TyrannosaurusTyrannosaurus — Tyrannosaurus 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 lived in 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. Several other genera of North American tyrannosaurids have also been synonymized with Tyrannosaurus.

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. While Tyrannosaurus was long thought to have only two digits on each hand, the discovery of a complete T. rex forelimb in 2007 showed that it in fact had three fingers, though the third was vestigial. 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 nearly 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 often subjects of debate.

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.

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 (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). Another recent histological study performed by different workers corroborates these results, finding that rapid growth began to slow at around 16 years of age. 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. Other tyrannosaurids exhibit extremely similar growth curves, although with lower growth rates corresponding to their lower adult sizes.

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. 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.

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