A reevaluation of Brachyprosopus broomi and Chelydontops altidentalis, dicynodonts (Therapsida, Anomodontia) from the middle Permian Tapinocephalus Assemblage Zone of the Karoo Basin, South Africa

ABSTRACT

Brachyprosopus broomi was described in 1937 based on a specimen from the Tapinocephalus Assemblage Zone (Karoo Basin, South Africa), but it was largely overlooked by subsequent workers. We have identified several new specimens that show that Brachyprosopus is a valid taxon. An autapomorphy for the taxon is a curled lateral edge of the squamosal that forms a lateral wall of the external adductor fossa. Other important characters are absence of anterior median palatal ridges; maxillary tooth rows bounded laterally by a shelf; unfused vomers; raised margins of the interpterygoid vacuity; broad intertemporal region; pineal boss; dentary tables; and a long, wide posterior dentary sulcus that extends posterior to the dentary teeth. Chelydontops altidentalis is a junior synonym of B. broomi. A phylogenetic analysis places Brachyprosopus among basal dicynodonts, not as a close relative of Endothiodon. It is noteworthy that some characters, such as well-developed medial maxillary tooth rows and the shape of the palatines, are shared by Brachyprosopus, Pristerodon, Endothiodon, and Niassodon, hinting that a final resolution of relationships among toothed dicynodonts remains to be achieved. Most Brachyprosopus specimens are from the upper Abrahamskraal Formation (Moordenaars and Karelskraal members). One specimen is from low in the Koonap Formation, just above the lowermost maroon mudrocks of the Beaufort Group, and an exact correlation between this level and the strata of the Abrahamskraal Formation is uncertain. Therefore, it is likely that the stratigraphic range of Brachyprosopus extends throughout the Tapinocephalus zone and possibly down into the Eodicynodon Assemblage Zone.

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Citation for this article: Angielczyk, K. D., B. S. Rubidge, M. O. Day, and F. Lin. 2016. A reevaluation of Brachyprosopus broomi and Chelydontops altidentalis, dicynodonts (Therapsida, Anomodontia) from the middle Permian Tapinocephalus Assemblage Zone of the Karoo Basin, South Africa. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1078342.

ACKNOWLEDGMENTS

We thank R. Smith for bringing SAM-PK-K7604a, SAM-PK-K7604b, and SAM-PK-K7607 to our attention, and for information on the stratigraphic occurrences of these specimens. B.S.R. and M.O.D. acknowledge the financial support of the Palaeontological Scientific Trust (PAST) and its Scatterlings of Africa program; the National Research Foundation (NRF); the DST/NRF Centre of Excellence in Palaeosciences (CoE-Pal); and the University of the Witwatersrand. F.L.'s work on this project was supported by NSF DBI-1156594 (to P. Sierwald and K. Angielczyk). R. Sadleir provided much advice and assistance with making the 3D reconstructions of FMNH UR 1770. We thank J. Aramini, C. Dube, S. Jirah, A. Shinya, and C. van Beek for fossil preparation. E. de Plessis drafted the drawings in Figures 3, 4, and 9. J. Choiniere assisted with photography. C. Kammerer and C. Sidor provided helpful reviews, and we thank J. Botha-Brink for her work as handling editor.

NOTE ADDED AT PROOF

While this paper was in press, Dr. R. Smith brought to our attention newly-prepared additional material of B. broomi that he collected in May, 2014. The material has the catalog number SAM-PK-K11281 and consists of a dorsoventrally flattened skull, a dorsoventrally flattened articulated skull and mandible, an isolated left postorbital, an isolated ?right squamosal, two isolated anterior mandibles, four articulated vertebrae, portions of multiple ribs, a left scapula, a left humerus, proximal left and distal right femurs, and unidentified partially prepared limb bones. A full description of this material is beyond the scope of this paper, but because SAM-PK-K11281 preserves previously unknown parts of the skeleton of B. broomi (e.g., scapula, femur), it should be taken into account in future work that considers the species. Specimen SAM-PK-K11281 is also important because it is a stratigraphically high occurrence of B. broomi that was collected in a measured stratigraphic section. The specimen was collected on the farm Beukesplaas, Fraserburg District, in the uppermost Karelskraal Member of the Abrahamskraal Formation, approximately four meters below the base of the Poortjie Member of the Teekloof Formation. The specimen corroborates our hypothesis that B. broomi was limited to the Tapinocephalus Assemblage Zone and did not extend into the overlying Pristerognathus Assemblage Zone.