Concept

Heterodontosauridae

Summary
Heterodontosauridae is a family of ornithischian dinosaurs that were likely among the most basal (primitive) members of the group. Their phylogenetic placement is uncertain but they are most commonly found to be primitive, outside of the group Genasauria. Although their fossils are relatively rare and their group small in numbers, they have been found on all continents except Australia and Antarctica, with a range spanning the Early Jurassic to the Early Cretaceous. Heterodontosaurids were fox-sized dinosaurs less than in length, including a long tail. They are known mainly for their characteristic teeth, including enlarged canine-like tusks and cheek teeth adapted for chewing, analogous to those of Cretaceous hadrosaurids. Their diet was herbivorous or possibly omnivorous. Among heterodontosaurids, only Heterodontosaurus itself is known from a complete skeleton. Fragmentary skeletal remains of Abrictosaurus are known but have not been fully described, while most other heterodontosaurids are known only from jaw fragments and teeth. Consequently, most heterodontosaurid synapomorphies (defining features) have been described from the teeth and jaw bones. Heterodontosaurus measured just over 1 meter (3.3 ft) in length, while the fragmentary remains of Lycorhinus may indicate a larger individual. Tianyulong from China appears to preserve filamentous integument which has been interpreted to be a variant of the proto-feathers found in some theropods. These filaments include a crest along its tail. The presence of this filamentous integument has been used to suggest that both ornithischians and saurischians were endothermic. Both Abrictosaurus and Heterodontosaurus had very large eyes. Underneath the eyes, the jugal bone projected sideways, a feature also present in ceratopsians. As in the jaws of most ornithischians, the anterior edge of the premaxilla (a bone at the tip of the upper jaw) was toothless and probably supported a keratinous beak (rhamphotheca), although heterodontosaurids did have teeth in the posterior section of the premaxilla.
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