Anatomy and affinities of large archosauromorphs from the lower Fremouw Formation (Early Triassic) of Antarctica

ABSTRACT

The vertebrate assemblage of the lower Fremouw Formation has been studied for nearly 50 years, but many components remain poorly known. We describe a partial presacral vertebra and the distal end of a left humerus collected just above the Permian–Triassic boundary in the Shackleton Glacier region of the central Transantarctic Mountains. Our identification of these specimens as archosauromorphs that represent at least one taxon of large-bodied archosauriform increases the known reptile diversity of the Fremouw Formation considerably, and provides the first definitive evidence for the presence of Archosauriformes in the Early Triassic of Antarctica. These records increase faunal similarities between the lower Fremouw Formation and other Early Triassic assemblages. Although the lower Fremouw assemblage is typically considered a subset of the coeval Lystrosaurus Assemblage Zone (LAZ) of South Africa, the discrepancy in inferred body size between the Antarctic specimens and Proterosuchus fergusi, coupled with the fact that the LAZ of the Karoo Basin has been sampled much more thoroughly, suggests a real disparity in the maximum body size of apex carnivores between the lower Fremouw assemblage and the LAZ. The lower Fremouw specimens also demonstrate that one or more lineages of archosauriform had attained the large body size characteristic of later members of the clade very soon after the end-Permian mass extinction. This offers a point of contrast with the global pattern of post-extinction terrestrial communities, which are typified by a marked reduction in body size (the ‘Lilliput effect’).

ACKNOWLEDGMENTS

We thank the 1977–1978 Ohio State University and Wayne State University field crews for the collection of vertebrate specimens and the 1995–1996 Ohio State University and Augustana College field crews for subsequent collection and stratigraphic refinement. We also thank USAP remote field crews, past and present, for scientific work in the central Transantarctic Mountains. Science and support staff based in McMurdo Station, Antarctica, also deserve thanks for making this research possible. M. Norell and C. Mehling (AMNH) are thanked for arranging the loan of AMNH 24262 and 24264. We thank L. Herzog (FMNH) for additional preparation of AMNH 24262. For access to collections in their care we also acknowledge O. Alcober and R. Martinez (PVSJ), C. Mehling and M. Norell (AMNH), J. Powell (Fundacíon Miguel Lillo, Tucumán, Argentina), and W. Simpson (FMNH). For constructive discussion on early drafts of the manuscript, we thank K. Angielczyk (FMNH), J. Fröbisch (FMNH), and S. Nesbitt (U. Texas, Austin). JVP editor Johannes Müller and reviewers Richard Butler and Sean Modesto also contributed comments and discussion that significantly improved the final version of the manuscript. We also thank S. Nesbitt for graciously providing specimen photographs of Euparkeria capensis (SAM 5867), Proterosuchus fergusi (NMQR 1484), and Tropidosuchus romeri (PVL 4601); and R. Irmis for providing specimen photographs of Saturnalia tupiniquim (MCP 3844-PV, 3845-PV, 3846-PV). J.R.C. and S.H. also acknowledge support for a summer internship at FMNH provided by Augustana College, FMNH, and NSF DBI-0849958 to P. Sierwald and P.J.M. Funding for this research was provided by NSF ANT-0838925 to P.J.M. and N.D.S., and NSF ANT-0837951 to W.R.H. N.D.S. was also supported by NSF DEB-0808250, the Field Museum of Natural History Brown Family Graduate Fellowship, and AMNH Frank M. Chapman Memorial Fund during this project.

Handling editor: Johannes Müller