Ornithischia the big picture

The fundamental split of Ornithischia is between the very primitive ornithischian Lesothosaurus and everything else ornithischian (Figure II.5). Lesothosaurus was a small, long-limbed Early Jurassic herbivore from South Africa (Figure II.6). It had a typical suite of diagnostic ornithischian characters including a jaw joint lower than the tooth row (see Figure II.4). That character hints at chewing; but mere hints won't be necessary for the rest of Ornithischia.

Genasauria

Ornithischia

Genasauria

Ornithischia

Dinosauria

Figure II. 5. Cladogram of Ornithischia. Derived characters include: at 1 (Ornithischia), opisthopubic pelvis, predentary bone, toothless and roughened tip of snout, reduced antorbital opening, palpebral bone, jaw joint set below level of the upper tooth row, cheek teeth with low subtriangular crowns, at least five sacral vertebrae, ossified tendons above the sacral region, small prepubic process along the pubis, long and thin preacetabular process on the ilium; at 2 (Genasauria), emarginated dentition (indicating large cheek cavities, and reduction in the size of the opening on the outside of the lower jaw (the external mandibular foramen); at 3 (Cerapoda), gap between the teeth of the premaxilla and maxilla, five or fewer premaxillary teeth, finger-like anterior trochanter; at 4, high-crowned cheek teeth, denticles on the margins restricted to the terminal third of the tooth crown, canine-like tooth in both the premaxilla and dentary.

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Figure II.6. Left lateral view of the skull (a) and skeleton (b) ofthe basal ornithischian Lesothosaurus.

and skeleton of Heterodontosaurus.

In contrast, "everything else ornithischian" is within the clade Genasauria (gena -cheek; see Figure II.5). They all share the derived characters of muscular cheeks, indicated by the deep-set position of the tooth rows, away from the sides of the face, a spout-shaped front to the mandibles, and reduction in the size of the opening on the outside of the lower jaw (the external mandibular foramen), among others. Because it's hard to understand cheeks without chewing, chewing should be thought of as a fundamental genasaur behavior. Then it only becomes a matter of how efficiently the various groups of genasaurs chewed. Genasaurs include the basal hetero-dontosaurids,2 and then the two great groups of ornithsichians, Thyreophora (Chapter Figure"8. Left lateral view of the sku" of Heterodontosaurus. 5), and Cerapoda (Marginocephalia + Ornithopoda; Chapters 6 and 7, respectively), as well as a few assorted forms with which we won't concern ourselves here.

Heterodontosaurids were small, bipedal ornithischians (Figure II.7). Despite their low position on the cladogram, however, in many respects they were not exactly primitive; for example, they evolved teeth bearing a high, chisel-shaped crown ornamented with tiny bumps (correctly termed "denticles") as well as a large canine-like tooth on both upper and lower jaws (the basis for the name "heterodontosaurid"; Figure II.8). Moreover, they chewed

2. A recent, important reanalysis of Ornithischia (Butler et al., 2008) removed Lesothosaurus from its conventional basal position in Ornithischia and made it a basal thyreophoran (see Chapter 5), closely related to stegosaurs and ankylosaurs.

Until this idea is generally accepted, however, we continue to regard it as among the most primitive of ornithischians.

In the same analysis, heterodontosaurs, traditionally considered ornithipods (see Chapter 7), were reassigned to an extremely basal position in Ornithischia; we follow this analysis here.

15 cm distinctively: they amplified the familiar vertical movement of the lower jaws with slight rotations of each side of the mandible about its long axis (Figure II.9). This allowed them to get a bit more grind out of each bite.

Like many ornithischians, the evolution of canine-like teeth of heterodontosaurids likely was related to combat between animals of the same species (males?), ritualized display, social ranking, and possibly even courtship (see Chapters 5 and 6). A modern analog is found in tusked tragulids, living mammals from southeastern Asia and Africa, closely related to deer. In these mammals, tusk development is tied to sexual maturity, and is a dimorphic feature that is, as we propose for heterodontosaurids, used for intraspecific combat, ritualized display, and social ranking. Similarly, the development of a bony boss in the cheek region (the jugal boss) in heterodontosaurids might also be interpreted as a form of visual display.

Thyreophora (thyreo - shield; phora -bearer; a reference to the fact that these animals have dermal armor) consists of those gena-saurs in which there are parallel rows of keeled dermal armor scutes (or bony plates) on the back surface of the body. The most familiar thy-reophorans are stegosaurs and ankylosaurs, but we'll encounter others along the way.

Cerapoda (kera - horn) are those genasaurs that share a pronounced diastem between the teeth of the premaxilla and maxilla among other derived characters (see Figure II.3). Marginocephalians (margin - margin; kephale - head), a group united by having, primitively, a distinctive, narrow shelf that extended over the back of the skull (see Figure II.7) consists of two well-known ornithischian taxa, the dome-headed pachycephalosaurs and ceratopsians (Chapter 6), the latter being the horned dinosaurs most famously known from the Late Cretaceous of North America.

Ornithopods took chewing to new levels, possibly unmatched in the history of life. Within this group are found the familiar duck-billed dinosaurs, as well as one of the very first dinosaurs ever recorded, Iguanodon. We'll visit these animals in Chapter 7.

Figure II.9. Jaw mechanics in Heterodontosauridae, showing mobility of the lower jaws. As the lower jaw closed, each side rotated slightly along its long axis.
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