Diagnostic features for Ornithischia abound. Among these, two stand out:
• In all ornithischians, at least a part of the pubis has rotated backward to lie close to and parallel with the ischium;1 this orientation is called opisthopubic (Figure II.1).
• All ornithischians had a unique bone, the predentary, an unpaired, scoop-shaped element that capped the front of the lower jaws (Figure II.2).
Figure II.1. Left lateral view of the ornithischian pelvis as exemplified by Stegosaurus. Note that the pubic bone is rotated backward to lie under the ischium (see arrow) in what is known as the opisthopubic condition.
Both of these adaptations were associated with food consumption and processing. The evolution of the rearward-directed pubis (recall that, primitively, the pubis points forward; see Figure 4.5) is believed to be associated with the development of a large stomach (or stomachs) and intestinal region (gut), the better for extracting nutrients from plants. Accommodating the large gut was a barrel-shaped torso, recognizable from the shape of the ribs. The predentary supported the lower portion of a beak, a characteristic feature of all ornithischians (see below).
Other ornithischian diagnostic characters include: a toothless, roughened front tip of the snout; a narrow bone (the palpebral) that crossed the outside of the eye socket; a jaw joint
1. Ornithischian dinosaurs are called, confusingly, "bird-hipped"; but birds themselves belong to Saurischia - the "lizard-hipped" clade (Saurischia) of dinosaurs (see Chapter 10).
Figure II.2. Left lateral view of the skull ofthe lambeosaurine hadrosaurid Corythosaurus. Predentary capping the front of the lower jaw is outlined in white.
set below the level of the upper tooth row; cheek teeth with low crowns somewhat triangular in shape; five or more vertebrae in the sacrum; and ossified tendons above the sacral region (and probably further along the vertebral column as well), for stiffening the backbone at the pelvis. The monophyly of Ornithischia is extremely well supported.
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