Redescription and new material of the Triassic cynodont Cistecynodon parvus and reassessment of its phylogeny

Abstract

Cynodonts are an important clade of non-mammaliaform therapsid that first occur in the late Permian and include the subclade Mammaliaformes, the latter ultimately giving rise to modern 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 the Probainognathia, Cynognathia, and non-eucynodont cynodonts, without any emerging consensus. Recently, a new specimen possibly referrable to this genus has been discovered on the farm Lemoenfontein, within the same subzone. Here the holotype of Cistecynodon parvus and the new specimen are described using CT scans of the skulls to produce 3D models. This new data is used to score a phylogenetic matrix which supports that Cistecynodon parvus is a basal, non-eucynodont. This basal position reflects on 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 subterranean ecology. The new specimen from Lemoenfontein is found to be closely related, but clearly distinct from Cistecynodon, and thus likely represents a new species of early Middle Triassic cynodont.