Ornithischia

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{{Taxobox | color = pink | name = Ornithischia | status = Conservation status: Fossil | image = Ornithischia.png | image_width = 200px | image_caption = Ornithischian pelvic structure | regnum = Animalia | phylum = Chordata | classis = Sauropsida | superordo = Dinosauria | ordo = Ornithischia | ordo_authority = Seeley, 1888 | subdivision_ranks = Suborders | subdivision = Thyreophora
Ornithopoda
Marginocephalia }}

Ornithischia or Predentata is an order of beaked, herbivorous dinosaurs. They are known as the "bird-hipped" dinosaurs because of their hip structure, even though birds actually descended from the "lizard-hipped" dinosaurs (the saurischians). Being herbivores that sometimes lived in herds, they were more numerous than the saurischians, as they were prey for the theropods and were smaller than the sauropods.

1 Characteristics
2 Classification
- 2.1 Taxonomy
3 External links

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Characteristics

The Dinosauria superorder was divided into the two orders Ornithischia and Saurischia by Harry Seeley in 1887. The division is based on the bird-like form of the pelvis, the possession of a predentary, details in the vertebrae and armor, and has been generally adopted. The predentary is an extra bone in the front of the lower jaw, and extends the dentary (the main lower jaw bone). The predentary coincides with the premaxilla in the upper jaw. Together they form a beak-like apparatus used to clip off plant material.

The ornithischian pubis bone points downward and toward the tail, while the saurischian pubis points downward, and towards the front. Ornithischians also had smaller holes in front of their eye sockets (antorbital fenestrae) than saurischians, and a wider, more stable pelvis. A bird-hip-like pubis, parallel to the vertebral column, independently evolved three times in dinosaur evolution, namely in the ornithischians, the therizinosauroids and in bird-like dromaeosaurids.

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Classification

The ornithischians are further divided in the two clades: the first are the Thyreophora and the second the Cerapoda. The Thyreophora include the Stegosauria (like the armored Stegosaurus) and the Ankylosauria (like Ankylosaurus). The Cerapoda include the Marginocephalia (Ceratopia like the frilled ceratopsidae and Pachycephalosauria) and the Ornithopoda (among which duck-bills (hadrosaurs) such as Edmontosaurus). The Cerapoda are a relatively recent grouping (Sereno, 1986), and may conceivably be identical to (synonymous with) the older group, Ornithopoda: most of these divisions are not true by definition.