A new archosaur from the Manda beds (Anisian, Middle Triassic) of southern Tanzania and its implications for character state optimizations at Archosauria and Pseudosuchia

ABSTRACT

The presence of early pseudosuchians and avemetatarsalians in Anisian beds of Africa demonstrates that the archosaur radiation was well underway by the beginning of the Middle Triassic. The rapid radiation produced a variety of forms, but many of the unique, well-diagnosed Late Triassic clades (e.g., Aetosauria, Ornithosuchidae) lack diagnosable members from the Middle Triassic. Here, we introduce a new Middle Triassic archosaur, Nundasuchus songeaensis, gen. et sp. nov., with an unusual mix of apomorphic character states found within Pseudosuchia and just outside Archosauria. The holotype consists of partial skeleton, including representative postcranial elements and parts of the skull. We added Nundasuchus songeaensis, gen. et sp. nov., into the two most comprehensive early archosaur phylogenetic data sets available, and in both analyses the new taxon falls within Pseudosuchia. However, a number of plesiomorphic archosaurian character states (e.g., posterolaterally directed tuber of the calcaneum) optimize as local autapomorphies of the new taxon within Pseudosuchia in our analyses. Therefore, we tested alternative hypotheses of relationships for the new taxon by utilizing constraint trees. The analyses resulted in little change in the relationships and structure of other Triassic archosaur clades, but changed optimizations of certain character states and character support at the base of Pseudosuchia and Archosauria. Our analyses suggest that the complex evolution of character-state changes at the base of Archosauria is inhibiting our understanding of the relationships of early Pseudosuchia and, in turn, Archosauria.

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ACKNOWLEDGMENTS

S.J.N. was funded through the National Science Foundation (NSF) (EAR 1024036 to C.A.S.), an Evolving Earth Grant, Stephen Jay Gould Grant (Paleontological Society), and support through the AMNH. We thank L. Herzog and B. Crowley for the preparation of the specimen. Our work in Tanzania has been supported by National Geographic Society grants 7787-05 and 8962-11 (to C.A.S.), with additional support from NSF DBI-0306158 and The Grainger Foundation (to K.D.A.). We thank C. Saanane (University of Dar es Salaam) as well as A. Tibaijuka and L. Nampunju (Antiquities Division, Ministry of Natural Resources and Tourism) for assistance in arranging and carrying out the field work. M. Abdalla, W. Simpson, S. Steyer, M. Stocker, and N. Tabor contributed to productive field seasons in Tanzania.