ORNITHISCHIAN

any member of an order (Ornithischia) of herbivorous dinosaurs with hip bones arranged like those of birds (hence their name, bird-hipped). The ornithischians were one of the two major orders of dinosaurs, the other being the saurischians. Their chief distinguishing feature was the structure of their pelvis, or hip, which had a backward-pointing pubis bone, like those of birds, in contrast to the pubis of saurischians, which extended forward like those of modern reptiles. Ornithischians were also distinguished by an extra bone at the tip of the lower jaw. Many species had a toothless, horny beak that was useful for nipping off vegetation, along with powerful cheek teeth for grinding it up. Many ornithischians also had a lattice of bony tendons that stiffened their tail bones. The ornithischians consisted of two distinct suborders: Cerapoda and Thyreophora. The cerapods are further subdivided into the infraorders Ornithopoda, Pachycephalosauria, and Ceratopsia. The ornithopods all walked on two legs but were otherwise quite diverse in habit, size, and distribution, including as they did the iguanodons and hadrosaurs (duck-billed dinosaurs). The closely related pachycephalosaurs were also bipedal but had dome-shaped heads with high foreheads and thick skulls. The Thyreophora consisted of the infraorders Stegosauria and Ankylosauria. The stegosaurs were four-legged dinosaurs with double rows of bony plates running along their backs. The ankylosaurs were heavily armoured with plates and spikes on their backs, flanks, and heads. The ceratopsians' defensive armour was confined to their massive head, which culminated in a great bony frill to protect the neck and shoulders. Ornithischians were more efficient plant-eaters than were saurischians, partly because their grinding teeth were set recessed within fleshy cheeks that prevented food from falling out of their mouths while they were chewing it. Because of these superior adaptations, the ornithischians outnumbered plant-eating saurischians by the end of the Cretaceous Period (66.4 million years ago).