Ornithischia (pronounced or Predentata
is an extinct order
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 descended from the 'lizard-hipped'
dinosaurs (the saurischians).
Being herbivores that sometimes lived in herds, they were more numerous than the
saurischians. They were prey animals for the theropods and were smaller than the sauropods.
The Dinosauria superorder was divided into the two orders Ornithischia and
Saurischia by Harry Seeley
in 1887. This division, which has generally
been accepted, is based on the evolution of the pelvis into a more bird-like structure (although birds
did not descend from these dinosaurs), details in the vertebrae and armor and the
possession of a 'predentary'
bone. The predentary is an extra bone in the front of the lower jaw, which 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 (backwards),
parallel with the ischium, with a forward-pointing process to support the
abdomen. This makes a four-pronged pelvic structure. In contrast to this, the
saurischian pubis points downward and toward the head (forwards), as in
ancestral lizard types. Ornithischians also had smaller antorbital fenestrae
(holes in front of their eye sockets) than did saurischians, and a wider, more
stable pelvis. A bird-like pubis arrangement, parallel to the vertebral column,
evolved independently three times in dinosaur evolution, namely in the ornithischians, in the therizinosauroids and in bird-like dromaeosaurids.
Ornithischians shifted from bipedal to quadrupedal posture at least three
times in their evolutionary history and have been shown to have been capable of
adopting both postures early in their evolutionary history
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