Abrictosaurus - Description

Description

Heterodontosaurids like Abrictosaurus were small, early ornithischians, named for their markedly heterodont dentition. They are best known for the large, canine-like tusks (often called caniniforms) in both upper and lower jaws. There were no teeth in the front of the jaws, where a hard beak was used to crop vegetation. There were three premaxillary teeth, with the first two small and conical and the third enlarged to form the upper caniniform, counterpart to the even larger lower caniniform, which was the first dentary tooth. In the upper jaw, a large gap (or diastema) accommodated the lower caniniform tooth and separated the premaxillary teeth from the wider chewing teeth of the maxilla. Similar teeth lined the remainder of the lower jaw.

Abrictosaurus is usually considered the most basal member of the family Heterodontosauridae. Lycorhinus and Heterodontosaurus both had high-crowned cheek teeth, which overlapped each other in the jaw, forming a continuous chewing surface analogous to those of Cretaceous hadrosaurids. Abrictosaurus had more widely-separated cheek teeth, with lower crowns, more similar to other early ornithischians. It has been suggested that Abrictosaurus lacked tusks and that this is another primitive feature. However, caniniforms were clearly present on one of the two specimens of Abrictosaurus. The upper caniniform measured 10.5 millimeters (0.4 inches) high, while the lower reached 17 mm (0.67 in). These caniniforms were serrated only on the anterior surface, unlike those of Lycorhinus and Heterodontosaurus, which were serrated on both anterior and posterior edges. Abrictosaurus also had smaller, less powerful forelimbs than Heterodontosaurus and one fewer phalanx bone in both the fourth and fifth digits of the forelimb.