Ornithischia

Ornithischia, meaning "bird-hipped," is one of the two great branches of dinosaurs, encompassing a mostly herbivorous radiation whose members are known from every continent and across an enormous range of sizes. The group takes its name from a pelvis superficially resembling that of birds, though this is a case of convergence, since birds are in fact theropods and only distantly related. Its major subgroups include the horn-faced ceratopsians, the thick-domed pachycephalosaurs, the armored thyreophorans such as stegosaurs and ankylosaurs, and the ornithopods, alongside a scattering of small, early-diverging forms near the base of the group. The earliest members were lightly built bipeds around one to two meters long with long counterbalancing tails, but later lineages grew far larger and many became quadrupedal. Most were browsers or grazers of ground-level plants, though a few were opportunistic omnivores. Several ornithischians are known to have lived in herds, sometimes with juveniles flocking apart from adults, and exceptionally preserved specimens reveal a range of skin coverings from mosaic scales to disputed filamentous, quill-like structures.