Revision of the basal mesoeucrocodylian Microsuchus schilleri from the Bajo de la Carpa Formation (Upper Cretaceous, Neuquén, Argentina)

Abstract

During Late Cretaceous times, Patagonia and the rest of Gondwana was inhabited by an astonishing fauna of crocodyliforms. After numerous studies a more complete idea of their phylogenetic relationships and evolutionary patterns was obtained, achieving important conclusions on biodiversity, distribution, paleobiology and paleoecology. However, the Argentinean taxon Microsuchus schilleri has been almost ignored. The type specimen of Microsuchus was recovered in the early 20?s from the Cretaceous outcrops of the Bajo de La Carpa Formation, in the area where the campus of the University of Comahue is actually located. This specimen is represented by an articulated individual which has hindlimb and axial remains although, the preservation of the materials is far from optimal. Besides the lack of details in its remains, Microsuchus has several traits that allow us to recognize it as a valid taxon, such as: the presence of lateral bulge lateral to the prezygapophyses of the second sacral vertebra; elongated posterior zeugopodia; proximal caudal centra with triangular cross section; rectangular dorsal osteoderms; among others. On the original description Microsuchus was assigned as a goniopholid based on the platycely of its vertebral centra, a character that is widely distributed among fossil crocodyliforms. A phylogenetical analysis using an expanded data matrix focusing on postcranial data was performed in which Microsuchus and another long-legged crocodyliform from the same locality, Neuquensuchus, were included. The position of Microsuchus is conflictive as it is depicted in a politomy as a basal mesoeucrocodylian, with its position as a neosuchian or notosuchian supported by a single synapomorphy on the different most parsimonious trees. Additionaly, Neuquensuchus is recovered well nested among notosuchians, unlike previous hypothesis. These findings further increase the crocodyliform diversity of the Candeleros Formation and emphasize the relevance of its fauna in studies of the diversification of mesoeucrocodylians and notosuchians. Furthermore, the occurrence of both Microsuchus and Neuquensuchus give evidence of a basal stack of basal mesoeucrocodylians with elongated limbs that was not previosly reported for northern Patagonia.