When I was in elementary school, I was taught that one of the ways to tell a reptile from a mammal was by looking at their teeth. Reptiles, like lizards and crocodiles, had a mouth full of nearly identical teeth, while mammals had more diverse dental toolkits. This “rule” might work some of the time, but there are plenty of exceptions to it. Some mammals, like dolphins, have teeth that are the same throughout their jaws, while animals that would be expected to have a full set of identical teeth have specialized teeth.
One such animal that broke the rule was Heterodontosaurus, a small ornithischian dinosaur that had some cone-shaped teeth in addition to grinders for mashing up plants. For many years paleontologists have thought that the sharp teeth may have been used in competition for mates, and were secondary sexual characteristics that became developed as the animals matured. It may be strange to think of herbivores as being “saber-toothed,” but it is not as uncommon as you might think. Male musk deer and muntjacs (see my photo above), for instance, have large canine teeth that are likely a product of sexual selection. During the mating season, when competition for mates can be fierce, males often use these teeth to inflict deep wounds on their opponents.
A new juvenile Heterodontosaurus skull described in the Journal of Vertebrate Paleontology provided an important test of this hypothesis. If the fangs of this dinosaur were used in competition for mates, juvenile specimens would not be expected to have large, canine-like teeth. And the teeth would be expected to be more developed in the sex that was fighting for access to mates. (In most but not all modern animals, that's the male.)
Contrary to these expectations, however, the juvenile Heterodontosaurus skull had large canine-like teeth. It is unlikely that they are the products of sexual selection. Why, then, did these dinosaurs have fangs?
The early appearance of the teeth, the authors of the paper suggest, is a crucial clue. If the sharp, conical teeth at the front of the dinosaur’s mouth appeared at such a young age, perhaps Heterodontosaurus was omnivorous. It's also possible that the teeth could have served a role in defense, although it seems more likely that they were primarily adaptations to feeding.
Unfortunately, the authors went no further than stating that the sexual selection hypothesis is not supported and that the omnivory hypothesis merits further investigation. Although it has been widely reported that these dinosaurs may have eaten meat, there is not yet enough information to confirm or refute that idea. Simply because an animal has sharp teeth does not mean that it was a meat-eater. Lemurs, for instance, have long canines, but they use them to open the tough outer coverings fruits. Perhaps Heterodontosaurus did something similar, using its sharp teeth to break into the tough skins or shells of some plant foods.
Either way, the sharp teeth of Heterodontosaurus allowed for the authors of the paper to make another prediction. The two great branches of the dinosaur family tree, the Ornithischia (to which Heterodontosaurus belongs) and Saurischia (sauropods and theropods) once shared a common ancestor. As far as is currently known, that common ancestor was probably a small, bipedal, carnivorous dinosaur. Given that Heterodontosaurus has been placed close to the bottom of the Ornithischian family tree, it may represent a transitional stage between carnivory and herbivory. This does not mean that Heterodontosaurus was ancestral to all later herbivorous ornithischians, but that it might illustrate the shift to herbivory that occurred in this group of dinosaurs.