New insights into the biology of the Permian genus Cistecephalus (Therapsida, Dicynodontia)

ABSTRACT

The taxonomy and paleobiology of the Upper Permian dicynodont Cistecephalus have been much debated over the last century. Fossils of Cistecephalus have been identified as belonging either to one species or up to six species and hypotheses concerning their lifestyle range from aquatic to arboreal and fossorial. Earlier studies of Cistecephalus focused mainly on macroanatomical characteristics, whereas the current assessment examines a combination of anatomical features, as well as bone histology and microanatomy to unravel its biology. The allometries of a skull growth series that were examined in the present study imply that all Cistecephalus specimens belong to a single species. Furthermore, our data suggest that the variability in the occurrence of supraorbital ridges, which are raised in some specimens and leveled in others, is a feature of sexual dimorphism. Histological thin-sections of a humerus, an ulna, a femur, and ribs from two Cistecephalus specimens were studied to evaluate life history traits of this taxon. The comparison of ribs from a subadult and a fully grown specimen allows an estimation that sexual maturity was attained when the skull length was between 5.9 and 6.5 cm. The compact microstructure of the sampled Cistecephalus bones implies aquatic and/or fossorial adaptations, refuting an arboreal lifestyle. We propose that the high degree of binocular vision evident in Cistecephalus developed in response to predatory (insectivory) and/or nocturnal habits and that it is unrelated to a scansorial lifestyle.

ACKNOWLEDGMENTS

We thank R. Smith, curator at the Iziko SAM (Cape Town), for suggesting a study of Cistecephalus and for his constructive comments. A. Crean is acknowledged for assisting in the removal of skeletal elements for thin-sectioning. We also thank B. Zipfel, curator at the BPI (University of the Witwatersrand, Johannesburg), and J. Neveling and E. de Kock from the Council for Geoscience (Pretoria), for the loans of the specimens. We are grateful to J. Botha-Brink (National Museum of Bloemfontein) for providing two pictures (Fig. 7K, L). Figure 2B is reproduced with permission from Iziko SAM of Cape Town and M. Cluver. This work was supported by the University of Cape Town through a Postgraduate Publication Initiative Award from the Faculty of Science to T.N., and a University Research Committee award to A.C. We thank B. Rubidge and two anonymous reviewers for their constructive comments on an earlier draft of this paper.

Handling editor: Bruce Rubidge