A heterodontosaurid ornithischian dinosaur from the Upper Triassic of Patagonia

Fragmentary remains of a new heterodontosaurid species, comparable to Heter odontosaurus Crompton and Charig, were discovered in concretions in the Lag una Colorada Formation, a Late Triassic continental sequence in Santa Cruz Province, Argentina. The material consists of a weathered, left posterior m axillary fragment with dentition, and, tentatively, an isolated caniniform with anterior and posterior serrations. The preserved three maxillary teeth bear flat wear facets, and are columnar and closely packed. The anterior a nd posterior surfaces of the crowns are in contact, a feature considered a synapomorphy of Heterodontosaurus and Lycorhinus from the Early Jurassic up per Stormberg Group of southern Africa. As in Heterodontosaurus tucki Cromp ton and Charig, the maxillary teeth lack a cingulum or a constriction separ ating crown and root, and the wear facets of adjoining teeth form a single, continuous surface. However, the posterior maxillary teeth bear more. nume rous and narrower ridges on their labial surfaces than those of H. tucki. T his new record of a heterodontosaurid extends the temporal range of this gr oup of small ornithischians and, considering the phylogeny of ornithischian s as now understood, indicates an extensive phyletic diversification of the se dinosaurs in the Late Triassic.