Heterodontosauridae

Heterodontosauridae were among the earliest and most primitive ornithischian dinosaurs. Exactly where they fit on the family tree is debated, but they usually fall outside the group Genasauria. Their fossils are scarce and their family was small, yet they managed to spread across every continent except Australia and Antarctica, living from the Early Jurassic into the Early Cretaceous. These dinosaurs were about the size of a fox, under 2 meters long including a long tail. They are best known for their unusual teeth: enlarged, canine-like tusks and cheek teeth built for chewing—a setup similar to the later hadrosaurs of the Cretaceous. They likely ate plants, though they may have eaten some meat as well.

Only one heterodontosaurid, *Heterodontosaurus*, is known from a complete skeleton. Others, like *Abrictosaurus*, survive only in fragments that haven’t been fully studied, and most are known just from jaw pieces and teeth. So most of the group’s defining traits come from those jaws and teeth. *Heterodontosaurus* itself was just over a meter long, while scraps of *Lycorhinus* hint at a larger animal. *Tianyulong* from China preserves what look like filaments on its body, possibly a kind of proto-feather similar to those found in some theropods, including a crest along its tail. This has been used to argue that both ornithischians and saurischians were warm-blooded.

The skulls of *Abrictosaurus* and *Heterodontosaurus* had very large eyes. Below each eye, the jugal bone stuck out sideways, a feature also seen in ceratopsians. Like most ornithischians, the front of the upper jaw (the premaxilla) had no teeth and likely supported a horny beak, though heterodontosaurids did have teeth farther back on the premaxilla. A large gap called a diastema separated those front teeth from the main upper jaw teeth, and in heterodontosaurids this gap was arched. The lower jaw ended in a predentary bone, unique to ornithischians, which also held a beak. All lower teeth sat on the dentary bone.

The group’s name comes from their strongly varied teeth. They had three premaxillary teeth. In the Early Jurassic species *Abrictosaurus*, *Heterodontosaurus*, and *Lycorhinus*, the first two were small and cone-shaped, while the third was much larger, like a carnivoran canine, often called a tusk. The lower tusk was even bigger and sat at the front of the dentary, fitting into the arched diastema when the mouth closed. These tusks were serrated on both edges in *Heterodontosaurus* and *Lycorhinus*, but only on the front edge in *Abrictosaurus*. In the Early Cretaceous *Echinodon*, there may have been two upper tusks, and they grew from the maxilla instead of the premaxilla. *Fruitadens* from the Late Jurassic may have had two lower tusks on each dentary.

The cheek teeth of more advanced heterodontosaurids were also unusual. In early ornithischians, small ridges called denticles lined the edges of cheek teeth for cropping plants. In heterodontosaurids, these denticles only went a third of the way down the crown, not farther toward the root like in other ornithischians. Basal forms like *Abrictosaurus* had widely spaced cheek teeth with low crowns and a strong ridge (cingulum) separating crown from root. More derived forms like *Lycorhinus* and *Heterodontosaurus* had chisel-shaped teeth with high crowns and no cingulum, so the crown and root were the same width. These teeth overlapped, forming a continuous grinding surface. The tooth rows sat slightly inward from the jaw edge, leaving a gap that may have held a muscular cheek, needed for chewing. Hadrosaurs, ceratopsians, and many herbivorous mammals later evolved similar dental batteries. Unlike hadrosaurs with their hundreds of constantly replaced teeth, heterodontosaurids replaced teeth very slowly, and some specimens have no replacement teeth at all. They also lacked the small openings (foramina) inside the jaw bones that helped tooth development in most other ornithischians. A unique ball-and-socket joint between the dentaries and predentary let the lower jaws rotate outward as the mouth closed, grinding the cheek teeth together. Because replacement was slow, this grinding wore teeth down heavily, often erasing most denticles, though the tall crowns gave each tooth a long lifespan.

The postcranial skeleton is known mainly from *Heterodontosaurus tucki*, which is considered the most advanced Early Jurassic heterodontosaurid, so it’s unclear how many of its features were shared. Its forelimbs were long for a dinosaur, over 70% of the hindlimb length.