A new silesaurid from the upper Ntawere Formation of Zambia (Middle Triassic) demonstrates the rapid diversification of Silesauridae (Avemetatarsalia, Dinosauriformes)

ABSTRACT

Recent discoveries have shown that non-dinosaurian dinosauromorphs were morphologically diverse, globally distributed, and have a stratigraphic range extending into the Upper Triassic. Silesauridae, the sister group to Dinosauria, contains at least seven species. Here we describe Lutungutali sitwensis, gen. et sp. nov., the first silesaurid from the upper portion of the Ntawere Formation of the Luangwa Basin, Zambia. The upper Ntawere Formation has been correlated with subzone C of the Cynognathus Assemblage Zone of the Karoo Basin in South Africa and the Lifua Member of the Manda beds in the Ruhuhu Basin in Tanzania, both of which are considered Anisian in age and the latter has yielded the silesaurid Asilisaurus kongwe. The results of our phylogenetic analysis, including a new pelvic character, allies Lutungutali with Upper Triassic silesaurids such as Silesaurus, Sacisaurus, and Eucoelophysis rather than with the possibly coeval Asilisaurus. The Zambian silesaurid shares a laterally oriented brevis fossa on the ilium and a transversely thin ischium in cross-section with Upper Triassic forms. Silesaurids were more diverse during their early evolution in the Anisian than previously suspected. Lutungutali and Asilisaurus are the two oldest known members of the bird-line archosaurs represented by body fossils. Together they show that a subclade of bird-line archosaurs was diversifying soon after its origin, building further support for the rapid diversification of Archosauria in the wake of the Permo-Triassic extinction.

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ACKNOWLEDGMENTS

We thank A. Goulding, J. Menke, K. Mwamulowe, S. Tolan, and R. Whatley for assistance in the field, N. Banda and B. Crowley for preparation of the specimen, and J. Swales for drawing Figures 2 and 4. We acknowledge the Zambian Wildlife Authority and NHCC for permits and administrative assistance. Field work in Zambia has been supported by National Geographic Society grant 8571-08 (to J.S.S.) and funding from The Field Museum/IDP Foundation, Inc. African Partners Program (to K.D.A.) and the Field Museum Department of Geology (to K.D.A.). B.R.P. was funded by a National Science Foundation Graduate Research Fellowship. S.J.N. was supported by NSF EAR-1024036. NMT RB159 was collected during 2012 field work supported by National Geographic Society grant 8962-11 (to C.A.S.). Special thanks are due to R. Butler and S. Brusatte for providing thorough and helpful reviews of the manuscript.

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