Tyrannosaurus

Tyrannosaurus ("tyrant lizard" ) is a genus of theropod dinosaur from the Late Cretaceous Period of North America. It is popularly known by its full binomial name, Tyrannosaurus rex (which is often shortened to T. rex).

Size
Tyrannosaurus was one of the largest theropod dinosaurs, and indeed land carnivores, of all time. The largest specimen so far discovered was 14 meters long and 4 meters tall at the hips. Estimates of its mass have varied widely from 4.5 to 7.2 metric tons, with modern estimates usually falling in the range of approximately 6 metric tons. Although Tyrannosaurus was once thought to be the largest carnivorous dinosaur ever, several other genera from Africa and South America are larger in both length or weight.

Skull
The skull of Tyrannosaurus could reach 1.5 meters in length, and was quite different from other non-tyrannosauroid theropods. The rear of its skull was wide but the front of the skull was narrow, giving it excellent binocular vision. Its nasal bones were fused yet flexible, giving Tyrannosaurus a powerful bite.

Its teeth varied in shape, and were generally better equipped for crushing rather than for slicing. The largest tooth of Tyrannosaurus yet found that includes the root is 30 centimeters long, which is the largest tooth of any carnivorous dinosaur currently known.

A 2012 study suggested that the bite force of Tyrannosaurus may have been the strongest of any terrestrial animal yet known, measuring from 35000 to 57000 newtons in the back teeth. This is three times the force estimated for a great white shark, and seven times the force estimated for the theropod dinosaur Allosaurus. A 2003 study estimated an even higher bite force, measuring 183,000 to 235,000 newtons.

Body
The arms of Tyrannosaurus were short and had only two clawed fingers, with the remnants of a third digit in the form of a tiny metacarpal bone. However, its hind limbs were among the longest in proportion to body size of all theropod dinosaurs. Its large head was balanced by a heavy tail, and many of its bones were pneumatized to reduce the animal's weight without losing strength.

Classification
Tyrannosaurus is an advanced tyrannosaurid, and is the type genus of the superfamily Tyrannosauroidea. Tyrannosaurids were originally believed to be descendants of the carnosaurs, but they are in fact gigantic descendants of the coelurosaur family.

The Asian tyrannosaurid Tarbosaurus was originally described as a species of Tyrannosaurus, and is still sometimes thought to be synonymous with that genus. However, the skull of Tarbosaurus is narrower than that of Tyrannosaurus, and it would have had a much smaller bite force.

Many smaller tyrannosaurids have been described from the same time and place as Tyrannosaurus, but virtually all of them are now considered to be juveniles of T. rex. The one exception may be the genus Nanotyrannus, which was originally classified as a Gorgosaurus in 1946. Many scientists consider it to be a juvenile Tyrannosaurus, but others suggest minor differences in the skull preclude Nanotyrannus being synonymized without additional fossil evidence.

Below is a cladogram of Tyrannosauridae based on the 2013 description of Lythronax, showing the position of Tyrannosaurus:

Early discoveries
The first teeth of the dinosaur we now know as Tyrannosaurus were found by Arthur Lakes in Colorado in 1874. Additional remains were collected from Wyoming in the early 1890s by John Bell Hatcher, but they were thought to come from a gigantic species of Ornithomimus, and were named O. grandis. Edward Drinker Cope found some vertebral fragments in 1892, and named them Manospondylus gigas (see Manospondylus, below).

Barnum Brown, a paleontologist working at the American Museum of Natural History in New York City, discovered a partial theropod skeleton in eastern Wyoming in 1900, and discovered another partial skeleton in the Hell Creek Formation of Montana two years later. In 1905, Henry Fairfield Osborn published a paper that described the Hell Creek specimen as Tyrannosaurus rex and the Wyoming specimen as Dynamosaurus imperiosus. A year later, Osborn realized the two genera were synonymous, and as Tyrannosaurus was described before Dynamosaurus in his paper he chose the former as the valid name. Today, the holotype specimen of Tyrannosaurus is at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, and Brown's largest specimen of T. rex (also discovered at Hell Creek, in 1908) remains at the American Museum of Natural History.

Although skeletal material of Tyrannosaurus is well known, only one footprint from the dinosaur was identified, located in New Mexico and identified in 1994.

Manospondylus
In 1892, Edward Drinker Cope described two partial vertebrae from South Dakota as Manospondylus gigas. At the time, he believed them to be from a ceratopsian. The remains were soon correctly identified as being from a theropod, and Henry Osborn realized that Manospondylus and Tyrannosaurus were quite similar. However, he did not synonymize the two genera, as Manospondylus was too fragmentary to be identified any further.

In June 2000, the Black Hills Institute of South Dakota re-discovered the original locality of Manospondylus, and found bones from the same 1892 individual that were identical to those of Tyrannosaurus. According to the general rules of the International Code of Zoological Nomenclature, the name Manospondylus should technically have priority over Tyrannosaurus, because it was named over a decade earlier, and the earliest used name takes priority under normal circumstances. However, the Fourth Edition of the ICZN which took effect in 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 ..." According to these revisions, Tyrannosaurus is the valid name and takes priority.

Notable specimens
An 85% complete specimen of Tyrannosaurus was discovered in the Hell Creek Formation of South Dakota in 1990, and named "Sue" after its discoverer. It was the subject of a legal battle over its ownership, and eventually ended up at auction, where it was purchased by the Field Museum of Natural History of Chicago for USD $7.6 million. "Sue" has been very well studied and has given paleontologists important information about Tyrannosaurus paleobiology.

Another Tyrannosaurus, named "Stan" (again after its discoverer) was identified in 1992 from South Dakota. It displays interesting pathologies and has been cast for museums around the world.

Paleoecology
Tyrannosaurus lived at the very end of the Mesozoic Era, during the Maastrichtian age of the Late Cretaceous Period. It ranged from southern Canada in the north to at least Texas and New Mexico in the south of western North America. Tyrannosaurid teeth that could be attributed to the Tyrannosaurus genus have also been found in the Lomas Coloradas Formation of Mexico. Fossils show that it lived in a wide variety of ecosystems.

The Hell Creek Formation preserves a number of well-known, well-preserved Tyrannosaurus specimens. During the time in which T. rex lived, this area was a subtropical floodplain with little seasonal change. The flora consisted largely of angiosperms, but there were also a variety of gymnosperms such as Metasequoia. The most abundant animal in the area was Triceratops, a common prey item of Tyrannosaurus. It also shared the ecosystem with the marginocephalians Torosaurus and Pachycephalosaurus, hadrosaurid Edmontosaurus, armored dinosaur Ankylosaurus, and the theropods Ornithomimus, Troodon, and Anzu.

Another formation with good Tyrannosaurus remains is the Lance Formation of Wyoming. This region was a bayou environment similar to the modern Gulf Coast. The fauna here was similar to that found in the Hell Creek Formation, although there were a few notable differences.

In its southern range, the habitat in which Tyrannosaurus lived consisted of semiarid plains, which may be due to the retreat of the Western Interior Seaway that occurred during this time. The fauna in this region included the titanosaurian sauropod Alamosaurus, Torosaurus, Edmontosaurus, and the pterosaur Quetzalcoatlus.

Diet
Tyrannosaurus was a carnivore with serrated teeth, likely ambushing prey and then striking the final blow. There is debate about whether T. rex was a hunter or a scavenger. Current studies suggest that it was both of these. Its teeth were the bluntest of any theropod, but this was because they were designed for crushing bone, not slicing meat.

In popular culture
Main article: Tyrannosaurus in Popular Culture

Tyrannosaurus is the dinosaur that appears most frequently in many media, and it has appeared in a wide variety of places from documentaries, to sci-fi movies, to kids' shows.