurus and the 'keyhole' shape of Tyrannosaurus.[6] [7] [8]

Daspletosaurus shared the same body form as other tyrannosaurids, with a short, S-shaped neck supporting the massive skull. It walked on its two thick hindlimbs, which ended in four-toed feet, although the first digit (the hallux) did not contact the ground. In contrast, the forelimbs were extremely small and bore only two digits, although Daspletosaurus had the longest forelimbs in proportion to body size of any tyrannosaurid. A long, heavy tail served as a counterweight to the head and torso, with the center of gravity over the hips.[1] [8]

[edit] Classification and systematics
Daspletosaurus belongs in the subfamily Tyrannosaurinae within the family Tyrannosauridae, along with Tarbosaurus, Tyrannosaurus and possibly Alioramus. Animals in this subfamily are more closely related to Tyrannosaurus than to Albertosaurus and are known for their robust build with proportionally larger skulls and longer femora than in the other subfamily, the Albertosaurinae.[8] [10]

Daspletosaurus is usually considered to be closely related to Tyrannosaurus rex, or even a direct ancestor through anagenesis.[11] Gregory Paul reassigned D. torosus to the genus Tyrannosaurus, creating the new combination Tyrannosaurus torosus,[2] but this has not been generally accepted.<sup class="reference" id="cite_ref-carr1999_5-1">[6] <sup class="reference" id="cite_ref-holtz2004_7-3">[8] Many researchers believe Tarbosaurus and Tyrannosaurus to be sister taxa or even to be the same genus, with Daspletosaurus a more basal relative.<sup class="reference" id="cite_ref-holtz2004_7-4">[8] <sup class="reference" id="cite_ref-carretal2005_8-1">[9] On the other hand, Phil Currie and colleagues find Daspletosaurus to be more closely related to Tarbosaurus and other Asian tyrannosaurids like Alioramus than to the North American Tyrannosaurus.<sup class="reference" id="cite_ref-currieetal2003_9-2">[10] The systematics (evolutionary relationships) of Daspletosaurus may become clearer once all the species have been described.

[edit] Discovery and naming
The type specimen of Daspletosaurus torosus (CMN 8506) is a partial skeleton including the skull, the shoulder, a forelimb, the pelvis, a femur and all of the vertebrae from the neck, torso and hip, as well as the first eleven tail vertebrae. It was discovered in 1921 by Charles Mortram Sternberg, who thought it was a new species of Gorgosaurus. It was not until 1970 that the specimen was fully described by Dale Russell, who made it the type of a new genus, Daspletosaurus, from the Greek stems δασπλητo-/daspleto- ('frightful') and σαυρος/sauros ('lizard').<sup class="reference" id="cite_ref-liddellscott_11-0">[12] The type species is D. torosus, which is Latin for 'muscular' or 'brawny.'<sup class="reference" id="cite_ref-russell1970_0-4">[1] Aside from the type, there is only one other well-known specimen, a complete skeleton discovered in 2001. Both specimens were recovered from the Oldman Formation in the Judith River Group of Alberta. A specimen from the younger Horseshoe Canyon Formation in Alberta has been reassigned to Albertosaurus sarcophagus.<sup class="reference" id="cite_ref-currie2003a_6-1">[7] The Oldman Formation was deposited during the middle Campanian stage of the Late Cretaceous, from about 77 to 76 Ma (million years ago).<sup class="reference" id="cite_ref-eberthhamblin1993_12-0">[13]

[edit] Unnamed species
Two or three additional species have been assigned to the genus Daspletosaurus over the years, although as of 2007 none of these species have received a proper description or scientific name. In the meantime, all are assigned to Daspletosaurus sp. although this does not imply that they all are the same species.<sup class="reference" id="cite_ref-currie2003a_6-2">[7] <sup class="reference" id="cite_ref-holtz2004_7-5">[8] Along with the holotype, Russell designated a specimen collected by Barnum Brown in 1913 as the paratype of D. torosus. This specimen (AMNH 5438) consists of parts of the hindleg, the pelvis and some of its associated vertebrae. It was discovered in the upper part of the Oldman Formation in Alberta.<sup class="reference" id="cite_ref-currie2003a_6-3">[7] This upper section has since been renamed the Dinosaur Park Formation, which dates back to the middle Campanian, from 76–74 Ma (million years ago).<sup class="reference" id="cite_ref-eberthhamblin1993_12-1">[13] In 1914, Brown collected a nearly complete skeleton and skull; forty years later his American Museum of Natural History sold this specimen to the Field Museum of Natural History in Chicago. It was mounted for display in Chicago and labeled as Albertosaurus libratus for many years, but after several skull features were later found to be modeled in plaster, including most of the teeth, the specimen (FMNH PR308) was reassigned to Daspletosaurus.<sup class="reference" id="cite_ref-carr1999_5-2">[6] A total of eight specimens have been collected from the Dinosaur Park Formation over the years since, most of them within the boundaries of Dinosaur Provincial Park. Phil Currie believes that the Dinosaur Park specimens represent a new species of Daspletosaurus, distinguished by certain features of the skull. Pictures of this new species have been published, but it still awaits a name and full description in print.<sup class="reference" id="cite_ref-currie2003a_6-4">[7]

A new tyrannosaur specimen (OMNH 10131), including skull fragments, ribs and parts of the hindlimb, was reported from New Mexico in 1990 and assigned to the now-defunct genus Aublysodon.<sup class="reference" id="cite_ref-lehmancarpenter1990_13-0">[14] Many later authors have reassigned this specimen, along with a few others from New Mexico, to yet another unnamed species of Daspletosaurus.<sup class="reference" id="cite_ref-currie2003a_6-5">[7] <sup class="reference" id="cite_ref-holtz2004_7-6">[8] <sup class="reference" id="cite_ref-carrwilliamson2000_14-0">[15] However, research published in 2010 showed that this species, from the Hunter Wash Member of the Kirtland Formation, is actually a more primitive tyrannosauroid, and was classified in the genus Bistahieversor.<sup class="reference" id="cite_ref-carrWilliamson2010_15-0">[16] There is currently disagreement over the age of the Kirtland Formation, with some workers claiming a late Campanian age,<sup class="reference" id="cite_ref-sullivanlucas2006_16-0">[17] while others suggest a younger age in the early Maastrichtian stage.<sup class="reference" id="cite_ref-ryan1997_17-0">[18]

In 1992, Jack Horner and colleagues published an extremely preliminary report of a tyrannosaurid from the upper parts of the Campanian Two Medicine Formation in Montana, which was interpreted as a transitional species between Daspletosaurus and the later Tyrannosaurus.<sup class="reference" id="cite_ref-horneretal1992_10-1">[11] Another partial skeleton was reported from the Upper Two Medicine in 2001, preserving the remains of a juvenile hadrosaur in its abdominal cavity. This specimen was assigned to Daspletosaurus but not to any particular species.<sup class="reference" id="cite_ref-varricchio2001_18-0">[19] The remains of at least three more Daspletosaurus have also been described in a Two Medicine bonebed.<sup class="reference" id="cite_ref-currieetal2005_19-0">[20] These specimens have not been described in detail, but Currie believes all of the Two Medicine material represents an as-yet-unnamed third species of Daspletosaurus.<sup class="reference" id="cite_ref-currie2003a_6-6">[7]

[edit] Coexistence with Gorgosaurus
In the late Campanian of North America, Daspletosaurus was a contemporary of the albertosaurine tyrannosaurid Gorgosaurus. This is one of the few examples of two tyrannosaur genera coexisting. In modern predator guilds, similar-sized predators are separated into different ecological niches by anatomical, behavioral or geographical differences that limit competition.<sup class="reference" id="cite_ref-farlowpianka2002_20-0">[21] Several studies have attempted to explain niche differentiation in Daspletosaurus and Gorgosaurus.

Dale Russell hypothesized that the more lightly built and more common Gorgosaurus may have preyed on the abundant hadrosaurs of the time, while the more robust and less common Daspletosaurus may have specialized on the less prevalent but better-defended ceratopsids, which may have been more difficult to hunt.<sup class="reference" id="cite_ref-russell1970_0-5">[1] However, a specimen of Daspletosaurus (OTM 200) from the Two Medicine Formation preserves the digested remains of a juvenile hadrosaur in its gut region.<sup class="reference" id="cite_ref-varricchio2001_18-1">[19] The higher and broader muzzles of tyrannosaurines like Daspletosaurus are mechanically stronger than the lower snouts of albertosaurines like Gorgosaurus, although tooth strengths are similar between the two groups. This may indicate a difference in feeding mechanics or diet.<sup class="reference" id="cite_ref-snivelyetal2006_21-0">[22]

Other authors have suggested that competition was limited by geographical separation. Unlike some other groups of dinosaurs, there appears to be no correlation with distance from the sea. Neither Daspletosaurus nor Gorgosaurus was more common at higher or lower elevations than the other.<sup class="reference" id="cite_ref-farlowpianka2002_20-1">[21] However, while there is some overlap, Gorgosaurus appears to be more common at northern latitudes, with species of Daspletosaurus more abundant to the south. The same pattern is seen in other groups of dinosaurs. Chasmosaurine ceratopsians and hadrosaurine hadrosaurs are also more common in the Two Medicine Formation and in southwestern North America during the Campanian. [http://en.wikipedia.org/wiki/Thomas_R._Holtz,_Jr. Thomas Holtz] has suggested that this pattern indicates shared ecological preferences between tyrannosaurines, chasmosaurines and hadrosaurines. Holtz notes that, at the end of the later Maastrichtian stage, tyrannosaurines like Tyrannosaurus rex, hadrosaurines and chasmosaurines like Triceratops were widespread throughout western North America, while albertosaurines and centrosaurines went extinct, and lambeosaurines were very rare.<sup class="reference" id="cite_ref-holtz2004_7-7">[8]

[edit] Social behavior
A young specimen of the Dinosaur Park Daspletosaurus species (TMP 94.143.1) shows bite marks on the face that were inflicted by another tyrannosaur. The bite marks are healed over, indicating that the animal survived the bite. A full-grown Dinosaur Park Daspletosaurus (TMP 85.62.1) also exhibits tyrannosaur bite marks, showing that attacks to the face were not limited to younger animals. While it is possible that the bites were attributable to other species, intraspecific aggression, including facial biting, is very common among predators. Facial bites are seen in other tyrannosaurs like Gorgosaurus and Tyrannosaurus, as well as in other theropod genera like Sinraptor and Saurornitholestes. Darren Tanke and Phil Currie hypothesize that the bites are due to intraspecific competition for territory or resources, or for dominance within a social group.<sup class="reference" id="cite_ref-tankecurrie1998_22-0">[23]

Evidence that Daspletosaurus lived in social groups comes from a bonebed found in the Two Medicine Formation of Montana. The bonebed includes the remains of three Daspletosaurus, including a large adult, a small juvenile, and another individual of intermediate size. At least five hadrosaurs are preserved at the same location. Geologic evidence indicates that the remains were not brought together by river currents but that all of the animals were buried simultaneously at the same location. The hadrosaur remains are scattered and bear numerous marks from tyrannosaur teeth, indicating that the Daspletosaurus were feeding on the hadrosaurs at the time of death. The cause of death is unknown. Currie speculates that the daspletosaurs formed a pack, although this cannot be stated with certainty.<sup class="reference" id="cite_ref-currieetal2005_19-1">[20] Other scientists are skeptical of the evidence for social groups in Daspletosaurus and other large theropods;<sup class="reference" id="cite_ref-eberthmccrea2001_23-0">[24] Brian Roach and Daniel Brinkman have suggested that Daspletosaurus social interaction would have more closely resembled the modern Komodo dragon, where non-cooperative individuals mob carcasses, frequently attacking and even cannibalizing each other in the process.<sup class="reference" id="cite_ref-BRDB07_24-0">[25]

[edit] Life history
Paleontologist Gregory Erickson and colleagues have studied the growth and life history of tyrannosaurids. Analysis of bone histology can determine the age of a specimen when it died. Growth rates can be examined when the age of various individuals are plotted against their size on a graph. Erickson has shown that after a long time as juveniles, tyrannosaurs underwent tremendous growth spurts for about four years midway through their lives. After the rapid growth phase ended with sexual maturity, growth slowed down considerably in adult animals. Erickson only examined Daspletosaurus from the Dinosaur Park Formation, but these specimens show the same pattern. Compared to albertosaurines, Daspletosaurus showed a faster growth rate during the rapid growth period due to its higher adult weight. The maximum growth rate in Daspletosaurus was 180 kilograms (400 lb) per year, based on a mass estimate of 1800 kilograms (2 tons) in adults. Other authors have suggested higher adult weights for Daspletosaurus; this would change the magnitude of the growth rate but not the overall pattern.<sup class="reference" id="cite_ref-ericksonetal2004_3-1">[4]

By tabulating the number of specimens of each age group, Erickson and his colleagues were able to draw conclusions about life history in a population of Albertosaurus. Their analysis showed that while juveniles were rare in the fossil record, subadults in the rapid growth phase and adults were far more common. While this could be due to preservation or collection biases, Erickson hypothesized that the difference was due to low mortality among juveniles over a certain size, which is also seen in some modern large mammals like elephants. This low mortality may have resulted from a lack of predation, since tyrannosaurs surpassed all contemporaneous predators in size by the age of two. Paleontologists have not found enough Daspletosaurus remains for a similar analysis, but Erickson notes that the same general trend seems to apply.<sup class="reference" id="cite_ref-ericksonetal2006_25-0">[26]

[edit] Paleoecology
All known Daspletosaurus fossils have been found in formations dating to the middle to late Campanian stage of the Late Cretaceous Period, between 77 and 74 million years ago. Since the middle of the Cretaceous, North America had been divided in half by the Western Interior Seaway, with much of Montana and Alberta below the surface. However, the uplift of the Rocky Mountains in the Laramide Orogeny to the west, which began during the time of Daspletosaurus, forced the seaway to retreat eastwards and southwards. Rivers flowed down from the mountains and drained into the seaway, carrying sediment along with them that formed the Two Medicine Formation, the Judith River Group, and other sedimentary formations in the region. About 73 million years ago, the seaway began to advance westwards and northwards again, and the entire region was covered by the Bearpaw Sea, represented throughout the western United States and Canada by the massive Bearpaw Shale.<sup class="reference" id="cite_ref-englishjohnston2004_26-0">[27] <sup class="reference" id="cite_ref-eberth1997_27-0">[28] <sup class="reference" id="cite_ref-rogers1997_28-0">[29]

Daspletosaurus lived in a vast floodplain along the western shore of the interior seaway. Large rivers watered the land, occasionally flooding and blanketing the region with new sediment. When water was plentiful, the region could support a great deal of plant and animal life, but periodic droughts also struck the region, resulting in mass mortality as preserved in the many bonebed deposits found in Two Medicine and Judith River sediments, including the Daspletosaurus bonebed.<sup class="reference" id="cite_ref-rogers1990_29-0">[30] Similar conditions exist today in East Africa.<sup class="reference" id="cite_ref-falconlang2003_30-0">[31] Volcanic eruptions from the west periodically blanketed the region with ash, also resulting in large-scale mortality, while simultaneously enriching the soil for future plant growth. It is these ash beds that allow precise radiometric dating as well. Fluctuating sea levels also resulted in a variety of other environments at different times and places within the Judith River Group, including offshore and nearshore marine habitats, coastal wetlands, deltas and lagoons, in addition to the inland floodplains.<sup class="reference" id="cite_ref-eberth1997_27-1">[28] The Two Medicine Formation was deposited at higher elevations farther inland than the other two formations.<sup class="reference" id="cite_ref-rogers1997_28-1">[29] The excellent vertebrate fossil record of Two Medicine and Judith River rocks resulted from a combination of abundant animal life, periodic natural disasters, and the deposition of large amounts of sediment. Many types of freshwater and estuarine fish are represented, including sharks, rays, sturgeons, gars and others. The Judith River Group preserves the remains of many aquatic amphibians and reptiles, including frogs, salamanders, turtles, Champsosaurus and crocodilians. Terrestrial lizards, including whiptails, skinks, monitors and alligator lizards have also been discovered. Azhdarchid pterosaurs, and neornithean birds like Apatornis flew overhead, while the enantiornithiform bird Avisaurus and several varieties of multituberculate, marsupial and placental mammals scurried beneath the feet of Daspletosaurus and other dinosaurs.<sup class="reference" id="cite_ref-eberth1997_27-2">[28]

In the Oldman Formation, Daspletosaurus torosus could have preyed upon hadrosaurs like Brachylophosaurus and Hypacrosaurus, small ornithopods like Orodromeus, ceratopsians like Centrosaurus, pachycephalosaurs, ornithomimids, therizinosaurs and possibly ankylosaurs. Other predators included troodonts, oviraptorosaurs, the dromaeosaur Saurornitholestes and possibly an albertosaurine tyrannosaur (genus currently unknown). The Dinosaur Park and Two Medicine Formations have faunas comparable to the Oldman, with the Dinosaur Park in particular preserving an unrivaled array of dinosaurs.<sup class="reference" id="cite_ref-eberth1997_27-3">[28] The albertosaurine Gorgosaurus lived alongside species of Daspletosaurus in the Dinosaur Park and Upper Two Medicine environments.<sup class="reference" id="cite_ref-farlowpianka2002_20-2">[21] Young tyrannosaurs may have filled the niches in between adult tyrannosaurs and smaller theropods, which were separated by two orders of magnitude in mass.<sup class="reference" id="cite_ref-russell1970_0-6">[1] <sup class="reference" id="cite_ref-holtz2004_7-8">[8] <sup class="reference" id="cite_ref-snivelyetal2006_21-1">[22] <sup class="reference" id="cite_ref-farlow1976_31-0">[32]

[edit] In popular culture
Daspletosaurus was featured in the Discovery Channel's Dinosaur Planet in the episode "Little Das' Hunt", where a family of daspletosaurs hunted a wounded adolescent Maiasaura.

Tyrannosaurus bataar (Maleev, 1955)

Skull length : 1.35 m.

Total length : 10 m.

Hip height : 2.9 m.

Weight : 5 tonnes

Tyrannosaurus bataar or commonly called Tarbosaurus (pronounced /ˌtɑrbɵˈsɔrəs/ TAR-bo-SAWR-əs; meaning "terrifying lizard") is a genus of tyrannosaurid theropod dinosaur that flourished in Asia between 70 and 65 million years ago, at the end of the Late Cretaceous Period. Fossils have been recovered in Mongolia with more fragmentary remains found further afield in parts of China. Although many species have been named, modern paleontologists recognize only one, T. bataar, as valid. Some experts contend that this species is actually an Asian representative of the North American genus Tyrannosaurus; if true, this would invalidate the genus Tarbosaurus altogether.

Tarbosaurus and Tyrannosaurus are considered closely related genera, even if they are not synonymous. Alioramus, also from Mongolia, is thought by some authorities to be the closest relative of Tarbosaurus. Like most known tyrannosaurids, Tarbosaurus was a large bipedal predator, weighing more than a ton and equipped with dozens of large, sharp teeth. It had a unique locking mechanism in its lower jaw and the smallest forelimbs relative to body size of all tyrannosaurids, renowned for their disproportionately tiny, two-fingered forelimbs.

Tarbosaurus lived in a humid floodplain criss-crossed by river channels. In this environment, it was an apex predator at the top of the food chain, probably preying on other large dinosaurs like the hadrosaur Saurolophus or the sauropod Nemegtosaurus. Tarbosaurus is very well-represented in the fossil record, known from dozens of specimens, including several complete skulls and skeletons. These remains have allowed scientific studies focusing on its phylogeny, skull mechanics, and brain structure. {| class="toc" id="toc"

Contents
[hide]*1 Description
 * 2 Classification and systematics
 * 3 Discovery and naming
 * 3.1 Possible synonyms
 * 4 Paleobiology
 * 4.1 Skull mechanics
 * 4.2 Brain structure
 * 5 Paleoecology
 * 6 References
 * 7 External links
 * }

[edit] Description
Although smaller than Tyrannosaurus, Tarbosaurus was one of the largest tyrannosaurids. The largest known individuals were between 10 and 12 meters (33 to 40 ft) long, each with a head held up to 5 meters (16.5 ft) above the ground.<sup class="reference" id="cite_ref-maleev1955b_0-0">[1] The mass of a fully grown individual has never been published, although it is considered comparable to or slightly smaller than Tyrannosaurus.<sup class="reference" id="cite_ref-holtz2004_1-0">[2]

The largest known Tarbosaurus skull is more than 1.3 meters (4 ft) long, larger than all other tyrannosaurids except Tyrannosaurus.<sup class="reference" id="cite_ref-holtz2004_1-1">[2] The skull was tall, like that of Tyrannosaurus, but not as wide, especially towards the rear. The unexpanded rear of the skull meant that Tarbosaurus eyes did not face directly forwards, suggesting that it lacked the binocular vision of Tyrannosaurus. Large fenestrae (openings) in the skull reduced its weight. Between 60 and 64 teeth lined its jaws, slightly more than in Tyrannosaurus but fewer than in smaller tyrannosaurids like Gorgosaurus and Alioramus. Most of its teeth were oval in cross section, although the teeth of the premaxilla at the tip of the upper jaw had a D-shaped cross section. This heterodonty is characteristic of the family. The longest teeth were in the maxilla (upper jaw bone), with crowns up to 85 millimeters (3.3 in) long. In the lower jaw, a ridge on the outer surface of the angular bone articulated with the rear of the dentary bone, creating a locking mechanism unique to Tarbosaurus and Alioramus. Other tyrannosaurids lacked this ridge and had more flexibility in the lower jaw.<sup class="reference" id="cite_ref-hurumsabath2003_2-0">[3]

Tyrannosaurids varied little in body form, and Tarbosaurus was no exception. The head was supported by an S-shaped neck, while the rest of the vertebral column, including the long tail, was held horizontally. Tarbosaurus had tiny, two-fingered forelimbs, which were smaller relative to its body size than those of any other member of the family. In contrast, the three-toed hindlimbs were long and thick, supporting the body in a bipedal posture. The long, heavy tail served as a counterweight to the head and torso and placed the center of gravity over the hips.<sup class="reference" id="cite_ref-maleev1955b_0-1">[1] <sup class="reference" id="cite_ref-holtz2004_1-2">[2]

[edit] Classification and systematics
Tarbosaurus is classified as a theropod in the subfamily Tyrannosaurinae within the family Tyrannosauridae. Other members include Tyrannosaurus and the earlier Daspletosaurus, both from North America,<sup class="reference" id="cite_ref-carretal2005_3-1">[4] and possibly the Mongolian genus Alioramus.<sup class="reference" id="cite_ref-hurumsabath2003_2-1">[3] <sup class="reference" id="cite_ref-currieetal2003_4-1">[5] Animals in this subfamily are more closely related to Tyrannosaurus than to Albertosaurus and are known for their robust build with proportionally larger skulls and longer femurs than in the other subfamily, the Albertosaurinae.<sup class="reference" id="cite_ref-holtz2004_1-3">[2]

Tarbosaurus bataar was originally described as a species of Tyrannosaurus,<sup class="reference" id="cite_ref-maleev1955a_5-0">[6] an arrangement that has been supported by more recent studies.<sup class="reference" id="cite_ref-carretal2005_3-2">[4] <sup class="reference" id="cite_ref-carpenter1992_6-0">[7] Others prefer to keep the genera separate, while still recognizing them as sister taxa.<sup class="reference" id="cite_ref-holtz2004_1-4">[2] A 2003 cladistic analysis based on skull features instead identified Alioramus as the closest known relative of Tarbosaurus, as the two genera share skull characteristics that are related to stress distribution and that are not found in other tyrannosaurines. If proven, this relationship would preclude Tarbosaurus from becoming a synonym for Tyrannosaurus and would suggest that separate tyrannosaurine lineages evolved in Asia and North America.<sup class="reference" id="cite_ref-hurumsabath2003_2-2">[3] <sup class="reference" id="cite_ref-currieetal2003_4-2">[5] The single known specimen of Alioramus, which shows juvenile characteristics, is not likely a juvenile Tarbosaurus because of its much higher tooth count (76 to 78 teeth) and the unique row of bony bumps along the top of its snout.<sup class="reference" id="cite_ref-currie2003_7-0">[8]

[edit] Discovery and naming
In 1946, a joint Soviet-Mongolian expedition to the Gobi Desert in the Mongolian Ömnögovi Province turned up a large theropod skull and some vertebrae in the Nemegt Formation. In 1955, Evgeny Maleev, a Russian paleontologist, made this specimen the holotype (PIN 551-1) of a new species, which he called Tyrannosaurus bataar.<sup class="reference" id="cite_ref-maleev1955a_5-1">[6] The species name is a misspelling of the Mongolian баатар/baatar ("hero").<sup class="reference" id="cite_ref-hurumsabath2003_2-3">[3] In the same year, Maleev also described and named three new theropod skulls, each associated with skeletal remains discovered by the same expedition in 1948 and 1949. The first of these (PIN 551-2) was named Tarbosaurus efremovi, a new generic name composed of the Ancient Greek ταρβος/tarbos ("terror", "alarm", "awe", or "reverence") and σαυρος/sauros ("lizard"),<sup class="reference" id="cite_ref-liddellscott_8-0">[9] and the species named after Ivan Yefremov, a Russian paleontologist and science fiction author. The other two (PIN 553-1 and PIN 552-2) were also named as new species and assigned to the North American genus Gorgosaurus (G. lancinator and G. novojilovi, respectively). All three of these latter specimens are smaller than the first.<sup class="reference" id="cite_ref-maleev1955b_0-2">[1] A 1965 paper by A.K. Rozhdestvensky recognized all of Maleev's specimens as different growth stages of the same species, which he believed to be distinct from the North American Tyrannosaurus. He created a new combination, Tarbosaurus bataar, to include all the specimens described in 1955 as well as newer material.<sup class="reference" id="cite_ref-rozhdestvensky1965_9-0">[10] Later authors, including Maleev himself,<sup class="reference" id="cite_ref-maleev1974_10-0">[11] agreed with Rozhdestvensky's analysis, although some used the name Tarbosaurus efremovi rather than T. bataar.<sup class="reference" id="cite_ref-barsbold1983_11-0">[12] American paleontologist Kenneth Carpenter re-examined the material in 1992. He concluded that it belonged to the genus Tyrannosaurus, as originally published by Maleev, and lumped all the specimens into the species Tyrannosaurus bataar except the remains that Maleev had named Gorgosaurus novojilovi. Carpenter thought this specimen represented a separate, smaller genus of tyrannosaurid, which he called Maleevosaurus novojilovi.<sup class="reference" id="cite_ref-carpenter1992_6-1">[7] George Olshevsky created the new generic name Jenghizkhan (after Genghis Khan) for Tyrannosaurus bataar in 1995, while also recognizing Tarbosaurus efremovi and Maleevosaurus novojilovi, for a total of three distinct, contemporaneous genera from the Nemegt Formation.<sup class="reference" id="cite_ref-olshevskyford1995_12-0">[13] A 1999 study subsequently reclassified Maleevosaurus as a juvenile Tarbosaurus.<sup class="reference" id="cite_ref-carr1999_13-0">[14] All research published since 1999 recognizes only a single species, which is either called Tarbosaurus bataar<sup class="reference" id="cite_ref-holtz2004_1-5">[2] <sup class="reference" id="cite_ref-currieetal2003_4-3">[5] <sup class="reference" id="cite_ref-xuetal2004_14-0">[15] or Tyrannosaurus bataar.<sup class="reference" id="cite_ref-carretal2005_3-3">[4]

After the original Russian-Mongolian expeditions in the 1940s, Polish-Mongolian joint expeditions to the Gobi Desert began in 1963 and continued until 1971, recovering many new fossils, including new specimens of Tarbosaurus from the Nemegt Formation.<sup class="reference" id="cite_ref-hurumsabath2003_2-4">[3] Expeditions involving Japanese and Mongolian scientists between 1993 and 1998,<sup class="reference" id="cite_ref-watabesuzuki2000_15-0">[16] as well as private expeditions hosted by Canadian paleontologist Phil Currie around the turn of the 21st century, discovered and collected further Tarbosaurus material.<sup class="reference" id="cite_ref-currie2001_16-0">[17] <sup class="reference" id="cite_ref-currie2002_17-0">[18] More than 30 specimens are known, including more than 15 skulls and several complete postcranial skeletons.<sup class="reference" id="cite_ref-holtz2004_1-6">[2]

[edit] Possible synonyms
Chinese paleontologists discovered a partial skull and skeleton of a small theropod (IVPP V4878) in the Xinjiang Autonomous Region of China in the mid-1960s. In 1977, Dong Zhiming described this specimen, which was recovered from the Subashi Formation, as a new genus and species, Shanshanosaurus huoyanshanensis.<sup class="reference" id="cite_ref-dong1977_18-0">[19] Gregory Paul recognized Shanshanosaurus as a tyrannosaurid in 1988, referring it to the now-defunct genus Aublysodon.<sup class="reference" id="cite_ref-paul1988_19-0">[20] Dong and Currie later re-examined the specimen and deemed it to be a juvenile of a larger species of tyrannosaurid. These authors refrained from assigning it to any particular genus but suggested Tarbosaurus as a possibility.<sup class="reference" id="cite_ref-curriedong2001_20-0">[21] Over the years, other Chinese localities have produced tyrannosaurid teeth and fragmentary remains, several of which have been given names. Albertosaurus periculosis, Tyrannosaurus luanchuanensis, Tyrannosaurus turpanensis and Chingkankousaurus fragilis are often considered synonyms of Tarbosaurus.<sup class="reference" id="cite_ref-holtz2004_1-7">[2]

Named in 1976 by Sergei Kurzanov, Alioramus is another genus of tyrannosaurid from slightly older sediments in Mongolia.<sup class="reference" id="cite_ref-kurzanov1976_21-0">[22] Several analyses have concluded Alioramus was quite closely related to Tarbosaurus.<sup class="reference" id="cite_ref-hurumsabath2003_2-5">[3] <sup class="reference" id="cite_ref-currieetal2003_4-4">[5] It was described as an adult, but its long, low skull is characteristic of a juvenile tyrannosaurid. This led Currie to speculate that Alioramus might represent a juvenile Tarbosaurus, but he noted that the much higher tooth count and row of crests on top of the snout suggested otherwise.<sup class="reference" id="cite_ref-currie2003_7-1">[8]

[edit] Paleobiology
Like several other large tyrannosaurids, Tarbosaurus is known from relatively abundant and well-preserved fossil material. In fact, one quarter of all fossils collected from the Nemegt Formation belong to Tarbosaurus.<sup class="reference" id="cite_ref-jerzykiewiczrussell1991_22-0">[23] Although Tarbosaurus has not been studied as thoroughly as the North American tyrannosaurids,<sup class="reference" id="cite_ref-hurumsabath2003_2-6">[3] the available material has allowed scientists to draw limited conclusions about its biology.

[edit] Skull mechanics
The skull of Tarbosaurus was completely described for the first time in 2003. Scientists noted key differences between Tarbosaurus and the North American tyrannosaurids. Many of these differences are related to the handling of stress by the skull bones during a bite. When the upper jaw bit down on an object, force was transmitted up through the maxilla, the primary tooth-bearing bone of the upper jaw, into surrounding skull bones. In North American tyrannosaurids, this force went from the maxilla into the fused nasal bones on top of the snout, which were firmly connected in the rear to the lacrimal bones by bony struts. These struts locked the two bones together, suggesting that force was then transmitted from the nasals to the lacrimals.<sup class="reference" id="cite_ref-hurumsabath2003_2-7">[3] Tarbosaurus lacked these bony struts, and the connection between the nasals and lacrimals was weak. Instead, a backwards projection of the maxilla was massively developed in Tarbosaurus and fit inside a sheath formed from the lacrimal. This projection was a thin, bony plate in North American tyrannosaurids. The large backwards projection suggests that force was transmitted more directly from the maxilla to the lacrimal in Tarbosaurus. The lacrimal was also more firmly anchored to the frontal and prefrontal bones in Tarbosaurus. The well-developed connections between the maxilla, lacrimal, frontal and prefrontal would have made its entire upper jaw more rigid.<sup class="reference" id="cite_ref-hurumsabath2003_2-8">[3]

Another major difference between Tarbosaurus and its North American relatives was its more rigid mandible (lower jaw). While many theropods, including North American tyrannosaurids, had some degree of flexibility between the bones in the rear of the mandible and the dentary in the front, Tarbosaurus had a locking mechanism formed from a ridge on the surface of the angular, which articulated with a square process on the rear of the dentary.<sup class="reference" id="cite_ref-hurumsabath2003_2-9">[3]

Some scientists have hypothesized that the more rigid skull of Tarbosaurus was an adaptation to hunting the massive titanosaurid sauropods found in the Nemegt Formation, which did not exist in most of North America during the Late Cretaceous. The differences in skull mechanics also have an impact on tyrannosaurid phylogeny. Tarbosaurus-like articulations between the skull bones are also seen in Alioramus from Mongolia, suggesting that it, and not Tyrannosaurus, is the closest relative of Tarbosaurus. Similarities between Tarbosaurus and Tyrannosaurus might therefore be related to their large size, independently developed through convergent evolution.<sup class="reference" id="cite_ref-hurumsabath2003_2-10">[3]

[edit] Brain structure
A Tarbosaurus skull found in 1948 by Soviet and Mongolian scientists (PIN 553-1, originally called Gorgosaurus lancinator) included the skull cavity that held the brain. Making a plaster cast, called an endocast, of the inside of this cavity allowed Maleev to make preliminary observations about the shape of a Tarbosaurus brain.<sup class="reference" id="cite_ref-maleev1965_23-0">[24] A newer polyurethane rubber cast allowed a more detailed study of Tarbosaurus brain structure and function.<sup class="reference" id="cite_ref-savelievalifanov2005_24-0">[25]

Tyrannosaurus rex brain structure has also been analyzed,<sup class="reference" id="cite_ref-brochu2000_25-0">[26] and Tarbosaurus was similar, differing only in the positions of some cranial nerve roots, including the trigeminal and accessory nerves. Tyrannosaurid brains were more similar to those of crocodilians and other reptiles than to birds. The total brain volume for a 12 meter (40 foot) Tarbosaurus is estimated at only 184 centimeters3 (11.2 in3). The large size of the olfactory bulbs, as well as the terminal and olfactory nerves, suggest that Tarbosaurus had a keen sense of smell, as was also the case with Tyrannosaurus. The vomeronasal bulb is large and differentiated from the olfactory bulb, indicating a well-developed Jacobsen's organ, which was used to detect pheromones. This may imply that Tarbosaurus had complex mating behavior. The auditory nerve was also large, suggesting good hearing, which may have been useful for auditory communication and spatial awareness. The nerve had a well-developed vestibular component as well, which implies a good sense of balance and coordination. In contrast, the nerves and brain structures associated with eyesight were smaller and undeveloped. The midbrain tectum, responsible for visual processing in reptiles, was very small in Tarbosaurus, as were the optic nerve and the oculomotor nerve, which controls eye movement. Unlike Tyrannosaurus, which had forward-facing eyes that provided some degree of binocular vision, Tarbosaurus had a narrower skull more typical of other tyrannosaurids in which the eyes faced primarily sideways. All of this suggests that Tarbosaurus relied more on its senses of smell and hearing than on its eyesight.<sup class="reference" id="cite_ref-savelievalifanov2005_24-1">[25]

[edit] Paleoecology
The vast majority of Tarbosaurus fossils are known from the Nemegt Formation of southern Mongolia. This geologic formation has never been dated radiometrically, but the fauna present in the fossil record indicate it was probably deposited during the Maastrichtian stage, at the end of the Late Cretaceous.<sup class="reference" id="cite_ref-jerzykiewiczrussell1991_22-1">[23] The Maastrichtian stage occurred 70 to 65 million years ago.<sup class="reference" id="cite_ref-gradsteinetal2005_26-0">[27] The Subashi Formation, in which Shanshanosaurus remains were discovered, is also Maastrichtian in age.<sup class="reference" id="cite_ref-shenmateer1992_27-0">[28]

Nemegt sediments preserve large river channels and soil deposits that indicate a far more humid climate than those suggested by the underlying Barun Goyot and Djadochta Formations. However, caliche deposits indicate at least periodic droughts. Sediment was deposited in the channels and floodplains of large rivers. Occasional mollusc fossils are found, as well as a variety of other aquatic animals like fish and turtles.<sup class="reference" id="cite_ref-jerzykiewiczrussell1991_22-2">[23] Crocodilians included several species of Shamosuchus, a genus with teeth adapted for crushing shells.<sup class="reference" id="cite_ref-efimov1983_28-0">[29] Mammal fossils are exceedingly rare in the Nemegt Formation, but many birds are known, including the enantiornithine Gurilynia and the hesperornithiform Judinornis, as well as Teviornis, an early representative of the still-existing Anseriformes (waterfowl), a bird order. Scientists have described many dinosaurs from the Nemegt, including ankylosaurids such as Tarchia and pachycephalosaurs such as Homalocephale and Prenocephale.<sup class="reference" id="cite_ref-jerzykiewiczrussell1991_22-3">[23] By far the largest predator of the time, adult Tarbosaurus most likely preyed upon large hadrosaurs such as Saurolophus and Barsboldia or sauropods such as Nemegtosaurus and Opisthocoelicaudia.<sup class="reference" id="cite_ref-hurumsabath2003_2-11">[3] Adults would have received little competition from small theropods such as troodontids (Borogovia, Tochisaurus, Saurornithoides), oviraptorosaurs (Elmisaurus, Nemegtomaia, Rinchenia) or Bagaraatan, sometimes considered a basal tyrannosauroid. Other theropods, like the gigantic Therizinosaurus, may have been herbivorous, and ornithomimosaurs such as Anserimimus, Gallimimus and Deinocheirus might have been omnivores that only took small prey and were therefore no competition for Tarbosaurus. However, as in other large tyrannosaurids as well as modern Komodo dragons, juveniles and subadult Tarbosaurus may have filled niches between the massive adults and these smaller theropods.<sup class="reference" id="cite_ref-holtz2004_1-8">[2]

Tyrannosaurus rex (Osborne, 1905)

Skull length : 1.75 m.

Total length : 12.4 m.

Hip height : 4.4 m.

Weight : 12 tonnes

Tyrannosaurus rex (pronounced /tɨˌrænɵˈsɔrəs/ or /taɪˌrænɵˈsɔrəs/, meaning 'tyrant lizard') from the Greek words τυραννος (tyrannos, meaning "tyrant") and σαυρος (sauros, meaning "lizard"), was a genus of theropod dinosaur. The species Tyrannosaurus rex ('rex' meaning 'king' in Latin), commonly abbreviated to T. rex, is a fixture in popular culture. It lived throughout what is now western North America, with a much wider range than other tyrannosaurids. Fossils are found in a variety of rock formations dating to the last three million years of the Cretaceous Period, approximately 68 to 65 million years ago. It was among the last non-avian dinosaurs to exist prior to the Cretaceous–Tertiary extinction event.

Like other tyrannosaurids, Tyrannosaurus was a bipedal carnivore with a massive skull balanced by a long, heavy tail. Relative to the large and powerful hindlimbs, Tyrannosaurus forelimbs were small, though unusually powerful for their size, and bore two clawed digits. Although other theropods rivaled or exceeded Tyrannosaurus rex in size, it was the largest known tyrannosaurid and one of the largest known land predators, measuring up to 12.8 m (42 ft) in length,<sup class="reference" id="cite_ref-henderson1999_0-0">[1] up to 4 metres (13 ft) tall at the hips,<sup class="reference" id="cite_ref-SueFMNH_1-0">[2] and up to 6.8 metric tons (7.5 short tons) in weight.<sup class="reference" id="cite_ref-ericksonetal2004_2-0">[3] By far the largest carnivore in its environment, Tyrannosaurus rex may have been an apex predator, preying upon hadrosaurs and ceratopsians, although some experts have suggested it was primarily a scavenger. The debate over Tyrannosaurus as apex predator or scavenger is among the longest running debates in paleontology.

More than 30 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 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, with some scientists considering Tarbosaurus bataar from Asia to represent a second species of Tyrannosaurus and others maintaining Tarbosaurus as a separate genus. Several other genera of North American tyrannosaurids have also been synonymized with Tyrannosaurus. {| class="toc" id="toc"

Contents
[hide]*1 Description
 * 2 Classification
 * 2.1 Manospondylus
 * 3 Paleobiology
 * 3.1 Life history
 * 3.2 Sexual dimorphism
 * 3.3 Posture
 * 3.4 Arms
 * 3.5 Soft tissue
 * 3.6 Skin and feathers
 * 3.7 Thermoregulation
 * 3.8 Footprints
 * 3.9 Locomotion
 * 3.10 Feeding strategies
 * 4 History
 * 4.1 Earliest finds
 * 4.2 Notable specimens
 * 5 Appearances in popular culture
 * 6 References
 * 7 Further reading
 * 8 External links
 * }

Description
Tyrannosaurus rex was one of the largest land carnivores of all time; the largest complete specimen, FMNH PR2081 ("Sue"), measured 12.8 metres (42 ft) long, and was 4.0 metres (13.1 ft) tall at the hips.<sup class="reference" id="cite_ref-SueFMNH_1-1">[2] Mass estimates have varied widely over the years, from more than 7.2 metric tons (7.9 short tons),<sup class="reference" id="cite_ref-henderson1999_0-1">[1] to less than 4.5 metric tons (5.0 short tons),<sup class="reference" id="cite_ref-andersonetal1985_3-0">[4] <sup class="reference" id="cite_ref-bakker1986_4-0">[5] with most modern estimates ranging between 5.4 and 6.8 metric tons (6.0 and 7.5 short tons).<sup class="reference" id="cite_ref-ericksonetal2004_2-1">[3] <sup class="reference" id="cite_ref-farlowetal1995_5-0">[6] <sup class="reference" id="cite_ref-seebacher2001_6-0">[7] <sup class="reference" id="cite_ref-christiansenfarina2004_7-0">[8] Although Tyrannosaurus rex was larger than the well known Jurassic theropod Allosaurus, it was slightly smaller than some other Cretaceous carnivores, such as Spinosaurus and Giganotosaurus.<sup class="reference" id="cite_ref-dalsassoetal2005_8-0">[9] <sup class="reference" id="cite_ref-calvocoria1998_9-0">[10]

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,<sup class="reference" id="cite_ref-brochu2003_10-0">[11] along with an additional small metacarpal representing the remnant of a third digit.<sup class="reference" id="cite_ref-CLKC08_11-0">[12] 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.<sup class="reference" id="cite_ref-brochu2003_10-1">[11]

The largest known Tyrannosaurus rex skulls measure up to 5 feet (1.5 m) in length.<sup class="reference" id="cite_ref-12">[13] Large fenestrae (openings) in the skull reduced weight and provided areas for muscle attachment, as in all carnivorous theropods. But in other respects Tyrannosaurus’ 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.<sup class="reference" id="cite_ref-Stevens2006Binocular_13-0">[14] <sup class="reference" id="cite_ref-jaffe_14-0">[15] 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.<sup class="reference" id="cite_ref-SnivelyHendersonPhillips2006FusedVaultedNasals_15-0">[16] <sup class="reference" id="cite_ref-GEetal96_16-0">[17] <sup class="reference" id="cite_ref-MM03_17-0">[18] 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.<sup class="reference" id="cite_ref-holtz1994_18-0">[19] <sup class="reference" id="cite_ref-paul1988_19-0">[20] The teeth of Tyrannosaurus rex displayed marked heterodonty (differences in shape).<sup class="reference" id="cite_ref-brochu2003_10-2">[11] <sup class="reference" id="cite_ref-Smith2005HeterodontyTRex_20-0">[21] 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.<sup class="reference" id="cite_ref-New_Scientist1998DinosaurDetectives_21-0">[22] 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 centimetres (12 in) long including the root when the animal was alive, making it the largest tooth of any carnivorous dinosaur.<sup class="reference" id="cite_ref-SueFMNH_1-2">[2]

Classification
Tyrannosaurus 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,<sup class="reference" id="cite_ref-currieetal2003_22-0">[23] <sup class="reference" id="cite_ref-holtz2004_23-0">[24] both of which have occasionally been synonymized with Tyrannosaurus.<sup class="reference" id="cite_ref-paul1988_19-1">[20] 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.<sup class="reference" id="cite_ref-holtz1994_18-1">[19] In 1955, Soviet paleontologist Evgeny Maleev named a new species, Tyrannosaurus bataar, from Mongolia.<sup class="reference" id="cite_ref-maleev1955_24-0">[25] By 1965, this species had been renamed Tarbosaurus bataar.<sup class="reference" id="cite_ref-rozhdestvensky1965_25-0">[26] Despite the renaming, many phylogenetic analyses have found Tarbosaurus bataar to be the sister taxon of Tyrannosaurus rex,<sup class="reference" id="cite_ref-holtz2004_23-1">[24] and it has often been considered an Asian species of Tyrannosaurus.<sup class="reference" id="cite_ref-holtz1994_18-2">[19] <sup class="reference" id="cite_ref-carpenter1992_26-0">[27] <sup class="reference" id="cite_ref-carretal2005_27-0">[28] 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.<sup class="reference" id="cite_ref-hurumsabath2003_28-0">[29] 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.<sup class="reference" id="cite_ref-currieetal2003_22-1">[23]

Other tyrannosaurid fossils found in the same formations as Tyrannosaurus rex were originally classified as separate taxa, including Aublysodon and Albertosaurus megagracilis,<sup class="reference" id="cite_ref-paul1988_19-2">[20] the latter being named Dinotyrannus megagracilis in 1995.<sup class="reference" id="cite_ref-Olshevsky1995_29-0">[30] However, these fossils are now universally considered to belong to juvenile Tyrannosaurus rex.<sup class="reference" id="cite_ref-carrwilliamson2004_30-0">[31] A small but nearly complete skull from Montana, 60 centimetres (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,<sup class="reference" id="cite_ref-gilmore1946_31-0">[32] but was later referred to a new genus, Nanotyrannus.<sup class="reference" id="cite_ref-bakkeretal1988_32-0">[33] Opinions remain divided on the validity of N. lancensis. Many paleontologists consider the skull to belong to a juvenile Tyrannosaurus rex.<sup class="reference" id="cite_ref-carr1999_33-0">[34] 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.<sup class="reference" id="cite_ref-holtz2004_23-2">[24] <sup class="reference" id="cite_ref-currie2003_34-0">[35]

Manospondylus
The first 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 and described as Manospondylus gigas. Osborn recognized the similarity between M. gigas and Tyrannosaurus rex as early as 1917 but, due to the fragmentary nature of the Manospondylus vertebrae, he could not synonymize them conclusively.<sup class="reference" id="cite_ref-osborn1917_35-0">[36]

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. 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. However, the Fourth Edition of the ICZN, which took effect on 1 January 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 ..."<sup class="reference" id="cite_ref-icznart23_36-0">[37] 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 was ever challenged, which it has not yet been. Manospondylus gigas would then be deemed a nomen oblitum ("forgotten name").<sup class="reference" id="cite_ref-taylor2002_37-0">[38]

Paleobiology
The 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 over 5,400 kg (12,000 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.<sup class="reference" id="cite_ref-ericksonetal2004_2-2">[3]

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).<sup class="reference" id="cite_ref-ericksonetal2004_2-3">[3] Another recent histological study performed by different workers corroborates these results, finding that rapid growth began to slow at around 16 years of age.<sup class="reference" id="cite_ref-hornerpadian2004_38-0">[39] This sudden change in growth rate 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.<sup class="reference" id="cite_ref-schweitzeretal2005_39-0">[40] Further study indicates an age of 18 for this specimen.<sup class="reference" id="cite_ref-LW08_40-0">[41] Other tyrannosaurids exhibit extremely similar growth curves, although with lower growth rates corresponding to their lower adult sizes.<sup class="reference" id="cite_ref-ericksonetal2006_41-0">[42]

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. However, 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.<sup class="reference" id="cite_ref-ericksonetal2006_41-1">[42]

Sexual dimorphism
As the number of 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.<sup class="reference" id="cite_ref-carpenter1990_42-0">[43] 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.<sup class="reference" id="cite_ref-larson1994_43-0">[44]

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 genders.<sup class="reference" id="cite_ref-ericksonetal2005_44-0">[45] 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.<sup class="reference" id="cite_ref-brochu2003_10-3">[11]

Only a single Tyrannosaurus rex specimen has been conclusively shown to belong to a specific gender. 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.<sup class="reference" id="cite_ref-schweitzeretal2005_39-1">[40] 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.<sup class="reference" id="cite_ref-schweitzeretal2007_45-0">[46]

Posture
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.<sup class="reference" id="cite_ref-leidy1865_46-0">[47] Henry Fairfield Osborn, former president of the American Museum of Natural History (AMNH) in New York City, who believed the creature stood upright, further reinforced the notion after unveiling the first complete Tyrannosaurus rex skeleton in 1915. It stood in this upright pose for nearly a century, until it was dismantled in 1992.<sup class="reference" id="cite_ref-amnhsite_47-0">[48] 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.<sup class="reference" id="cite_ref-newman1970_48-0">[49] 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)<sup class="reference" id="cite_ref-49">[50] until the 1990s, when films such as Jurassic Park introduced a more accurate posture to the general public. Modern 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.<sup class="reference" id="cite_ref-paul1988_19-3">[20]

Arms
When Tyrannosaurus rex was first discovered, the humerus was the only element of the forelimb known.<sup class="reference" id="cite_ref-osborn1905_50-0">[51] For the initial mounted skeleton as seen by the public in 1915, Osborn substituted longer, three-fingered forelimbs like those of Allosaurus.<sup class="reference" id="cite_ref-osborn1917_35-1">[36] However, a year earlier, Lawrence Lambe described the short, two-fingered forelimbs of the closely related Gorgosaurus.<sup class="reference" id="cite_ref-lambe1914_51-0">[52] 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").<sup class="reference" id="cite_ref-hornerlessem1993_52-0">[53] The remains of "Sue" also include complete forelimbs.<sup class="reference" id="cite_ref-brochu2003_10-4">[11] Tyrannosaurus rex arms are very small relative to overall body size, measuring only 1 metre (3.3 ft) long. However, they are not vestigial but instead 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.<sup class="reference" id="cite_ref-osborn1906_53-0">[54] It has also been suggested that the forelimbs were used to assist the animal in rising from a prone position.<sup class="reference" id="cite_ref-newman1970_48-1">[49] Another possibility is that the forelimbs held struggling prey while it was dispatched by the tyrannosaur's enormous jaws. This hypothesis may be supported by biomechanical analysis. Tyrannosaurus rex forelimb bones exhibit extremely thick cortical bone, indicating that they were developed to withstand heavy loads. The biceps brachii muscle of a full-grown Tyrannosaurus rex was capable of lifting 199 kilograms (439 lb) by itself; this number would only increase with other muscles (like the brachialis) acting in concert with the biceps. A Tyrannosaurus rex forearm also had a reduced 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, extreme strength of the muscles, and limited range of motion may indicate a system designed to hold fast despite the stresses of a struggling prey animal.<sup class="reference" id="cite_ref-carpentersmith2001_54-0">[55] Bronze cast of the wishbone of the "Sue" specimen, Field Museum

In 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 68-million-year-old Tyrannosaurus. 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.<sup class="reference" id="cite_ref-smithsonian-fields_55-0">[56] 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.<sup class="reference" id="cite_ref-MHSetalb_56-0">[57] 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 merely be the result of people assuming preserved tissue was impossible, therefore simply not looking. Since the first, two more tyrannosaurs and a hadrosaur have also been found to have such tissue-like structures.<sup class="reference" id="cite_ref-smithsonian-fields_55-1">[56] Research on some of the tissues involved has suggested that birds are closer relatives to tyrannosaurs than other modern animals.<sup class="reference" id="cite_ref-57">[58]

In studies reported in the journal 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."<sup class="reference" id="cite_ref-58">[59]

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."<sup class="reference" id="cite_ref-59">[60]

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.<sup class="reference" id="cite_ref-60">[61] 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.<sup class="reference" id="cite_ref-61">[62]

Skin and feathers
In 2004, the scientific journal Nature published a report describing an early tyrannosauroid, Dilong paradoxus, from the famous Yixian Formation of China. 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. It has also been proposed that Tyrannosaurus and other closely related tyrannosaurids had such protofeathers. However, skin impressions from large tyrannosaurid specimens show mosaic scales.<sup class="reference" id="cite_ref-GSP08_62-0">[63] While it is possible that protofeathers existed on parts of the body which have not been preserved, a lack of insulatory body covering is consistent with modern multi-ton mammals such as elephants, hippopotamus, and most species of rhinoceros. As an object increases in size, its ability to retain heat increases due to its decreasing surface area-to-volume ratio. Therefore, as large animals evolve in or disperse into warm climates, a coat of fur or feathers loses its selective advantage for thermal insulation and can instead become a disadvantage, as the insulation traps excess heat inside the body, possibly overheating the animal. Protofeathers may also have been secondarily lost during the evolution of large tyrannosaurids like Tyrannosaurus, especially in warm Cretaceous climates.<sup class="reference" id="cite_ref-xuetal2004_63-0">[64]

Thermoregulation
Main article: Physiology of dinosaurs

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.<sup class="reference" id="cite_ref-bakker1968_64-0">[65] <sup class="reference" id="cite_ref-bakker1972_65-0">[66] Tyrannosaurus rex itself was claimed to have been endothermic ("warm-blooded"), implying a very active lifestyle.<sup class="reference" id="cite_ref-bakker1986_4-1">[5] 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.<sup class="reference" id="cite_ref-hornerpadian2004_38-1">[39]

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.<sup class="reference" id="cite_ref-barrettshowers1994_66-0">[67] 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).<sup class="reference" id="cite_ref-truemanetal2003_67-0">[68] 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).<sup class="reference" id="cite_ref-barrickshowers1999_68-0">[69] Ornithischian dinosaurs also showed evidence of homeothermy, while varanid lizards from the same formation did not.<sup class="reference" id="cite_ref-barrickstevens1997_69-0">[70] 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.<sup class="reference" id="cite_ref-paladinoetal1997_70-0">[71] <sup class="reference" id="cite_ref-chinsamyhillenius2004_71-0">[72]

Footprints
Probable 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 centimetres (33 in) long by 71 centimetres (28 in) wide.<sup class="reference" id="cite_ref-lockley.26hunt1994_72-0">[73]

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 76 centimetres (30 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. Further study of the track (a full description has not yet been published) will compare the Montana track with the one found in New Mexico.<sup class="reference" id="cite_ref-rextrack2007_73-0">[74]

Locomotion
Replica of a sequence of theropod footprints attributed to Megalosaurus. No such sequence has yet been reported for tyrannosaurs, making gait and speed estimates difficult.There 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 (see below).

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 tail-less, can spin in a fraction of a second.<sup class="reference" id="cite_ref-74">[75] 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 — 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.<sup class="reference" id="cite_ref-CarrierWalterLee2000TurningPerformance_75-0">[76]

Scientists have produced a wide range of maximum speed estimates, mostly around 11 metres per second (40 km/h; 25 mph), but a few as low as 5–11 metres per second (18–40 km/h; 11–25 mph), and a few as high as 20 metres 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.<sup class="reference" id="cite_ref-76">[77] 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. Additionally, some have argued that Tyrannosaurus had relatively larger leg muscles than any animal alive today, which could have enabled fast running 40–70 kilometres per hour (25–43 mph).<sup class="reference" id="cite_ref-HutchinsonGarcia2002TrexSlow_77-0">[78]

Jack 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.<sup class="reference" id="cite_ref-hornerlessem1993_52-1">[53] However, 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.<sup class="reference" id="cite_ref-Holtz1998TaxonomyCoelurosauria_78-0">[79] Femur (thigh bone)Tibia (shin bone)Metatarsals (foot bones)DewclawPhalanges (toe bones)Skeletal anatomy of a T. rex right legChristiansen (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 metres 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.<sup class="reference" id="cite_ref-Christiansen1998Strength_79-0">[80]

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.<sup class="reference" id="cite_ref-farlowetal1995_5-1">[6] However, giraffes have been known to gallop at 50 kilometres 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.<sup class="reference" id="cite_ref-80">[81] <sup class="reference" id="cite_ref-81">[82] Thus it is quite possible that Tyrannosaurus also moved fast when necessary and had to accept such risks.<sup class="reference" id="cite_ref-Alexander2006DinoBioMechanics_82-0">[83] <sup class="reference" id="cite_ref-Hanna2002MultipleInjuriesBigAl_83-0">[84]

Most recent research on Tyrannosaurus locomotion does not support speeds faster than 40 kilometres per hour (25 mph), i.e. moderate-speed running. For example, a 2002 paper in the journal Nature used a mathematical model (validated by applying it to three living animals, alligators, chickens, and humans; additionally later eight more species including emus and ostriches<sup class="reference" id="cite_ref-84">[85] ) to gauge the leg muscle mass needed for fast running (over 40 km/h or 25 mph).<sup class="reference" id="cite_ref-HutchinsonGarcia2002TrexSlow_77-1">[78] They found that proposed top speeds in excess of 40 kilometres per hour (25 mph) were unfeasible, 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 kilometres per hour (11 mph) walking/jogging might have been possible.<sup class="reference" id="cite_ref-HutchinsonGarcia2002TrexSlow_77-2">[78]

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 metres per second (29 km/h; 18 mph). An average professional football (soccer) player would be slightly slower, while a human sprinter can reach 12 metres per second (43 km/h; 27 mph). Note that these computer models predict a top speed of 17.8 metres per second (64 km/h; 40 mph) for a 3-kilogram (6.6 lb) Compsognathus<sup class="reference" id="cite_ref-SellersManning2007ProcRSocB_85-0">[86] <sup class="reference" id="cite_ref-86">[87] (probably a juvenile individual).<sup class="reference" id="cite_ref-compysize_87-0">[88]

Those who argue that Tyrannosaurus was incapable of running estimate the top speed of Tyrannosaurus at about 17 kilometres per hour (11 mph). This is still faster than its most likely prey species, hadrosaurids and ceratopsians.<sup class="reference" id="cite_ref-HutchinsonGarcia2002TrexSlow_77-3">[78] 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.<sup class="reference" id="cite_ref-manning2008_88-0">[89] However, 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.<sup class="reference" id="cite_ref-PaulChristiansen2000NeoceratopsianForelimbPosture_89-0">[90] 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.<sup class="reference" id="cite_ref-Hanna2002MultipleInjuriesBigAl_83-1">[84]

Feeding strategies
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.<sup class="reference" id="cite_ref-Lambe1917Gorgosaurus_90-0">[91] This argument is no longer taken seriously, because theropods replaced their teeth quite rapidly. Ever since the first discovery of Tyrannosaurus most scientists have agreed that it was a predator, although like modern large predators it would have been happy to scavenge or steal another predator's kill if it had the opportunity.<sup class="reference" id="cite_ref-FarlowHoltz2002FossilRecordPredation_91-0">[92]

Noted hadrosaur expert Jack Horner is currently the major advocate of the idea that Tyrannosaurus was exclusively a scavenger and did not engage in active hunting at all.<sup class="reference" id="cite_ref-hornerlessem1993_52-2">[53] <sup class="reference" id="cite_ref-Horner1994SteakKnives_92-0">[93] <sup class="reference" id="cite_ref-BBC2003TrexOnTrial_93-0">[94] Horner has presented several arguments to support the pure scavenger hypothesis: Cast of the braincase at the Australian Museum, Sydney*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.<sup class="reference" id="cite_ref-94">[95] The eye-sockets faced mainly forwards, giving it good binocular visionOther 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. He also pointed out that the tyrannosaur lineage had a history of steadily improving binocular vision. It is hard to see how 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.<sup class="reference" id="cite_ref-Stevens2006Binocular_13-1">[14] <sup class="reference" id="cite_ref-jaffe_14-1">[15] In modern animals, binocular vision is found mainly in predators. Restoration (based on MOR 980) with parasite infections, which might be the cause of scars seen in the skulls of several specimens that were previously explained by intraspecific attacksA 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.<sup class="reference" id="cite_ref-carpenter1998_100-0">[101] 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.<sup class="reference" id="cite_ref-JH08_101-0">[102] 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.<sup class="reference" id="cite_ref-TC98_102-0">[103] However, 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.<sup class="reference" id="cite_ref-Horner1994SteakKnives_92-2">[93] A 2009 study showed that holes in the skulls of several specimens might have been caused by Trichomonas-like parasites that commonly infect avians.<sup class="reference" id="cite_ref-103">[104]
 * 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.<sup class="reference" id="cite_ref-95">[96] 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.<sup class="reference" id="cite_ref-OnlyModernScavengersBirds_96-0">[97] However, 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 Warm-bloodedness of dinosaurs). They also suggested that modern ecosystems like 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.<sup class="reference" id="cite_ref-RuxtonHouston2003TRexScavenger_97-0">[98]
 * 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 dung) 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.<sup class="reference" id="cite_ref-ChinEtal1998KingSizeCoprolite_98-0">[99]
 * 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.<sup class="reference" id="cite_ref-Horner1994SteakKnives_92-1">[93] <sup class="reference" id="cite_ref-dinodictionary_99-0">[100] 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.<sup class="reference" id="cite_ref-HutchinsonGarcia2002TrexSlow_77-4">[78] <sup class="reference" id="cite_ref-manning2008_88-1">[89]

Some researchers argue that if Tyrannosaurus were a scavenger, another dinosaur had to be the top predator in the Amerasian Upper Cretaceous. Top prey was the larger marginocephalians and ornithopods. The other tyrannosaurids share so many characteristics that only small dromaeosaurs 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.<sup class="reference" id="cite_ref-dinodictionary_99-1">[100] Most paleontologists accept that Tyrannosaurus was both an active predator and a scavenger like all large carnivores.

History
Skeletal restoration by William D. Matthew from 1905, the first reconstruction of this dinosaur ever published<sup class="reference" id="cite_ref-LindaHall_104-0">[105] 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 τυραννος (tyrannos, meaning "tyrant") and σαυρος (sauros, meaning "lizard"). Osborn used the Latin word rex, meaning "king", for the specific name. The full binomial therefore translates to "tyrant lizard king," emphasizing the animal's size and perceived dominance over other species of the time.<sup class="reference" id="cite_ref-osborn1905_50-1">[51]

Earliest finds
Teeth from what is now documented as a Tyrannosaurus rex were found in 1874 by A. Lakes near Golden, Colorado. In the early 1890s, J. B. 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. Vertebral fragments found by E. D. Cope in western South Dakota in 1892 and named as Manospondylus gigas have also been reclassified as Tyrannosaurus rex.<sup class="reference" id="cite_ref-quinlanetal2007_105-0">[106] Scale model of the never-completed exhibit planned for the American Museum of Natural History by H.F. OsbornBarnum 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.<sup class="reference" id="cite_ref-106">[107] Had it not been for page order, Dynamosaurus would have become the official name. The original Dynamosaurus material resides in the collections of the Natural History Museum, London.<sup class="reference" id="cite_ref-Breithaup_107-0">[108]

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.<sup class="reference" id="cite_ref-hornerlessem1993_52-3">[53]

Although there are numerous skeletons in the world, only one track has been documented — at Philmont Scout Ranch in northeast New Mexico. It was discovered in 1983 and identified and documented in 1994.<sup class="reference" id="cite_ref-108">[109]

Notable specimens
Main article: Specimens of Tyrannosaurus"Sue" specimen, Field Museum of Natural History, ChicagoSue Hendrickson, amateur paleontologist, discovered the most complete (approximately 85%) and, until 2001, the largest, Tyrannosaurus fossil skeleton known in the Hell Creek Formation near Faith, South Dakota, on 12 August 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 USD 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.<sup class="reference" id="cite_ref-Sueprep_109-0">[110] 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.<sup class="reference" id="cite_ref-Ericksonetal2004TyrannosaurGigantism_110-0">[111] 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.<sup class="reference" id="cite_ref-Brochu2003_111-0">[112] 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.<sup class="reference" id="cite_ref-112">[113]

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. After 30,000 man-hours of digging and preparing, a 65% complete skeleton emerged. Stan is currently on display in the Black Hills Museum of Natural History Exhibit in Hill City, South Dakota, after an extensive world tour. 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. Both "Stan" and "Sue" were examined by Peter Larson.

In the summer of 2000, Jack Horner discovered five Tyrannosaurus skeletons near the Fort Peck Reservoir in Montana. One of the specimens, dubbed "C. rex," was reported to be perhaps the largest Tyrannosaurus ever found.<sup class="reference" id="cite_ref-bbc-horner_113-0">[114] "Jane" specimen, Burpee Museum, Rockford, IllinoisIn 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.<sup class="reference" id="cite_ref-114">[115] 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.<sup class="reference" id="cite_ref-115">[116] <sup class="reference" id="cite_ref-visitjane_116-0">[117]

In a press release on 7 April 2006, Montana State University revealed that it possessed the largest Tyrannosaurus skull yet discovered. Discovered in the 1960s and only recently reconstructed, the skull measures 59 inches (150 cm) long compared to the 55.4 inches (141 cm) of "Sue's" skull, a difference of 6.5%.<sup class="reference" id="cite_ref-117">[118] <sup class="reference" id="cite_ref-118">[119]

Appearances in popular culture
Main 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.<sup class="reference" id="cite_ref-brochu2003_10-5">[11] Robert T. Bakker notes this in The Dinosaur Heresies and explains that a name like

<p style="text-align:center">"Tyrannosaurus rex is just irresistible to the tongue."<sup class="reference" id="cite_ref-bakker1986_4-2">[5]

Tyrannosaurus lancensis (Gilmore, 1946)

Skull length : 0.6 m.

Total length : 5.1 m.

Hip height : 1.9 m.

Weight : 0.81 tonnes

Tyrannosaurus lancensis  or Nanotyrannus ("dwarf tyrant") is a genus of tyrannosaurid dinosaur, and is possibly a juvenile specimen of Tyrannosaurus. It is based on CMN 7541, a skull collected in 1942 and described by Charles W. Gilmore described in 1946, who gave it the new species Gorgosaurus lancensis.<sup class="reference" id="cite_ref-gilmore1946_0-0">[1] In 1988, the specimen was re-described by Robert T. Bakker, Phil Currie, and Michael Williams, then the curator of paleontology at the Cleveland Museum of Natural History, where the original specimen was housed and is currently on display. Initial research indicated that the skull bones were fused, and that it therefore represented an adult specimen. In light of this, Bakker and colleagues assigned the skull to a new genus, which they named Nanotyrannus for its apparently small size.<sup class="reference" id="cite_ref-bakkeretal1988_1-0">[2] However, subsequent work has cast doubt on this, and some paleontologists no longer consider it a valid genus—since the fossil was a contemporary of Tyrannosaurus rex, many paleontologists now believe it to be a juvenile T. rex, especially since the discovery in 2001 of a new Nanotyrannus specimen, nicknamed "Jane." The original Nanotyrannus specimen is estimated to have been around 17 feet (5.2 meters) long when it died.

In 2001, a more complete juvenile tyrannosaur ("Jane", catalogue number BMRP 2002.4.1), belonging to the same species as the original Nanotyrannus specimen, was uncovered. In 2005, a conference on tyrannosaurs focused on the issues of Nanotyrannus validity brought about by the discovery of the Jane specimen, was held at the Burpee Museum of Natural History. Several paleontologists, such as Phil Currie and Donald M. Henderson, saw the discovery of Jane as a confirmation that Nanotyrannus was a juvenile T. rex or closely related species.<sup class="reference" id="cite_ref-currieetal2005_2-0">[3] <sup class="reference" id="cite_ref-henderson2005_3-0">[4] Peter Larson, on the other hand, continued to support a separate genus for Nanotyrannus.<sup class="reference" id="cite_ref-larson2005_4-0">[5] The actual scientific study of Jane, set to be published by Bakker, Larson, and Currie, may help determine whether Nanotyrannus is a valid genus, whether it simply represents a juvenile T. rex, or whether it is a new species of a previously identified genus of tyrannosaur.<sup class="reference" id="cite_ref-5">[6]

Bakker has stated he believes Nanotyrannus hunted in packs. Teeth from multiple Nanotyrannus have been found in the bones of herbivorous dinosaurs.<sup class="reference" id="cite_ref-Nordquist_6-0">[7]

[edit] Popular culture
The "Quintaglios" from Robert J. Sawyer's Quintaglio Ascension Trilogy are a race of highly evolved, sentient descendants of tyrannosaurs descended from Nanotyrannus.

In 2008, Nanotyrannus was featured in the second episode of Jurassic Fight Club, a pseudo-documentary about prehistoric predators.<sup class="reference" id="cite_ref-HistoryChannel_7-0">[8] The episode addressed the ongoing scientific debate on the validity of the Nanotyrannus genus, presenting a speculative battle between two juvenile Tyrannosaurus and one Nanotyrannus (which was possibly a juvenile Tyrannosaurus). The episode depicted both genera as having pronated hands (hands with downward or backward-facing palms), something tyrannosaurids could not do.<sup class="reference" id="cite_ref-SenterRobins05_8-0">[9] The episode relied heavily on speculation to determine who would be the victor in the battle between the similar (or possibly synonymous) genera.