|Cast of the holotype specimen SAM-PK-K1332 of Heterodontosaurus tucki, Cape Town|
They have often been considered basal ornithopods, although recent studies suggest they may have been more closely related to marginocephalians. Although their fossils are rare, they lived around the globe beginning in the Upper Triassic and may have persisted into the Lower Cretaceous.
Jaws and teeth change
The mandible (lower jaw) was tipped by the predentary, a bone unique to ornithischia. This bone also supported a beak similar to the one found on the premaxilla. All the teeth in the lower jaw were found on the dentary bone.
Heterodontosaurids are called that because they have two different kinds of teeth. There were three premaxillary teeth. In the early Jurassic Abrictosaurus, Heterodontosaurus, and Lycorhinus, the first two premaxillary teeth were small and conical, while the much larger third tooth resembled the canines of carnivoran mammals and is often called the caniniform or 'tusk'.
Several early studies suggested that heterodontosaurs were primitive ornithischians. The main hypothesis in recent years has put heterodontosaurids in as basal ornithopods. However, others have suggested that heterodontosaurs share a common ancestor with Marginocephalia (ceratopsians and pachycephalosaurs). This has support from some recent studies. A clade containing heterodontosaurs and marginocephalians has been named Heterodontosauriformes. Heterodontosaurs may even be basal to both ornithopods and marginocephalians. In 2007, a cladistic analysis suggested that heterodontosaurs are basal to all known ornithischians except Pisanosaurus, a result that echoes some of the very earliest work on the family.
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