Reconstruction of the Masticatory Apparatus of the Holotype of the RhynchosaurHyperodapedon sanjuanensisfrom the Late Triassic of Argentina: Implications for the Diagnosis of the Species

Rhynchosaurs are a clade of robust, quadruped, and herbivorous stem-archosauromorph diapsids restricted to the Triassic Period. Most species have a specialized masticatory apparatus composed of a blade-and-groove occlusion and multiple longitudinal tooth rows. Rhynchosaurs are the most numerically abundant tetrapods of the Upper Triassic Ischigualasto Formation of NW Argentina. However, there is currently a single nominal species, Hyperodapedon sanjuanensis, described for this unit and the entire country. Though the dental morphology constitutes one of the most important sources of taxonomic and phylogenetic information of the group, this anatomical region remains undescribed in the holotype of Hyperodapedon sanjuanensis. The full occlusion of the skull and lower jaw obscures most of the morphology of the masticatory apparatus in this specimen. Here we present the first description of the maxillary tooth plate and dentary dentition of this specimen based on CT scan data and a three-dimensional rendering. The maxillary tooth plate of the holotype of Hyperodapedon sanjuanensis possesses two tooth-bearing areas that are separated by a single longitudinal groove. The lateral tooth-bearing area of the maxilla is wider than the medial one. The dentary possesses a single cutting blade and tooth row. Both maxilla and dentary lack lingual dentition. The information gathered here was used to emend the diagnosis of the species, review the taxonomy of some specimens, and check the dental scorings for Hyperodapedon sanjuanensis—based on the morphology of the holotype—in three independent phylogenetic data sets.