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Tyrannosaurus

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Manetho
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« on: July 23, 2007, 01:22:27 pm »


Tyrannosaurus (IPA pronunciation [tænss] or [to-], meaning 'tyrant lizard') is a genus of theropod dinosaur. The species Tyrannosaurus rex, commonly abbreviated to T. rex, is one of the dinosaurs most often featured in popular culture around the world. It hails from what is now western North America. Some scientists consider Tarbosaurus bataar from Asia to represent a second species of Tyrannosaurus, while others maintain Tarbosaurus as a separate genus.

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 and they retained only two digits. Although other theropods rivaled or exceeded T. rex in size, it was the largest known tyrannosaurid and one of the largest known land predators, measuring over 13 metres (43 feet) in length and up to 6.8 metric tons (7.5 short tons) in weight.

Fossils of T. rex have been found in North American rock formations dating to the last three million years of the Cretaceous Period at the end of the Maastrichtian stage, approximately 68.5 to 65.5 million years ago; it was among the last dinosaurs to exist prior to the Cretaceous-Tertiary extinction event. More than 30 specimens of T. rex have been identified, some of which are nearly complete skeletons. Some researchers have discovered soft tissue as well. 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 T. rex are a few of the topics which remain controversial.
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Manetho
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« Reply #1 on: July 23, 2007, 01:23:24 pm »



Various specimens of Tyrannosaurus rex with a human for scale.
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« Reply #2 on: July 23, 2007, 01:24:38 pm »

Description

Tyrannosaurus rex was one of the largest land carnivores of all time, measuring 12 to 13 meters (40 to 43.3 feet) long, and 4.5-5 m (14-16.6 ft) tall, when fully-grown.[1] Mass estimates have varied widely over the years, from more than 7,200 kilograms (8 tons),[2] to less than 4,500 kg (5 tons),[3][4] with most modern estimates ranging between 5,400 and 6,800 kg (between 6 and 7.5 tons).


The largest known T. rex skulls measure up to 1.5 m (5 ft) in length. Compared to other theropods, the skull was heavily modified. The skull was extremely wide posteriorly, with a narrow snout, allowing some degree of binocular vision. Some of the bones, such as the nasals, were fused, preventing movement between them. Large fenestrae (openings) in the skull reduced weight and provided areas for muscle attachment. The bones themselves were massive, as were the serrated teeth which, rather than being bladelike, were oval in cross-section. Like other tyrannosaurids, T. rex displayed marked heterodonty, with the premaxillary teeth at the front of the upper jaw closely-packed and D-shaped in cross-section. Large bite marks found on bones of other dinosaurs indicate that these teeth could penetrate solid bone. T. rex had the greatest bite force of any dinosaur and one of the strongest bite forces of any animal. Worn or broken teeth are often found, but unlike those of mammals, tyrannosaurid teeth were continually replaced throughout the life of the animal.[1]

The neck of T. rex formed a natural S-shaped curve like that of other theropods, but was short and muscular to support the massive head. The two-fingered forelimbs were very small relative to the size of the body, but heavily built. In contrast, the hindlimbs 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. This reduced the weight of the skeleton while maintaining much of the strength of the bones
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Manetho
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« Reply #3 on: July 23, 2007, 01:25:38 pm »



T. rex head reconstruction at the Oxford University Museum of Natural History.
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« Reply #4 on: July 23, 2007, 01:26:39 pm »

Classification

Tyrannosaurus is the type genus of the superfamily Tyrannosauroidea, the family Tyrannosauridae, and the subfamily Tyrannosaurinae. Other members of the tyrannosaurine subfamily include the North American Daspletosaurus and the Asian Tarbosaurus,[9][10] both of which have occasionally been synonymized with Tyrannosaurus.[11] 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.[12]

In 1955, Soviet paleontologist Evgeny Maleev named a new species, Tyrannosaurus bataar, from Mongolia.[13] By 1965, this species had been renamed Tarbosaurus bataar.[14] Despite the renaming, many phylogenetic analyses have found Tarbosaurus bataar to be the sister taxon of Tyrannosaurus rex,[10] and it has often been considered an Asian species of Tyrannosaurus.[12][15][16] 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.[17] 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.[9]

Other tyrannosaurid fossils found in the same formations as T. rex were originally classified as separate taxa, including Aublysodon and Albertosaurus megagracilis,[11] the latter being named Dinotyrannus megagracilis in 1995.[18] However, these fossils are now universally considered to belong to juvenile T. rex.[19] A small but nearly complete skull from Montana, 60 cm (2 ft) long, may be an exception. This skull was originally classified as a species of Gorgosaurus (G. lancensis) by Charles W. Gilmore in 1946,[20] but was later referred to a new genus, Nanotyrannus.[21] Opinions remain divided on the validity of N. lancensis. Many paleontologists consider the skull to belong to a juvenile T. rex.[22] 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.
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« Reply #5 on: July 23, 2007, 01:28:53 pm »



Skull of T. rex, type specimen at the Carnegie Museum of Natural History. This was heavily and inaccurately restored with plaster after Allosaurus, and has since been disassembled.
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« Reply #6 on: July 23, 2007, 01:30:02 pm »

Manospondylus controversy

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 T. rex as early as 1917 but, due to the fragmentary nature of the Manospondylus vertebrae, he could not synonymize them conclusively.[24]

Controversy erupted in June 2000 after 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 T. 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.[25] However, the Fourth Edition of the ICZN, which took effect on January 1, 2000, states that "the prevailing usage must be maintained" when "the senior synonym or homonym has not been used as a valid name after 1899" and "the junior synonym or homonym has been used for a particular taxon, as its presumed valid name, in at least 25 works, published by at least 10 authors in the immediately preceding 50 years..."[26] Tyrannosaurus rex easily qualifies 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").


Paleobiology

As with all dinosaurs known only from the fossil record, much of Tyrannosaurus biology, including behavior, coloration, ecology, and physiology, remains unknown. However, many new specimens have been discovered in the last twenty years, which has allowed some informed speculation on growth patterns, sexual dimorphism, biomechanics, and metabolism.

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« Reply #7 on: July 23, 2007, 01:31:01 pm »



A graph showing the hypothesized growth curves (body mass versus age) of four tyrannosaurids. Tyrannosaurus rex is drawn in black. Based on Erickson et al. 2004.
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« Reply #8 on: July 23, 2007, 01:32:22 pm »

Life history

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 29.9 kg (66 lb), while the largest, such as FMNH PR2081 ("Sue") most likely weighed over 5400 kg (6 short tons). Histologic analysis of T. 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.[8]

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 T. rex growth curve is S-shaped, with juveniles remaining under 1800 kg (2 short tons) until approximately 14 years of age, when body size began to increase dramatically. During this rapid growth phase, a young T. rex would gain an average of 600 kg (1600 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).[8] Another recent histological study performed by different workers corroborates these results, finding that rapid growth began to slow at around 16 years of age.[28] This sudden change 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 T. 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.[29] Other tyrannosaurids exhibit extremely similar growth curves, although with lower growth rates corresponding to their lower adult sizes.[30]

Over half of the known T. 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 T. 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.
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« Reply #9 on: July 23, 2007, 01:32:55 pm »

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, similarly 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.[31] 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.[32]

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 T. rex genders.[33] 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 T. 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.[1]

Only a single T. 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 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.[29] 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.
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« Reply #10 on: July 23, 2007, 01:33:50 pm »



Replica at Senckenberg Museum, showing modern view of posture.
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« Reply #11 on: July 23, 2007, 01:34:45 pm »

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.[35] 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 T. rex skeleton in 1915. It stood in this upright pose for nearly a century, until it was dismantled in 1992.[36] 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.[37] Despite its inaccuracies, the 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) 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 T. rex with its body approximately parallel to the ground and tail extended behind the body to balance the head.
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« Reply #12 on: July 23, 2007, 01:35:33 pm »



Outdated reconstruction (by Charles R. Knight), showing 'tripod' pose.
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« Reply #13 on: July 23, 2007, 01:36:40 pm »

Arms



When Tyrannosaurus rex was first discovered, the humerus was the only element of the forelimb known.[38] For the initial mounted skeleton as seen by the public in 1915, Osborn substituted longer, three-fingered forelimbs like those of Allosaurus.[24] However, a year earlier, Lawrence Lambe described the short, two-fingered forelimbs of the closely-related Gorgosaurus.[39] This strongly suggested that T. rex had similar forelimbs, but this hypothesis was not confirmed until the first complete T. rex forelimbs were identified in 1989, belonging to MOR 555 (the "Wankel rex").[40] The remains of "Sue" also include complete forelimbs.[1] T. rex 'arms' are very small relative to overall body size, measuring only 1 m (3 ft 3 in) 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.[41] It has also been suggested that the forelimbs were used to assist the animal in rising from a prone position.[37] 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. T. 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 kg (438 lb) by itself; this number would only increase with other muscles (like the brachialis) acting in concert with the biceps. A T. 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.
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« Reply #14 on: July 23, 2007, 01:37:46 pm »



Closeup of forelimb; specimen at National Museum of Natural History, Washington, DC.
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