Heterodontosauridae - Paleobiology

Paleobiology

Most heterodontosaurid fossils are found in geologic formations that represent arid to semi-arid environments, including the Upper Elliot Formation of South Africa and the Purbeck Beds of southern England. It has been suggested that heterodontosaurids underwent seasonal aestivation or hibernation during the driest times of year. Due to the lack of replacement teeth in most heterodontosaurids, it was proposed that the entire set of teeth was replaced during this dormant period, as it seemed that continual and sporadic replacement of teeth would interrupt the function of the tooth row as a single chewing surface. However, this was based on a misunderstanding of heterodontosaurid jaw mechanics. It was thought that heterodontosaurids actually did replace their teeth continually, though more slowly than in other reptiles, but CT scanning of skulls from juvenile and mature Heterodontosaurus shows no replacement teeth. There is currently no evidence that supports the hypothesis of aestivation in heterodontosaurids, but it cannot be rejected, based on the skull scans.

While the cheek teeth of heterodontosaurids are clearly adapted for grinding tough plant material, their diet may have been omnivorous. The pointed premaxillary teeth and sharp, curved claws on the forelimbs suggest some degree of predatory behavior. It has been suggested that the long, powerful forelimbs of Heterodontosaurus may have been useful for tearing into insect nests, similarly to modern anteaters. These forelimbs may have also functioned as digging tools, perhaps for roots and tubers.

The length of the forelimb compared to the hindlimb suggests that Heterodontosaurus might have been partially quadrupedal, and the prominent olecranon process and hyperextendable digits of the forelimb are found in many quadrupeds. However, the manus is clearly designed for grasping, not weight support. Many features of the hindlimb, including the long tibia and foot, as well as the fusion of the tibiofibiotarsus and tarsometatarsus, indicate that heterodontosaurids were adapted to run quickly on the hindlegs, so it is unlikely that Heterodontosaurus moved on all four limbs except perhaps when feeding.

The short tusks found in all known heterodontosaurids strongly resemble tusks found in modern musk deer, peccaries and pigs. In many of these animals (as well as the longer-tusked walrus and Asian elephants), this is a sexually dimorphic trait, with tusks only found in males. The type specimen of Abrictosaurus lacks tusks and was originally described as a female. While this remains possible, the unfused sacral vertebrae and short face indicate that this specimen represents a juvenile animal, while a second, larger specimen of Abrictosaurus clearly possesses tusks. Therefore, it is possible that tusks are found only in adults, rather than being a secondary sexual characteristic of males. These tusks could have been used for combat or display with members of the same species or with other species. The absence of tusks in juvenile Abrictosaurus could also be another characteristic separating it from other heterodontosaurids as well, as tusks are known in juvenile Heterodontosaurus. Other proposed functions for the tusks include defense and use in an occasionally omnivorous diet.