Redescription of the Triassic cynodont <scp> <i>Cistecynodon parvus</i> </scp> and reassessment of its phylogeny

Abstract

Cynodontia is an important subclade of Therapsida that first occurred in the late Permian. It includes extinct subclades which are the non-mammaliaform cynodonts and Mammaliaformes, with the latter ultimately giving rise to crown mammals. The systematics of non-mammaliaform cynodonts has been extensively studied and is relatively well-resolved, however, there are still many problematic taxa that are difficult to identify and place confidently into the cynodont phylogenetic tree. Cistecynodon parvus is one such taxon, known only from a single specimen, the holotype skull which was found in the Middle Triassic Burgersdorp Formation of South Africa, in strata assigned to the Trirachodon-Kannemeyeria Subzone of the Cynognathus Assemblage Zone. Over the past century Cistecynodon has been variously referred to Eucynodontia (which includes the two major subclades Probainognathia and Cynognathia) and to non-eucynodont cynodonts, without any emerging consensus. Here the holotype of Cistecynodon parvus (BP/1/2520) is re-described based on computed tomography (CT) digital reconstruction of the specimen. This new data is applied to determine a phylogenetic matrix which supports that C. parvus is a basal (non-eucynodont) cynodont. This basal position is reflected in the anatomy of its secondary palate, which is not closed despite the specimen being a subadult. The inner ear, trigeminal canal, parietal foramen and carotid foramina are uniquely derived, likely as adaptations to an obligate fossorial lifestyle, confirming the validity of the taxon and its inferred subterranean habitat.