The earliest cynodont therapsids from the Late Permian of the Karoo Basin, South Africa

Abstract

Non-mammaliaform cynodonts were a widespread and successful group of advanced therapsids that arose in the early late Permian. The oldest described species are Charassognathus gracilis and Abdalodon diastematicus, both of which are known from single specimens from the Endothiodon Assemblage Zone (Tropidostoma-Gorgonops Subzone) of the Beaufort Group of the Main Karoo Basin, South Africa. Here we present two undescribed cynodont specimens from the same subzone, which comprise a second specimen of A. diastematicus and a new species of a small-bodied cynodont, housed at the palaeontological collections of the Council for Geosciences, Pretoria, South Africa and the Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg, South Africa, respectively. Three of these four early cynodont specimens, excluding Charassognathus, were CT scanned using a Nikon Metrology XTH 225/320 LC dual source industrial CT system at the Evolutionary Studies Institute at the University of the Witwatersrand (Johannesburg, South Africa), enhancing the assessment of their craniodental morphology. Remarkable features of the new species include: the presence of a platform on the dentary, lateral to the lower postcanines; the absence of a masseteric fossa; two forms of sectorial postcanine teeth, whereby the anterior postcanines possess a posterior accessory cusp and the posterior postcanines have an additional anterior accessory cusp; and the presence of two small, canine-like postcanines in the dentary immediately distal to the canines. X-ray images of the new material of A. diastematicus suggest alternating dental replacement of the complex upper postcanines, which also seems to be the case for the lower incisors. A preliminary phylogeny retrieves Abdalodon as closely related to a monophyletic clade formed by the cosmopolitan Procynosuchus and the Russian Dvinia, whereas the new taxon and Charassognathus are successive sister species of remaining cynodonts.