Heterodontosaurids (= "different toothed lizard"), which lived only during the Early Jurassic and mostly in southern Africa, derive their name from their differentiated teeth, an unusual condition for any dinosaurian clade. In fact, some of their teeth are morphologically distinctive enough that they are known only in heterodontosaurids. These teeth, occurring in the areas of the former cheeks, have been described as "chisel-like" because they come to an edge at their crowns, which are also adorned with denticles. Other teeth that are radically different from the cheek teeth are caninelike tusks on the predentary and dentary (Fig. 11.3). Although these teeth might look menacing, and were probably visible externally when the mouth was closed,

FIGURE 11.3 Skull of the Early Jurassic Heterodontosaurus, a heterodontosaurid of South Africa. From Cowen (2000), History of Life, 3e, Blackwell Science, Inc., Maiden, MA, p. 220, fig. 12.10. (After Charig and Crompton.)

they were not used for predation. A more likely explanation is that they served for display within their species. A few paleontologists have suggested that they may even be indicators of sexual dimorphism and present in only one gender. Heterodontosaurid males are assumed to have been the possessors of these attributes, although this assignment is arbitrary.

Parts of the appendicular skeleton of heterodontosaurids indicate that they had well-developed arms and hands, which are interpreted as adaptations to feeding, such as pulling or holding on to plants, or even for digging. No other evidence, such as trace fossils or former gut contents, has verified these interpretations, but they are reasonably inferred on the basis of other adaptations that clearly favor an herbivorous habit. The best-known heterodontosaurid species, the eponymous Heterodontosaurus, had a large number of caudal vertebrae, which resulted in a tail that made up more than half the total length of the body. It also has hind limbs with fused distal elements, specifically in the tibia, fibula, and tarsals. These traits are interpreted as aids to fast movement. Such an adaptation certainly would have been advantageous for a small herbivore in an Early Jurassic world already inhabited by large ceratosaurs (Chapter 9).

Heterodontosaurids also included Abrictosaurus, Lycorhinus, and Echinodon, although the latter two are only identified from sparse skeletal material. This scarcity is probably related to their apparent biogeographic restriction, relatively short geologic range, and taphonomic factors in their preferred environments. However, heterodontosaurid remains have been found in arid-climate facies, which normally are conducive to preserving skeletal remains in the event of quick burial (Chapter 7). Faced with such a disappointingly small number of bones, paleontologists could then turn to trace fossils. Although ornithopod tracks from the Early Jurassic are common in some places, none have been attributed specifically to heterodontosaurids. Likewise, no other trace fossils of heterodontosaurids, such as nests, tooth marks, gastroliths, or coprolites, have been recognized. Embryonic remains of heterodontosaurids are also unknown. Thus, paleontologists who study heterodontosaurids do not have much material for interpreting the lifestyles of these seminal ornithopods.

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