Ornithischia

Ornithischia (/ɔːrnᵻˈθɪskiə/ or-ni-THISS-kee-ə) or Predentata is an extinct clade of beaked, herbivorous dinosaurs. The name ornithischia is derived from the Greek ornitheos (ορνιθειος) meaning 'of a bird' and ischion (ισχιον) meaning 'hip joint'. They are known as the 'bird-hipped' dinosaurs because of their bird-like hip structure, even though birds actually belong to the 'lizard-hipped' dinosaurs (the saurischians).

Some commonly known Ornithischians include the horned dinosaurs (ceratopsians), armored dinosaurs (stegosaurs and ankylosaurs), Iguanodon, and the "duck-billed" dinosaurs (hadrosaurids). Being herbivores that sometimes lived in herds, they were more numerous than the saurischians. Many were prey animals for the theropods and were usually smaller than the sauropods.

Characteristics

The clade Dinosauria was divided into the two orders Ornithischia and Saurischia by Harry Seeley in 1887. This division, which has generally been accepted, is based in part on the evolution of the pelvis into a more bird-like structure (although birds did not descend from ornithischian dinosaurs), details in the vertebrae and armor, and the possession of a predentary bone. The predentary is a single toothless bone that lies in the front of the dentaries, extending the lower jaw. The predentary coincides with the premaxilla in the upper jaw. Together they form a beak-like apparatus used to clip off plant material.