Tyrannosauridae

Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family of coelurosaurian theropod dinosaurs which comprises two subfamilies containing up to six 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.

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, one of the largest known land predators, which measured up to 13 meters (43 feet) in length and up to 6.8 metric tons (7.5 short tons) in weight. Tyrannosaurids were bipedal carnivores with massive skulls filled with large teeth. Despite their large size, their legs were long and proportioned for fast 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 tyrannosaurids. This has allowed a variety of research into their biology. Scientific studies have focused on their ontogeny, biomechanics and ecology, among other subjects. Soft tissue, both fossilized and intact, has been reported from one specimen of Tyrannosaurus rex.

Taxonomy and systematics
Tyrannosaurus was named by Henry Fairfield Osborn in 1905, along with the family Tyrannosauridae. The name is derived from the Ancient Greek words τυραννος/tyrannos ('tyrant') and σαυρος/sauros ('lizard'). The very common suffix -idae is normally appended to zoological family names and is derived from the Greek suffix -ιδαι/-idai, which indicates a plural noun. The family name Deinodontidae was often used by scientists up until the 1920s, based on the genus Deinodon, which was named after isolated teeth from Montana. This taxon, however, is now considered a nomen dubium so the name Tyrannosauridae is preferred by all modern experts.

Tyrannosauridae is a family in rank-based Linnaean taxonomy, within the superfamily Tyrannosauroidea and the suborder Theropoda. With the advent of phylogenetic taxonomy in vertebrate paleontology, Tyrannosauridae has been given several explicit definitions. The original was produced by Paul Sereno in 1998, and included all tyrannosauroids closer to Tyrannosaurus than to either Alectrosaurus, Aublysodon or Nanotyrannus. . However, Nanotyrannus is often considered to be a juvenile Tyrannosaurus rex, while Aublysodon is usually regarded as a nomen dubium unsuitable for use in the definition of a clade. Definitions since then have been based on more well-established genera.

A 2003 attempt by Christopher Brochu included Albertosaurus, Alectrosaurus, Alioramus, Daspletosaurus, Gorgosaurus, Tarbosaurus and Tyrannosaurus in the definition. Holtz redefined the family in 2004 to use all of the above as specifiers except for Alioramus and Alectrosaurus, which his analysis could not place with certainty. However, in the same paper, Holtz also provided a completely different definition, including all theropods more closely related to Tyrannosaurus than to Eotyrannus. The most recent definition is that of Sereno in 2005, which defined Tyrannosauridae as the least inclusive clade containing Albertosaurus, Gorgosaurus and Tyrannosaurus.

Classification
Carr et al. 2005

* Note : Carr et al. regard Gorgosaurus libratus as a species of Albertosaurus and Tarbosaurus bataar as a species of Tyrannosaurus

Currie et al. 2003

FAMILY TYRANNOSAURIDAE
 * Subfamily Albertosaurinae
 * Albertosaurus
 * Gorgosaurus
 * Subfamily Tyrannosaurinae
 * Alioramus
 * Daspletosaurus
 * Tarbosaurus
 * Tyrannosaurus

Phylogeny
Tyrannosauridae is uncontroversially divided into two subfamilies. Albertosaurinae comprises the North American genera Albertosaurus and Gorgosaurus, while Tyrannosaurinae includes Daspletosaurus, Tarbosaurus and Tyrannosaurus itself. Some authors include the species Gorgosaurus libratus in the genus Albertosaurus and Tarbosaurus bataar in the genus Tyrannosaurus,  while others prefer to retain Gorgosaurus and Tarbosaurus as separate genera. Albertosaurines are characterized by more slender builds, lower skulls, and proportionately longer tibiae than tyrannosaurines. In tyrannosaurines, the sagittal crest on the parietals continues forward onto the frontals.

Cladistic analyses of tyrannosaurid phylogeny often find Tarbosaurus and Tyrannosaurus to be sister taxa, with Daspletosaurus more basal than either. A close relationship between Tarbosaurus and Tyrannosaurus is supported by numerous skull features, including the pattern of sutures between certain bones, the presence of a crescent-shaped crest on the postorbital bone behind each eye, and a very deep maxilla with a noticeable downward curve on the lower edge, among others. An alternative hypothesis was presented in a 2003 study by Phil Currie and colleagues, which found weak support for Daspletosaurus as a basal member of a clade also including Tarbosaurus and Alioramus, both from Asia, based on the absence of a bony prong connecting the nasal and lacrimal bones. Alioramus was found to be the closest relative of Tarbosaurus in this study, based on a similar pattern of stress distribution in the skull.

A related study also noted a locking mechanism in the lower jaw shared between the two genera. In a separate paper, Currie noted the possibility that Alioramus might represent a juvenile Tarbosaurus, but stated that the much higher tooth count and more prominent nasal crests in Alioramus suggest it is a distinct genus. Similarly, Currie uses the high tooth count of Nanotyrannus to suggest that it may be a distinct genus, rather than a juvenile Tyrannosaurus as most other experts believe.

Feathers
Long filamentous structures have been preserved along with skeletal remains of numerous coelurosaurs from the Early Cretaceous Yixian Formation and other nearby geological formations from Liaoning, China. These filaments have usually been interpreted as "protofeathers," homologous with the branched feathers found in birds and some non-avian theropods, although other hypotheses have been proposed. A skeleton of Dilong was described in 2004 that included the first example of "protofeathers" in a tyrannosauroid. Similarly to down feathers of modern birds, the "protofeathers" found in Dilong were branched but not pennaceous, and may have been used for insulation.

It has also been theorized that tyrannosaurids had such protofeathers. However, rare skin impressions from adult tyrannosaurids in Canada and Mongolia show pebbly scales typical of other dinosaurs. While it is possible that protofeathers existed on parts of the body which have not been preserved, a lack of insulatory body covering is consistent with modern multi-ton mammals such as elephants, hippopotamus, and most species of rhinoceros. As an object increases in size, its ability to retain heat increases due to its decreasing surface area-to-volume ratio. Therefore, as large animals evolve in or disperse into warm climates, a coat of fur or feathers loses its selective advantage for thermal insulation and can instead become a disadvantage, as the insulation traps excess heat inside the body, possibly overheating the animal. Protofeathers may also have been secondarily lost during the evolution of large tyrannosaurids, especially in warm Cretaceous climates.