Angonisaurus and Shansiodon, dicynodonts (Therapsida, Anomodontia) from subzone C of the Cynognathus Assemblage Zone (Middle Triassic) of South Africa

ABSTRACT

Dicynodonts are the most abundant therapsids from the Beaufort Group (Karoo Basin, South Africa), and they have been used as index fossils to define most of the Beaufort biozones. Although speciose during the Late Permian, dicynodont diversity was greatly reduced by the end-Permian mass extinction. The Cynognathus Assemblage Zone, the uppermost biozone of the Beaufort Group, is traditionally known to host only the dicynodonts Kannemeyeria simocephalus and Kombuisia frerensis. Recent biostratigraphic research in this biozone has revealed the presence of two more poorly known dicynodont genera, Angonisaurus and Shansiodon, which are biostratigraphically constrained to the uppermost Cynognathus Assemblage Zone (subzone C). Although these taxa have been reported from subzone C of the Cynognathus Assemblage Zone previously, we provide the first detailed description of the material in question and justifications for the taxonomic identifications. The presence of Angonisaurus in the Cynognathus subzone C allows a direct correlation between this assemblage and that of the Lifua Member of the Manda beds in Tanzania. A correlation with the Antarctic upper Fremouw Formation also may be possible, but is less certain. The presence of Shansiodon allows a direct correlation with the upper Ermaying Formation of China, but we reject the previously proposed global ‘Shansiodon biochron’ because Shansiodon sensu stricto only occurs in China and South Africa. Biogeographically, Angonisaurus appears to have been widespread but rare in southern Gondwana, and the unusual distribution of Shansiodon mirrors that of Diictodon feliceps in the Permian. Southern African dicynodont assemblages display increased provinciality in the Middle Triassic.

ACKNOWLEDGMENTS

We gratefully acknowledge the supportive role of the Morgan family of the Bamboeshoek Valley who generously provided access to their farms and accommodation during field seasons. This work benefited greatly from discussions with F. Abdala, A. Cruickshank, A. Keyser, J. Kitching, R. Damiani, S. Lucas, M. Shishkin, R. Smith, and A. Yates. J. Botha-Brink, J. Fröbisch, and S. Modesto also provided helpful reviews and editorial suggestions. We acknowledge the assistance of G. Modisi and C. Dube who prepared the specimens described here and E. de Plessis for illustrations of the Shansiodon and Angonisaurus specimens. Financial support from the University of the Witwatersrand, DST, NRF, and the Palaeontological Scientific Trust (PAST) and its Scatterlings of Africa programmes (PAST) is acknowledged.

Handling editor: Sean Modesto.