Ornithischia (/ɔrnɨˈθɪskiə/ or-ni-THISS-kee-ə) or Predentata is an extinct order of beaked, herbivorous (though a minor few are known to be omnivorous) 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 birdlike hip structure, even though birds actually belong to the 'lizard-hipped' dinosaurs (the Saurischians).
Some commonly known Ornithischians include the horned ceratopsians, armored thyreophorans, Iguanodon, and the "duck-billed" hadrosaurs. Being generally herbivores that sometimes lived in herds, they were more numerous than the saurischians in Cretaceous times (though there were not that many in the Triassic). Many were prey animals for the theropods and were usually smaller than the sauropods.
They died out in the Cretaceous-Paleogene Extinction Event.
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