"Basal ornithischians" (Fabrosauridae)

Pisanosaurus

Fabrosaurus

Lesothosaurus

Genasauria

Thyreophora

Cerapoda

Ornithopoda

Marginocephalia

Heterodontosauridae

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.

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 (antiorbital 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: Thyreophora, which includes the armored Stegosaurus and Ankylosaurus; Cerapoda, which includes the frilled ceratopsids and pachycephalosaurids, and duck-bills (hadrosaurs) like Edmontosaurus. The Cerapoda is a relatively recent grouping (Sereno, 1986), and may be identical to (synonymous with) the older group, Ornithopoda.

External links

• Ornithischia, from Palæos. (cladogram, characteristics)