Tyrannosaurus (meaning "tyrant lizard") is a genus of coelurosaurian 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 Maastrichtian age of the upper Cretaceous Period, 67 to 65.5 million years ago. It was among the last non-avian dinosaurs to exist before the Cretaceous–Paleogene 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, Tyrannosaurusforelimbs 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 13-14 m (40-43 ft) in length, up to 6 metres (13 ft) tall at the hips, and up to 4-6 tons in weight. By far the largest carnivore in its environment, Tyrannosaurus rex may have been an apex predator, preying upon hadrosaurs and ceratopsians,
abundance of fossil material has allowed significant research into many aspects of its biology, including life history and biomechanics. The feeding habits, physiologyand 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 withTyrannosaurus. Tyrannosaurus rex was one of the largest land carnivores of all time; the largest complete specimen, FMNH PR2081 ("Sue"), measured 14 metres (46 ft) long, and was 6 metres (20 ft) tall at the hips. Mass estimates have varied widely over the years, from more than 7.2 metric tons (7.9 short tons), to less than 4.5 metric tons (5.0 short tons), with most modern estimates ranging between 5.4 and 6.8 metric tons (6.0 and 7.5 short tons). Packard et al.(2009) tested dinosaur mass estimation procedures on elephants and concluded that dinosaur estimations are flawed and produce over-estimations; thus, the weight of Tyrannosaurus could be much less than usually estimated. Other estimations have concluded that the largest knownTyrannosaurus specimens had a weight exceeding 9 tonnes.
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, along with an additional small metacarpal representing the remnant of a third digit. 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.
The largest known Tyrannosaurus rex skulls measure up to 5 feet (1.5 m) in length. Largefenestrae (openings) in the skull reduced weight and provided areas for muscle attachment, as in all carnivorous theropods. But in other respects Tyrannosaurus's skull was significantly different from those of large non-tyrannosauroid theropods. It was extremely wide at the rear but had a narrow snout, allowing unusually good binocular vision. 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 thetyrannosaurid trend towards an increasingly powerful bite, which easily surpassed that of all non-tyrannosaurids. 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.
The teeth of Tyrannosaurus rex displayed marked heterodonty (differences in shape). Thepremaxillary 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. 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 yet found. 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, both of which have occasionally been synonymized with Tyrannosaurus. 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.
In 1955, Soviet paleontologist Evgeny Maleev named a new species, Tyrannosaurus bataar, fromMongolia. By 1965, this species had been renamed Tarbosaurus bataar. Despite the renaming, many phylogenetic analyses have found Tarbosaurus bataar to be the sister taxon ofTyrannosaurus rex, and it has often been considered an Asian species ofTyrannosaurus. 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. A related cladistic analysis found that Alioramus, not Tyrannosaurus, was the sister taxon of Tarbosaurus, which, if true, would suggest that Tarbosaurus and Tyrannosaurusshould remain separate.