Tyrannosauridae - Description

Description

10 meters 3 m Tyrannosaurus rex Tarbosaurus bataar Albertosaurus sarcophagus Daspletosaurus torosus Gorgosaurus libratus Various Tyrannosaurids with a human for scale

The known tyrannosaurids were all large animals. 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 10 metres (26 and 33 ft) long, while Tarbosaurus reached lengths of 12 metres (39 ft) from snout to tail. The massive Tyrannosaurus was the largest, approaching 13 metres (43 ft) in the longest specimens.

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 metre (3 ft 3 in) in length, The largest discovered Tyrannosaurus skull measures over 1.5 metres (4 ft 11 in) 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, 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.

Humerus Radius Metacarpal II Phalanx II-1 Phalanx II-2 Comparison of Tyrannosaurids forelimb element proportions

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. 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.

It is unclear when the arctometatarsus first evolved; it was not present in the earliest tyrannosauroids like Dilong, but was found in the later Appalachiosaurus. This structure also characterized troodontids, ornithomimids and caenagnathids, but its absence in the earliest tyrannosauroids indicates that it was acquired by convergent evolution.