Bird-hipped dinosaurs mostly chewed plants, not meat
Ornithischia is an extinct clade of mostly plant-eating dinosaurs whose pelvis loosely resembles a bird's, hence the name bird-hipped. It gathered horned ceratopsians, dome-headed pachycephalosaurs, armoured stegosaurs and ankylosaurs, and duck-billed ornithopods. Yet birds descend from theropods, so the hip likeness is a coincidence rather than a sign of close kinship.
Shared traits unite the group: a predentary bone capping the lower jaw, a palpebral bone over the eye, extra hip vertebrae, no belly ribs and a backward-pointing pubis. Members lived on every continent. Some probably supplemented plants with other food, and bonebeds show herds in which juveniles flocked apart from adults. Filaments on Tianyulong, Psittacosaurus and Kulindadromeus fuel debate over whether these were early feathers.
The first ornithischians were light, fast two-legged runners 1 to 2 m long, with big sideways eyes, short arms, a shin longer than the thigh and a tail making up almost half the body as a counterweight. Much of that picture comes from Lesothosaurus, known from several skulls and skeletons from Early Jurassic Lesotho. Size grew over time: armoured forms reached about 7.5 m and 9 tonnes, horned ones 8.5 m and 11 tonnes, and crested duckbills around 15 m and 13.5 tonnes.
Skulls diverged dramatically. Ankylosaurs closed off skull openings, fused their sutures under textured armour and developed elaborate sinuses. Heterodontosaurus grew canine-like front teeth, and hadrosaurs evolved dental batteries of constantly replaced teeth, many adding crests built from several skull bones.
Pachycephalosaurs, once linked with ornithopods, are now grouped with ceratopsians; their thick domes, fused from two roof bones, are often all that survives. Ceratopsians added a rostral bone forming the upper beak and a frill at the back of the skull, and later ceratopsids grew brow and nose horns. Torosaurus and Pentaceratops have skulls over 2 m long, the largest of any known land vertebrate.
Source: Ornithischia
