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Dinosaurs Tetanurae. Theropods that featured large heads, like Allosaurus and Tyrannosaurus, had long, strong lower jaws that
undoubtedly were powered by massive jaw muscles. The skull was a highly fenestrated strut work, both for
lightness and for strength, providing ample attachment areas for muscles. The jaws are noted for their
complement of sharp, bladelike teeth. In nearly all theropods these laterally compressed blades had
steak-knife-like serrations along the rear edge and often along the front edge as well. Among the predatory
adaptations displayed by most kinds of theropods, the characteristic teeth were the most conspicuous. The
diversity of the suborder Theropoda, with its various modes of predation and carnivory, is suggested by the
following summary of the group's two infraorders.
Tetanurae. These comprise all the nonceratosaurian theropods. The tetanuran theropods are subdivided into five distinct categories: the coelurosaurs, the ornithomimosaurs, the maniraptors, the segnosaurs, and the carnosaurs. The coelurosaurs were small to medium-size carnivores, the smallest known being Compsognathus. Coelurosaurs had very long legs of cursorial proportions and had forelimbs and hands ranging from short (Compsognathus) to long and grasping (Ornitholestes). Ornithomimosaurs were medium-size to large theropods. They were toothless and apparently beaked, with very long legs and arms. A well-known example is Struthiomimus. Most were ostrich-size and were designed for fast running. The largest kind was Deinocheirus from Asia, known only from one specimen consisting of complete arms and hands almost three metres (nine feet) long‹nearly four times longer than those of Struthiomimus. These animals' cursorial design, toothlessness, and hands unsuited for seizing prey leave their lifestyle and feeding habits unclear. The maniraptors are also known as deinonychosaurs and include the oviraptors and troodontids. These medium-size predators had long, grasping arms and hands, moderately long legs, and a specialized tail that could be held high for active balance control. Their feet bore the primary killing device, large slicing talons on the inside toes. The best-known examples are Deinonychus of North America and Velociraptor of Asia. Segnosaurs were medium-size Asian theropods known only from a few examples. The mouth had bladelike teeth at the back but apparently no teeth at the front. The pelvis differed markedly from the normal saurischian design. They are very inadequately understood but seem to have been unlike all other theropods. The carnosaurs were large to very large (up to 6- or 7-ton) carnivores with blade-toothed jaws, twice or more as long as the arms and hands, powerful hind legs, and taloned feet. It is not certain whether they were predators or carrion feeders. Tyrannosaurus is the most commonly cited example. Ornithischia The order Ornithischia, unlike the Saurischia, appears to be a natural group of closely related animals. All were plant eaters, and all are thought to have descended from a common ancestor. The rationale for postulating such an ancestor is based on the common existence of a uniquely ornithischian feature‹a median predentary bone that joined the two lower jaws at the symphysis. Further, a distinctive tooth form, crenulated along the upper edges, occurred in some members of all suborders. Collectively, these features point to a close common ancestry. The order has traditionally been divided into four suborders. However, recent studies have regrouped the members of this order into two major categories, the suborders Cerapoda and Thyreophora. Ornithischia Cerapoda The suborder Cerapoda is divided into three infraorders: Ornithopoda, Pachycephalosauria, and Ceratopsia. Ornithopoda The ornithopods ranged in size from the small fabrosaurids, 1 to 2 metres long, to the huge duck-billed types that reached lengths of 10 to 12 metres or more. Ornithopods appear to have flourished the longest of the Dinosaur varieties, thriving from the Late Triassic to the latest part of the Cretaceous. They inhabited all land areas except Antarctica. Ornithopod families included the early and somewhat primitive fabrosaurids, mostly from Eurasia and southern
Africa, and heterodontosaurs, also largely from southern Africa. In the latter group was the oldest of the
ornithischians, Pisanosaurus, a single very fragmentary specimen from the Late Triassic of Argentina. Better
known are the slightly larger hypsilophodonts and much larger iguanodonts, mostly from North America and
Europe. Representative of these groups are Hypsilophodon, about three metres in length, and the famed
Iguanodon, about nine metres long. Another group, the hadrosaurs, sometimes called trachodonts, were the
large duck-billed ornithopods of the Late Cretaceous, especially of North America and Eurasia. An abundant and
diverse family, they included more than two dozen known genera. (The Late Triassic or Early Jurassic
Scutellosaurus had some similarities to the ornithopods, but its affinities are still uncertain.)
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