Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family of coelurosaurian dinosaurs that comprises two subfamilies containing up to fifteen genera, including the eponymous Tyrannosaurus. The exact number of genera is controversial, with some experts recognizing as few as three. All of these animals lived near the end of the Cretaceous period and their fossils have been found only in North America and Asia. Well-known genera include Tyrannosaurus, Albertosaurus, Gorgosaurus, Daspletosaurus and Tarbosaurus. Although descended from smaller ancestors, tyrannosaurids were almost always the largest predators in their respective ecosystems, putting them at the apex of the food chain. The largest species was Tyrannosaurus rex, the most massive known terrestrial predator, which measured over 13 metres (43 ft) in length and according to most modern estimates up to 8.87 metric tons (9.78 short tons) in weight. Tyrannosaurids were bipedal carnivores with massive skulls filled with large teeth. Despite their large size, their legs were (outside of adults of the largest taxa) relatively long and cursorial for swift movement. In contrast, their arms were very small, bearing only two functional digits. Unlike most other groups of dinosaurs, very complete remains have been discovered for most known tyrannosaurids. This has allowed a variety of research into their biology. Scientific studies have focused on their ontogeny, biomechanics and ecology, among other subjects.
History of discovery
The first remains of tyrannosaurids were uncovered during expeditions led by the Geological Survey of Canada, which located numerous scattered teeth. These distinctive dinosaur teeth were given the name Deinodon ("terrible tooth") by Joseph Leidy in 1856. The first good specimens of a tyrannosaurid were found in the Horseshoe Canyon Formation of Alberta, and consisted of nearly complete skulls with partial skeletons. These remains were first studied by Edward Drinker Cope in 1876, who considered them a species of the eastern tyrannosauroid Dryptosaurus. In 1905, Henry Fairfield Osborn recognized that the Alberta remains differed considerably from Dryptosaurus, and coined a new name for them: Albertosaurus sarcophagus ("flesh-eating Alberta lizard"). Cope described more tyrannosaur material in 1892, in the form of isolated vertebrae, and gave this animal the name Manospondylus gigas. This discovery was mostly overlooked for over a century, and caused controversy in the early 2000s when it was discovered that this material actually belonged to, and had name priority over, Tyrannosaurus rex.
In his 1905 paper naming Albertosaurus, Osborn described two additional tyrannosaur specimens that had been collected in Montana and Wyoming during a 1902 expedition of the American Museum of Natural History, led by Barnum Brown. Initially, Osborn considered these to be distinct species. The first, he named Dynamosaurus imperiosus ("emperor power lizard"), and the second, Tyrannosaurus rex ("king tyrant lizard"). A year later, Osborn recognized that these two specimens actually came from the same species. Despite the fact that Dynamosaurus had been found first, the name Tyrannosaurus had appeared one page earlier in his original article describing both specimens. Therefore, according to the International Code of Zoological Nomenclature (ICZN), the name Tyrannosaurus was used. Barnum Brown went on to collect several more tyrannosaurid specimens from Alberta, including the first to preserve the shortened, two-fingered forelimbs characteristic of the group (which Lawrence Lambe named Gorgosaurus libratus, "balanced fierce lizard", in 1914). A second significant find attributed to Gorgosaurus was made in 1942, in the form of a well-preserved, though unusually small, complete skull. The specimen waited until after the end of World War II to be studied by Charles W. Gilmore, who named it Gorgosaurus lancesnis. This skull was re-studied by Robert T. Bakker, Phil Currie, and Michael Williams in 1988, and assigned to the new genus Nanotyrannus. It was also in 1946 that paleontologists from the Soviet Union began expeditions into Mongolia, and uncovered the first tyrannosaur remains from Asia. Evgeny Maleev described new Mongolian species of Tyrannosaurus and Gorgosaurus in 1955, and one new genus: Tarbosaurus ("terrifying lizard"). Subsequent studies, however, showed that all of Maleev's tyrannosaur species were actually one species of Tarbosaurus at different stages of growth. A second species of Mongolian tyrannosaurid was found later, described by Sergei Kurzanov in 1976, and given the name Alioramus remotus ("remote different branch"), though its status as a true tyrannosaurid and not a more primitive tyrannosaur is still controversial.
Description
The tyrannosaurids were all large animals, with all species capable of weighing at least 1 metric ton. A single specimen of Alioramus of an individual estimated at between 5 and 6 metres (16 and 20 ft) long has been discovered, although it is considered by some experts to be a juvenile. Albertosaurus, Gorgosaurus and Daspletosaurus all measured between 8 and 9 metres (26 and 30 ft) long, while Tarbosaurus reached lengths of 11 metres (36 ft) from snout to tail. The massive Tyrannosaurus reached 13 metres (43 ft) in the largest specimen, RSM P2523.8. Tyrannosaurids are united by numerous synapomorphies, including an interfenestral strut that is less than half the rostrocaudal length of the maxillary fenestra, a lacrimal horn with a distinct apex in lateral view, and a pronounced lateral ridge on the postorbital ramus of the jugal that braces its articulation with the postorbital, among many other derived features.
Skull and dentition
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