An integrative approach to distinguishing the late permian dicynodont species oudenodon bainii and tropidostoma microtrema (therapsida: Anomodontia)

The dicynodonts Oudenodon bainii and Tropidostoma microtrema are remarkably similar in most aspects of their morphology. The most obvious distinguishing feature is the presence of tusks and/or postcanine teeth in T. microtrema and their absence in O. bainii. However, some specimens of T. microtrema lack tusks or postcanine teeth. This variability raises the possibility that O. bainii and T. microtrema are not distinct species, but rather endpoints on a morphological continuum. Resolution of this uncertainty is necessary because both species play important roles in Upper Permian terrestrial biostratigraphy. Here we address this issue using several types of data. Our results show that the bone histology of T. microtrema is comparable with that of O. bainii, emphasizing their similarity. However, a geometric morphometric analysis of snout shape and a traditional morphometric analysis of skull dimensions can reliably differentiate tuskless specimens from those with tusks and/or postcanine teeth. We examine several scenarios (e.g. anagenesis, sexual dimorphism) that could explain the observed distinction, but multiple lines of evidence, including stratigraphic range data, suggest that the two morphotypes are regarded best as distinct species. Because O. bainii specimens have been collected at Tropidostoma Assemblage Zone localities in the Karoo Basin of South Africa, the first appearance of O. bainii can no longer be used to define the base of the Cistecephalus Assemblage Zone and Oudenodon-based correlations with other basins should be made with caution.