TAXONOMY OF DICYNODON (THERAPSIDA) AND LATEST PERMIAN TETRAPOD BIOSTRATIGRAPHY

The therapsid reptile Dicynodon has been regarded as a taxon that became extinct at the end of the Permian. This dicynodont has also long been an index fossil of an interval of latest Permian time (Dicynodon assemblage zone in the Karoo basin and correlatives; Platbergian land-vertebrate faunachron). Fossils of Dicynodon have been recovered from upper Permian strata in South Africa, Tanzania, Zambia, the United Kingdom, Laos, China and Russia, making it a cosmopolitan genus across late Permian Pangea with a limited stratigraphic range, and thus a good index fossil. However, recent cladotaxonomic revision of Dicynodon has split the genus into 11 genera, almost all of which are endemic to single basins. This revision did not evaluate the variation in population samples of Dicynodon, particularly with regard to ontogenetic variation. For example, differences in intertemporal bar morphology, judged very significant by the cladotaxonomy, have long been known to be notoriously variable and subject to substantial change ontogenetically. The endemism of the 11 genera that were once Dicynodon is striking, because of the cosmopolitanism of other dicynodont genera of the middle-late Permian and Early Triassic, including Tropidostoma, Endothiodon, Oudenodon, Diictodon and Lystrosaurus. Resurrection of the genus Daptocephalus has also led to renaming the Dicynodon assemblage zone in the Karoo basin the Daptocephalus assemblage zone. This, despite the fact that the few cranial characters that supposedly distinguish Daptocephalus from Dicynodon cannot be verified on most of the dicynodont material found in the assemblage zone. Thus, retention of the genus Dicynodon (not the 11 genera) and maintaining a Dicynodon assemblage zone are recommended. We need a taxonomic revision of Dicynodon based on sound principles of evolutionary taxonomy, using available population samples and, particularly, aimed at identifying variation that has ontogenetic and epigenetic bases. Such a revision will make it possible to evaluate further the paleobiogeographic distribution and the biostratigraphic utility of Dicynodon, as well as its relevance to the end-Permian extinctions.