ornithischia

Ornithischia is a major clade of dinosaurs characterized by a pelvis in which the pubis points backward, giving a “bird-hipped” appearance. This group is defined largely by this pelvic anatomy, along with features commonly associated with herbivory such as beaks and specialized teeth. Ornithischians first appeared in the Late Triassic and became widespread in Jurassic and Cretaceous ecosystems, with fossils found on most continents. They were predominantly herbivores, adapted to processing plant material through beaks, a predentary bone at the lower jaw, and, in many lineages, dental batteries for grinding.

The clade subdivides into two broad lineages: Thyreophora, which includes armored dinosaurs such as Stegosauria and Ankylosauria; and Cerapoda, which further splits into Ornithopoda (including iguanodontids and hadrosaurs) and Marginocephalia (comprising Pachycephalosauria and Ceratopsia). Ornithopods ranged from small, agile forms to large, duck-billed hadrosaurs with complex chewing surfaces. Marginocephalians display notable head ornamentation, with ceratopsians bearing horns and frills and pachycephalosaurs known for thick domed skulls. Thyreophorans developed extensive body armor, from plates and spikes to protective osteoderms.

Ecology and evolution: Ornithischians occupied diverse herbivorous niches across ecosystems, employing various adaptations for feeding, display, and defense. Many groups show elaborate cranial and postcranial ornamentation, likely used in species recognition and mate competition. The lineage persisted from the Late Triassic through the end of the Cretaceous, at which point all non-avian dinosaurs except birds became extinct at the Cretaceous–Paleogene boundary. Birds are the sole surviving descendants of theropod saurischians, not Ornithischia.