Redescription and reevaluation of the taxonomical status of Microsuchus schilleri (Crocodyliformes: Mesoeucrocodylia) from the Upper Cretaceous of Neuquén, Argentina

Abstract

Fragmentary remains of Microsuchus schilleri (Crocodyliformes: Mesoeucrocodylia) from the Bajo de La Carpa Formation (Santonian, Late Creteceous) of northern Patagonia, Argentina, are decribed and reevaluated taxonomically. The type specimen is an articulated individual with hindlimb and axial remains. Microsuchus has several traits that allow to recognize it as a valid taxon (the presence of bulge lateral to the prezygapophyses of the second sacral vertebra; elongated posterior zeugopodia; proximal caudal centra with triangular cross section). Microsuchus was originally assigned as a goniopholidid 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-limbed crocodyliform from the same location, Neuquensuchus, were included. Microsuchus 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 well nested among notosuchians, in contrast to previous hypothesis. These findings further increase the crocodyliform diversity of the Bajo de la Carpa 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 gives evidence of a basal stock of mesoeucrocodylians with elongated limbs that was not previously reported for northern Patagonia.