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.

 

Systematics

The ornithischians are further divided in the two clades: the first are the Thyreophora, which include the Stegosauria (like the armored Stegosaurus) and the Ankylosauria (like Ankylosaurus); the second the Cerapoda, which 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.