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Tyrannosauridae Fossil range: Late Cretaceous
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Sauropsida Superorder: Dinosauria Order: Saurischia Suborder: Theropoda Superfamily: Tyrannosauroidea Family: Tyrannosauridae Osborn, 1905
Genera
Albertosaurus Alioramus Daspletosaurus Gorgosaurus Tarbosaurus Tyrannosaurus (type)
Synonyms
Deinodontidae Brown, 1914 (=Dinodontidae Cope, 1866)

Tyrannosauridae (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 nearly 13 metres (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, most tyrannosaurids are known from very complete remains. This has allowed a wide 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.

[edit] Description

The known tyrannosaurids were all large animals.^[1] Alioramus is known from the remains of an individual estimated at between 5 and 6 meters (16.5 to 20 ft) long,^[2] although it is considered by some experts to be a juvenile.^[1][3] Albertosaurus, Gorgosaurus and Daspletosaurus all measured between 8 and 10 meters (26 and 33 ft) long,^[4] while Tarbosaurus reached lengths of 12 meters (40 ft) from snout to tail.^[5] The massive Tyrannosaurus was the largest, approaching 13 meters (43 ft) in the longest specimens.^[6]

Tyrannosaurid skull anatomy is well understood as complete skulls are known for all genera but Alioramus, which is known only from partial skull remains. Tyrannosaurus, Tarbosaurus, and Daspletosaurus had skulls which exceeded 1 meter (3.3 ft) in length, with the largest Tyrannosaurus skull measuring over 1.5 meters (5 ft) long. Adult tyrannosaurids had tall, massive skulls, with many bones fused and reinforced for strength. At the same time, hollow chambers within many skull bones and large openings (fenestrae) between those bones helped to reduce skull weight. Many features of tyrannosaurid skulls were also found in their immediate ancestors, including tall premaxillae and fused nasal bones. Tyrannosaurid skulls had many unique characteristics, however, including fused parietal bones with a prominent sagittal crest, which ran longitudinally along the sagittal suture and separated the two supratemporal fenestrae on the skull roof. Behind these fenestrae, tyrannosaurids had a characteristically tall nuchal crest, which also arose from the parietals but ran along a transverse plane rather than longitudinally. The nuchal crest was especially well-developed in Tyrannosaurus, Tarbosaurus and Alioramus. Albertosaurus, Daspletosaurus and Gorgosaurus had tall crests in front of the eyes on the lacrimal bones, while Tarbosaurus and Tyrannosaurus had extremely thickened postorbital bones forming crescent-shaped crests behind the eyes. Alioramus had a row of six bony crests on top of its snout, arising from the nasal bones; lower crests have been reported on some specimens of Daspletosaurus and Tarbosaurus, as well as the more basal tyrannosauroid Appalachiosaurus.^[3][7] Tyrannosaurids, like their ancestors, were heterodont, with premaxillary teeth D-shaped in cross section and smaller than the rest. Unlike earlier tyrannosauroids and most other theropods, however, the maxillary and mandibular teeth of mature tyrannosaurid s are not blade-like but extremely thickened and often circular in cross-section.^[1] Tooth counts tend to be consistent within species, and larger species tend to have lower tooth counts than smaller ones. For example, Alioramus had 76 to 78 teeth in its jaws, while Tyrannosaurus had between 54 and 60.^[8]

The skull was perched at the end of a thick, S-shaped neck, and a long, heavy tail acted as a counterweight to balance out the head and torso, with the center of mass over the hips. Tyrannosaurids are known for their proportionately very small two-fingered forelimbs, although remnants of a vestigial third digit are sometimes found.^[1][9] Tarbosaurus had the shortest forelimbs compared to its body size, while Daspletosaurus had the longest. Tyrannosaurids walked exclusively on their hindlimbs, so their leg bones were massive. In contrast to the forelimbs, the hindlimbs were longer compared to body size than almost any other theropods. Juveniles and even some smaller adults, like more basal tyrannosauroids, had longer tibiae than femora, a characteristic of fast-running dinosaurs like ornithomimids. Larger adults had leg proportions characteristic of slower-moving animals, but not to the extent seen in other large theropods like abelisaurids or carnosaurs. The third metatarsals of tyrannosaurids were pinched between the second and fourth metatarsals, forming a structure known as the arctometatarsus.^[1] It is unclear when the arctometatarsus first evolved; it was not present in the earliest tyrannosauroids like Dilong,^[10] but was found in the later Appalachiosaurus.^[7] This structure also characterized troodontids, ornithomimids and caenagnathids,^[11] but its absence in the earliest tyrannosauroids indicates that it was acquired by convergent evolution.^[10]

[edit] Taxonomy and systematics

Tyrannosaurus was named by Henry Fairfield Osborn in 1905, along with the family Tyrannosauridae.^[12] 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.^[13] The family name Deinodontidae was often used by scientists up until the 1920s,^[14] based on the genus Deinodon, which was named after isolated teeth from Montana.^[15] This taxon, however, is now considered a nomen dubium so the name Tyrannosauridae is preferred by all modern experts.^[1]

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.^[16]. 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.^[1] 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.^[17] 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 defintion, including all theropods more closely related to Tyrannosaurus than to Eotyrannus.^[1] The most recent definition is that of Sereno in 2005, which defined Tyrannosauridae as the least inclusive clade containing Albertosaurus, Gorgosaurus and Tyrannosaurus.^[18]

[edit] Classification

FAMILY TYRANNOSAURIDAE

• Subfamily Albertosaurinae
  • Albertosaurus
  • Gorgosaurus
• Subfamily Tyrannosaurinae
  • Alioramus
  • Daspletosaurus
  • Tarbosaurus
  • Tyrannosaurus

[edit] 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.^[1] Some authors include the species Gorgosaurus libratus in the genus Albertosaurus and Tarbosaurus bataar in the genus Tyrannosaurus,^[7][19][20] while others prefer to retain Gorgosaurus and Tarbosaurus as separate genera.^[1][3] Albertosaurines are characterized by more slender builds, lower skulls, and proportionately longer tibiae than tyrannosaurines.^[1] In tyrannosaurines, the sagittal crest on the parietals continues forward onto the frontals.^[3]

Within Tyrannosaurinae, Tarbosaurus and Tyrannosaurus are often considered 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.^[1][7] 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.^[8] 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.^[21] 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,^[3] rather than a juvenile Tyrannosaurus as most other experts believe.^[1][22]

[edit] Distribution

While earlier tyrannosauroids are found on all three northern continents, tyrannosaurid fossils are known only from North America and Asia.

Tyrannosaurid remains are only found in Asia and western North America. The exact time and place of origin of the family remain unknown due to the poor fossil record in the middle part of the Cretaceous on both continents, although the earliest confirmed tyrannosaurids lived in the early Campanian stage in western North America.^[1] Tyrannosaurid remains have never been recovered from eastern North America, while more basal tyrannosauroids like Dryptosaurus and Appalachiosaurus persisted there until the end of the Cretaceous, indicating that tyrannosaurids must have evolved in or dispersed into western North America after the continent was divided in half by the Western Interior Seaway in the middle of the Cretaceous.^[7] Tyrannosaurid fossils have been found in Alaska, which may have provided a route for dispersal between North America and Asia.^[23] Alioramus and Tarbosaurus are found to be related in one cladistic analysis, forming a unique Asian branch of the family.^[8]

Of the two subfamilies, tyrannosaurines appear to have been more widespread. Albertosaurines are unknown in Asia, which was home to the tyrannosaurines Tarbosaurus and Alioramus. Both subfamilies were present in the Campanian and early Maastrichtian stages of North America, with tyrannosaurines like Daspletosaurus ranging throughout the Western Interior, while the albertosaurines Albertosaurus and Gorgosaurus are currently known only from the northwestern part of the continent. By the late Maastrichtian, albertosaurines appear to have gone extinct, while the tyrannosaurine Tyrannosaurus roamed from Saskatchewan to Texas. This pattern is mirrored in other North American dinosaur taxa. During the Campanian and early Maastrichtian, lambeosaurine hadrosaurs and centrosaurine ceratopsians are common in the northwest, while hadrosaurines and chasmosaurines were more common to the south. By the end of the Cretaceous, centrosaurines are unknown and lambeosaurines are rare, while hadrosaurines and chasmosaurines were common throughout the Western Interior.^[1]

[edit] Paleobiology

[edit] References

[edit] External Links