Of all the Ornithischia (Table 2), the Ornithopoda showed the greatest tendency towards bipedality. Like all ornithischians, they were plant-eating dinosaurs. In most cases, their cheek teeth were slightly inset from the edges of the jaws. The spaces outside were probably enclosed by fleshy cheeks which prevented food from falling out while they were chewing it. It seems quite possible that the evolution of cheeks may have been responsible for the outstanding success of small and medium-sized herbivorous dinosaurs during the Jurassic and Cretaceous periods. Linked to this was the failure to compete and consequent extinction of their cheekless rivals, the prosauropod saurischians (Sect. 10.3) of the Triassic.
Fabrosaurids and Heterodontosaurids
The Fabrosauridae, which radiated throughout the world during the Lower Jurassic period, may well have been the ancestral group from which the other ornithischians evolved. Fabrosaurs were lizard-like reptiles that looked superficially somewhat like small theropods. Numerous fossilised bones have been discovered, but only Lesothosaurus (Fig. 111) from South Africa is represented by a reasonably complete skeleton. This lightly-built ornithischian predated the Heterodontosauridae (Fig. 88) which were indisputably ornithopods. Leso-
thosaurus had strong arms which could have been used to gather vegetation and pull branches down into its mouth where well-spaced, serrated teeth chopped the tough leaves. Like other fabrosaurs, it lacked the cheek pouches of the later ornithopods. The Heterodontosauridae ('varied-toothed lizards') had differentiated teeth - 2 incisors, 1 canine and 12 molars - on each side of the jaws. The lower canines fitted into deep notches in the upper jaws while the upper canines of the males formed tusks. Heterodontosaurus was able to make sideways chewing movements of its lower jaw - a step towards complete mouth processing and disappearance of the need to use gastroliths.
The Fabrosauridae and Heterodontosauridae appear to have been well equipped for survival in a hostile world. Some, like Lesothosaurus, were swift and agile. They would have been able to escape from predators by flight. In contrast, Scutellosaurus was less speedy but partly armoured. Rows of horny knobs covered its back and flanks, while its long tail would have helped to balance the body on the hind legs while the animal defended itself with its powerful arms. These arms were longer than those of the other known fabrosaurids. This also suggests that Scutellosaurus may have browsed on all four legs, relying on its armour for defence in the first instance. This dinosaur's short head was equipped with canine teeth like those of the heterodontosaurids. Some skulls have been found without canine tusks and could have belonged to females. The tusks might well have been a secondary sexual character, used both in sexual display and for defence. Perhaps it was the larger males who defended family groups against attack.
In general, it appears that the heterodontosaurids were speedy and agile bipeds, similar to the fabrosaurids that lived contemporaneously with them, but were able to eat a much greater variety of plant food. They chewed this food with special cheek teeth and held it in their mouths with their fleshy cheeks. Their large and powerful hands may have been used for digging up roots and termite nests as well as for pulling down branches. In 1978, Tony Thulborn postulated that both fabrosaurids and heterodontosaurids might have aestivated during the annual dry season, a view subsequently refuted by Jim Hopson. The controversy has been summarised by Norman (1985).
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