THE SKULL OF THE BASAL STEGOSAUR HUAYANGOSAURUS TAIBAII AND A CLADISTIC DIAGNOSIS OF STEGOSAURIA

The recent discovery of the Middle Jurassic stegosaur, Huayangosaurus taibaii has shed new light on the early phylogeny of Stegosauria. The skull of this taxon is described in detail and compared to those of other thyreophorans. Cranial and postcranial autapomorphies of Huayangosaurus include high premaxillary and maxillary tooth counts (7 and 25-30, respectively), small postorbital horn, intercostal articular flanges, and a carpus composed of a single coossified block. Nineteen synapomorphies unite Huayangosaurus and other stegosaurs and constitute a revised cladistic diagnosis for Stegosauria. Huayangosaurus is the sister-taxon to all other stegosaurs, which have lost premaxillary teeth, reduced the size of the antorbital fossa, increased the length of the prepubic process of the pubis, and lost lateral scute rows along the trunk.