A Basal Nonmammaliaform Cynodont from the Permian of Zambia and the Origins of Mammalian Endocranial and Postcranial Anatomy

ABSTRACT

Nonmammaliaform cynodonts were a diverse group of Permo-Triassic synapsids whose morphological evolution documented the beginning of many classic mammalian traits. Here, we describe a new basal cynodont from the upper Permian Madumabisa Mudstone Formation of Zambia’s Luangwa Basin as Nshimbodon muchingaensis gen. et sp. nov. The holotype, a relatively complete and undistorted cranium and articulated mandible with associated postcranial elements, is interpreted as the most complete and well-preserved example of a charassognathid cynodont, and preserves hitherto unknown details of charassognathid endocranial and postcranial anatomy. A phylogenetic analysis of 111 morphological characters from 25 therapsid taxa (including 15 Permo-Triassic cynodonts) supports a sister-taxon relationship between Nshimbodon and Abdalodon, including them with Charassognathus in a monophyletic Charassognathidae, and placing the family near the base of Cynodontia. In addition to its phylogenetic importance, Nshimbodon provides evidence of correlated transformations in the feeding system, neck, and shoulder, which are consistent with novel mammal-like locomotor and feeding mechanics in the earliest cynodonts. Lastly, given previous reports of charassognathids in the Karoo Basin of South Africa, the occurrence of Nshimbodon indicates that charassognathids, like the basal cynodont Procynosuchus, were geographically widespread in southern Pangea by Lopingian times. Continued collecting in the Madumabisa Mudstone Formation will lead to a better understanding of the formation’s Permian cynodont diversity and biostratigraphy, as well as the biogeographic structure of southern Pangean vertebrate assemblages prior to the Permo-Triassic mass extinction.

http://zoobank.org/urn:lsid:zoobank.org:pub:143CE9D3-5742-4E92-B48F-164685F0907C

ACKNOWLEDGMENTS

We thank the members of the 2009 expedition to the Luangwa Basin for their assistance and companionship in the field, especially R. Smith for discovering the specimen described here. Fieldwork in 2009 supported by National Geographic Society grant 8571-08 (to S. Steyer) and Field Museum/IDP Foundation Inc. funding (to K. Angielczyk), with more recent research supported by NSF EAR-1337569 (to C.A.S.). Our colleagues at NHCC have provided research and collecting permits since 2009 and we especially thank J. Museba for his assistance with all aspects of this collaborative project. We acknowledge B. Crowley and K. Abrams (Burke Museum) for preparation of the specimen and M. Rich for taking the photographs used in Figure 8. We are particularly grateful to C. Shin for the interpretive drawings and life restoration. Finally, we thank the editor and two reviewers, L. Gaetano and C. Kammerer, for their constructive suggestions on the submitted manuscript. Virtual models of NHCC LB277 are curated on MorphoSource.org (under project number P1072).