|
Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family of coelurosaurian theropod dinosaurs which comprises two subfamilies containing up to eleven 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 in length and up to in weight.〔 and corresponding body mass estimates ranged from 29.9 to 5654 kg〕 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 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. Soft tissue, both fossilized and intact, has been reported from one specimen of ''Tyrannosaurus rex''. ==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 long has been discovered, although it is considered by some experts to be a juvenile.〔 ''Albertosaurus'', ''Gorgosaurus'' and ''Daspletosaurus'' all measured between long, while ''Tarbosaurus'' reached lengths of from snout to tail. The massive ''Tyrannosaurus'' reached in the largest specimen, FMNH PR2081.〔() 〕 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 in length,〔 The largest discovered ''Tyrannosaurus'' skull measures over 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''.〔 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.〔〔 (only )〕 ''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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Tyrannosauridae」の詳細全文を読む スポンサード リンク
|