New insights on the cranial anatomy of Exaeretodon: intraspecific variation and taxonomic implications

Abstract

Exaeretodon is a very abundant non-mammalian cynodont from the Upper Triassic found in South America and India. Presently, two species are considered valid: E. argentinus (Cabrera, 1943) from Argentina and E. riograndensis Abdala et al. 2002 from Brazil. We studied several unpublished specimens tentatively assigned to E. argentinus from the Ischigualasto Fm. (San Juan Province, Argentina) that range from ~130mm to ~400mm in basal skull length. Regardless of the individual size, two morphotypes were identified on the basis of differences in the zygomatic region, including: the presence/absence of a process of the jugal; the shape, relative orientation, and posterior extension of the dorsal and ventral zygomatic rami of the jugal; and the morphology and orientation of the zygomatic portion of the squamosal. The holotype of E. argentinus (formerly Belesodon argentinus Cabrera, 1943) is an incomplete left lower jaw. Only the holotypes of the once valid E. frenguelli Cabrera, 1943 and Theropsis robusta Cabrera, 1943 (presently considered junior synonyms of E. argentinus) and E. riograndensis preserve the relevant portions of the zygomatic region. These specimens represent only one of the morphotypes whereas the second morphotype has not been previously recognized. In addition, there are differences among the specimens of the first morphotype that were originally interpreted to be due to preservational artifacts but could instead account for intraspecific variation. Our preliminary results suggest that the second morphotype might characterize a new taxon, highlighting the importance of an integral re-evaluation of Exaeretodon Cabrera, 1943 considering all the evidence available.