Abstract

The partial skull and postcranial skeleton of a subadult Cricodon metabolus is described as the first record of a trirachodontid cynodont from the upper Ntawere Formation of northeastern Zambia. On the basis of features in the type and referred specimen, Cricodon metabolus can be diagnosed by the presence of a posterior cingulum in the lower postcanines that opens labially in a V-shaped notch between a large basal cuspule and the steeply descending posterior ridge of the principal labial cusp and by a pair of large foramina on the maxillary shelf. The subadult specimen shows evidence for four series of postcanine teeth, including a wave of small gomphodont teeth being replaced by a younger wave of larger gomphodont teeth. Cladistic analysis of 75 characters in 27 cynodonts recovers Trirachodontidae as a clade that is the sister taxon of Traversodontidae. Trirachodontidae includes four species, with Langbergia modisei as the earliest diverging taxon. Our analysis also recovers "Trirachodon" kannemeyeri as the sister taxon of Cricodon metabolus, leading us to transfer the former species to the genus Cricodon. Trirachodon berryi is determined to be a valid taxon that is sister to Cricodon. Two reputed trirachodontids from China, Sinognathus gracilis and Beishanodon youngi, do not appear as trirachodontids in our analysis, but rather resolve as sister taxa within Traversodontidae. Although this hypothesis of relationships is not supported by the current analysis, we suggest that the two Chinese species might be probainognathians that convergently evolved expanded postcanine teeth rather than gomphodontians.