A new basal archosauriform diapsid from the Lower Triassic of Argentina

ABSTRACT

The best-known South American Early Triassic archosauriform belongs to a putative proterosuchid briefly reported by José Bonaparte in 1981, collected from the Quebrada de los Fósiles Formation (Puesto Viejo Group, Argentina). This specimen consists of well-preserved natural external molds of a partial postcranium that preserve dorsal vertebrae, osteoderms, a dorsal rib, a possible gastralium, a chevron, a humerus, an ilium, two metapodials, and an ungual. We re-describe this specimen and identify autapomorphies that allow us to recognize Koilamasuchus gonzalezdiazi, gen. et sp. nov. The presence of an iliac blade with a slightly convex dorsal margin and with a maximum length more than 3 times its maximum height places Koilamasuchus within Archosauriformes. A cladistic analysis of basal Archosauriformes positions Koilamasuchus more crownwards than Proterosuchus, Sarmatosuchus, Fugusuchus, and Osmolskina, as the sister taxon of the clade that includes Erythrosuchidae and Archosauria. Proterosuchidae is found to be paraphyletic. The presence of an iliac preacetabular process, a pubic peduncle that forms an angle lower than 45° to the longitudinal axis of the ilium, and dorsal body osteoderms positions Koilamasuchus in Archosauriformes more crownwards than proterosuchids. Koilamasuchus is more basal than erythrosuchids within Archosauriformes because of the presence of dorsal ribs with a poorly developed proximal end. Koilamasuchus importantly increases the diversity of Archosauriformes during the biotic recovery following the Permo-Triassic mass extinction.

ACKNOWLEDGMENTS

We thank A. Kramarz who loaned us the holotype of Koilamasuchus and several people who allowed us to study specimens under their care: J. Powell (PVL), S. Devinvenzi Gonzáles (MCNAM), P. Holroyd (UCMP), A. Da-Rosa (UFSM), A. Kramarz (MACN), L. Steel, S. Chapman, and P. Barrett (NHM). We thank J. Desojo (MACN) for comments and discussion of the type material of Koilamasuchus and other basal archosauriforms. The comments of three anonymous reviewers and editor S. Modesto improved the quality of this paper. Thanks are also extended to M. Miñana and M. Iberlucea who performed latex casts of the holotype of Koilamasuchus. Access to the free version of TNT 1.1 was granted by the Willi Hennig Society.