A Kannemeyeriiform (Synapsida: Dicynodontia) Occipital Plate from the Middle Triassic Upper Fremouw Formation of Antarctica

Nathan D. Smith, Peter J. Makovicky, Christian A. Sidor, William R. Hammer

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A kannemeyeriiform dicynodont is described on the basis of an occipital plate from the upper Fremouw Formation (Middle Triassic) Gordon Valley locality in the Beardmore Glacier region of Antarctica. The Antarctic specimen is comparable in size to Kannemeyeria simocephalus from the well-known Cynognathus Assemblage Zone of the Beaufort Group of South Africa, and represents the largest therapsid currently known from the upper Fremouw Formation. The presence of an occipital condyle with distinct contributions from the exoccipital and the basioccipital; a wide, tri-radiate occipital condyle; and a well-developed tympanic process of the paroccipital, which is situated below the level of the occipital condyle, represent a combination of character states hitherto unknown among Kannemeyeriiformes. Combined with the possible autapomorphic feature of slender, dorsoventrally elongate basal tubera, this may suggest the Antarctic specimen represents a new kannemeyeriiform taxon. This specimen represents the most complete, and only the fourth definitive, dicynodont specimen known from the upper Fremouw Formation, and the contradictory phylogenetic character data from these specimens adds support for the presence of multiple (at least two) kannemeyeriiform taxa within the upper Fremouw tetrapod assemblage. Taken together, these kannemeyeriiform specimens provide additional support for a correlation with the Cynognathus Assemblage Zone, particularly the Trirachodon-Kannemeyeria or Cricodon-Ufudocyclops subzones (= subzones B or C), as well as an Anisian or younger age for the upper Fremouw tetrapod fauna.

Original languageEnglish (US)
Article numbere1829634
JournalJournal of Vertebrate Paleontology
Issue number5
StatePublished - 2020

Bibliographical note

Funding Information:
For access to specimens in their collections, we thank M. Norell, and C. Mehling (AMNH), and M. Walsh (LACM), as well as the University of Michigan’s Online Repository of Fossils (UMORF), which includes a 3D scan of Kannemeyeria sp. (UMMP VP 14530). C. Kammerer (North Carolina Museum of Natural Sciences) is thanked for providing access to numerous specimen photos and insight into dicynodont anatomy and phylogeny. We would like to thank past and current United States Antarctic Program field and support staff and contractors for enabling remote field research in the central Transantarctic Mountains. We also wish to thank JVP Senior Editors A. López-Arbarello and A. Turner, and Associate Editor P. Viglietti, for their constructive feedback on the manuscript. P. Viglietti and C. Kammerer are also thanked for their thoughtful and insightful reviews, which benefited the final version. Funding for field work was provided by NSF DPP 88-17023 to W.R.H., and this research was also supported by NSF ANT-1341475 to NDS, NSF ANT-1341304 to C.A.S., and NSF ANT-1341645 to P.J.M.

Publisher Copyright:
© 2020, by the Society of Vertebrate Paleontology.


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