Heterodontosaurus tucki Crompton and Charig, 1962

Nesbitt, Sterling J., 2011, The Early Evolution Of Archosaurs: Relationships And The Origin Of Major Clades, Bulletin of the American Museum of Natural History 2011 (352), pp. 1-292 : 50

Heterodontosaurus tucki Crompton and Charig, 1962

AGE: Early Jurassic ( Olsen and Galton, 1984).

OCCURRENCE: Clarens Formation (5 Cave Sandstone) and upper Elliot Formation, Herschel, Cape Province, South Africa.

HOLOTYPE: SAM-K-337, partial skull.

REFERRED MATERIAL: SAM-K-1332, complete skull and skeleton.

REMARKS: Although Heterodontosaurus is only represented by two unambiguous specimens, SAM-K-1332 remains one of the most well-preserved and most complete dinosaurs known to date. Santa Luca (1980) fully described the postcrania of SAM-K-1332, but a full description of the skull has yet to be published.

The phylogenetic position of Heterodontosaurus , though highly debated in the literature, is critical to the understanding of early dinosaur and ornithischian relationships. As summarized by Butler et al. (2008b), Heterodontosaurus was has been classified as a basal ornithopod, as the sister taxon to Margincephalia, as the sister taxon to Margincephalia + Ornithopoda, and as one of the basalmost ornithischians. Most recently, Heterodontosaurus was found as a basal ornithischian near Pisanosaurus ( Butler et al., 2008b) . This position better reflects the fossil record of Ornithischia and suggests that some of the ‘‘odd’’ features (e.g., the hand) of Heterodontosaurus present in non-ornithischian dinosaurs (e.g., Herrerasaurus ) may represent plesiomorphies of Dinosauria rath- er than autapomorphies of Heterodontosaurus . A further discussion of these potentially plesiomorphic features were presented by Butler et al. (2008b).

Butler et al. (2008b) provided the following diagnosis of Heterodontosaurus : dorsal process of premaxilla does not form contact with nasals; anterior, accessory opening present within the antorbital fossa; squamosal-quadratojugal contact is anteroposteriorly broad; paroccipital processes are very deep dorsoventrally; paired, deep recesses on the ventral surface of the basisphenoid; basisphenoid processes are extremely elongated; cingulum is completely absent on cheek teeth; ischium with elongate flange on lateral margin.

KEY REFERENCES: Crompton and Charig, 1962, Santa Luca et al., 1976; Santa Luca, 1980; Butler et al., 2008b.