Ornithischia (/ɔːrnɪˈθɪskiə/) is an extinct clade of mainly herbivorous dinosaurscharacterized by a pelvic structure superficially similar to that of birds. The name Ornithischia, or "bird-hipped", reflects this similarity and is derived from the Greek stem 'ornith-' (ὀρνιθ-), meaning "of a bird", and 'ischion' (ἴσχιον), plural 'ischia', meaning "hip joint". However, birds are only distantly related to this group as birds are theropod dinosaurs.
Ornithischians with well known anatomical adaptations include the ceratopsians or "horn-faced" dinosaurs (e.g. Triceratops), armored dinosaurs (Thyreophora) such as stegosaurus and ankylosaurs, pachycephalosaurids and the ornithopods. There is strong evidence that certain groups of ornithischians lived in herds, often segregated by age group, with juveniles forming their own flocks separate from adults. Some were at least partially covered in filamentous (hair- or feather- like) pelts, and there is much debate over whether these filaments found in specimens of Tianyulong, Psittacosaurus, and Kulindadromeusmay have been primitive feathers.