A heterodontosaurid ornithischian dinosaur from the Upper Triassic of Patagonia

Abstract

Fragmentary remains of a new heterodontosaurid species, comparable to Heterodontosaurus Crompton and Charig, were discovered in concretions in the Laguna Colorada Forrnation, a Late Triassic continental sequence in Santa Cruz Province, Argentina. The material consists of a weathered, left posterior maxillary fragment with dentition, and, tentatively, an isolated caniniform with anterior and posterior serrations. The preserved three maxillary teeth bear fIat wear facets, and are colurnnar and closely packed. The anterior and posterior surfaces of the crowns are in contact, a feature considered a synapomorphy of Heterodontosaurus and Lycorhynus from the Early Jurassic upper Stormberg Group of southern Africa. As in Heterodontosaurus tucki Crompton and Charig. the maxillary teeth lack a cingulum or a constriction separating crown and root, and the wear facets of adjoining teeth form a single, continuous surface. However, the posterior maxillary teeth bear more numerous and narrower ridges on their labial surfaces than those of H. tucki. This new record of a heterodontosaurid extends the temporal range of this group of small ornithischians and, considering the phylogeny of ornithischians as now understood, indicates an extensive phyletic diversification of these dinosaurs in the Late Triassic.