The Diversity Of Ornithischian Dinosaurs

The Ornithischia are the second major dinosaurian clade and they are relatively easy to diagnose. They have a pubis that points backwards (Figure 8.3(b)) as well as over 30 other derived characters of the skull and skeleton (Sereno, 1986,1999).

The ornithischians arose during the Carnian (Late Triassic, 230-220Myr ago), or earlier, but fossils are extremely rare until the Jurassic. The ornithischians were all herbivorous and they divide into two main groups, the Cerapoda (the bipedal ornithopods, bone-headed pachycephalosaurs and horned ceratopsians) and the Thyreophora (the armoured ankylosaurs and stegosaurs) (see Box 8.5).

8.4.1 Pisanosaurus—the first possible ornithischian

Pisanosaurus from the Carnian of Argentina is known from only its jaws, neck and a few limb elements (Bonaparte, 1976). The cheek teeth (Figure 8.11(a)) have low triangular crowns with a well-developed narrow neck beneath and they are set over to the inside of the jaws, leaving a broad shelf on the outside. This sug

Stegosaurus Bone Names

Fig. 8.11 Early ornithischians: (a) Pisanosaurus, maxilla fragment and partial lower jaw in lateral view; (b-d) the fabrosaurid Lesothosaurus,skeleton, skull and tooth; (e-g) the heterodontosaurid Heterodontosaurus,skeleton, skull in lateral view, maxillary tooth row. [Figure (a) after Bonaparte, 1976; (b-d) after Galton, 1978; (e) after Santa Luca, 1980; (f, g) after Charig and Crompton, 1974.]

Fig. 8.11 Early ornithischians: (a) Pisanosaurus, maxilla fragment and partial lower jaw in lateral view; (b-d) the fabrosaurid Lesothosaurus,skeleton, skull and tooth; (e-g) the heterodontosaurid Heterodontosaurus,skeleton, skull in lateral view, maxillary tooth row. [Figure (a) after Bonaparte, 1976; (b-d) after Galton, 1978; (e) after Santa Luca, 1980; (f, g) after Charig and Crompton, 1974.]

gests that Pisanosaurus had cheeks, pouches of skin that lay on either side of the tooth rows, that could retain unchewed plant material while other food was being processed. Cheeks are typical of ornithischians and other reptiles in which the skin of the face is firmly attached to the jaw margins just below the tooth rows. If Pisanosaurus is an ornithischian, it is followed by a long gap in their fossil record: the next ornithischians appear only some 20 Myr later.

8.4.2 Fabrosauridae

Small ornithischians, often called fabrosaurids, have been reported from the Lower Jurassic of several parts of the world (Galton, 1978; Knoll, 2002), but only Lesothosaurus from southern Africa is reasonably complete. It is a lightly built animal 0.9 m long, with long hindlimbs and short arms (Figure 8.11(c)). It has the typical ornithischian pelvis, an ilium with a narrow anterior process and fusion of the ischia and pubes at their tips. The skull (Figure 8.11(b)) shows even more or-nithischian characters. The tip of the premaxilla is toothless and roughened and it is matched by an entirely new bone in the lower jaw, the unpaired preden-tary.The orbit also contains a new bone, the palpebral. The teeth (Figure 8.11(d)) are more typically ornithis-chian than those of Pisanosaurus because they have a bulbous base to the crown and rounded denticles on the edges. The wear facets lie symmetrically on either side of the pointed tip of the crown, which suggests an up and down jaw action with no possibility of back and forwards or side-to-side chewing.

8.4.3 Basal ornithopods

The ornithopods were the largest and most successful ornithischian group, comprising more than 100 species and achieving great abundance in Cretaceous faunas. There are four main groups, the heterodontosaurids, hypsilophodontids, 'iguanodontids' and hadrosaurids.

The heterodontosaurids lived at the same time as the fabrosaurs. Heterodontosaurus, from southern Africa (Charig and Crompton, 1974; Santa Luca, 1980), just over 1 m long, is similar to Lesothosaurus in many ways. The bodily proportions (Figure 8.11(e)) differ only in the slightly longer arms and the shorter body. The skull (Figure 8.11(f)) shows the most unusual features. Heterodontosaurus (literally 'different tooth reptile') has differentiated teeth, two incisors, a canine and about 12 cheek teeth. The canines are long and the lower one fits into a deep notch in the upper jaw. One specimen has no tusks and it has been suggested that their presence may be a secondary sexual character of males. If so, the canine tusks may have been used for defence and for sexual display, as in modern herbivorous mammals with tusks, such as certain pigs and the musk deer.

Heterodontosaurus shows several features that group it with later ornithopods: the tooth-bearing edge of the premaxilla is a step down from the maxilla, the pre-maxilla extends back to contact the prefrontal and lacrimal, the jaw joint is set well below the level of the tooth rows to increase the duration and force of the bite (convergent with other herbivorous dinosaurs and synapsids (see pp. 121, 189), the cheek teeth wear against the opposite teeth of the lower jaw forming a straight line at the crest of the teeth (Figure 8.11(g)), and the outer surfaces of the lower teeth fit inside the upper teeth and wear them from the inside. Unlike Lesothosaurus, Heterodontosaurus was capable of a small amount of sideways chewing by rotation of the lower jaw about its long axis (see Box 8.4).

The hypsilophodontids, typified by Hypsilophodon from the Lower Cretaceous of England (Galton, 1974), but known from the Late Jurassic to the Late Cretaceous, ranged in length from 3 to 5 m or so. The bodily proportions (Figure 8.12(a)) and the skull (Figure 8.12(b, c)) are similar to those of Heterodontosaurus,ex-cept that the skull lacks tusks and is narrower in the midline. The ventral view (Figure 8.12(c)) shows the extent of the cheeks, represented by the broad area of the maxilla lying outside the tooth rows.

An early view of Hypsilophodon was that it lived in trees, grasping the branches with its feet, but the foot (Figure 8.12(d)) was incapable of grasping, being a typical elongate running foot with hoof-like 'claws'. Further, the end of the tail is sheathed in ossified tendons that stiffened it and caused it to act as a stabilizer during running, as in Deinonychus (Figure 8.8(a)). The limb proportions of Hypsilophodon are similar to those

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