New information on the postcranial skeleton of Gracilisuchus stipanicicorum (Archosauria: Suchia) and reappraisal of its phylogenetic position

Abstract

Gracilisuchus stipanicicorum Romer, 1972a, the only species in its genus, is a small basal suchian, recovered from the Middle?Late Triassic Chañares Formation (La Rioja, Argentina). This species has been interpreted as related to vari- ous taxa within Archosauria, including Ornithosuchidae, Rauisuchidae and Crocodylomorpha. Recent phylogenetic analyses are concordant in recovering it in a basal position within Suchia, and the most recent analysis recovered it within the clade Gracilisuchidae. Six specimens of G. stipanicicorum are known and all of them were studied and included in a phylogenetic analysis of Archosauriformes. Re-examination of the specimens allowed us to improve the diagnosis, identify new diagnostic characters (e.g. first sacral rib with a slight distal expansion relative to the proximal portion) and, in particular, redescribe the postcranial skeleton. The phylogenetic data matrix is an updated version of a previous study with the inclusion of additional characters and taxa. The strict consensus supports the most recent phylogenetic results, recovering Gracilisuchus as a basal suchian within Gracilisuchidae. The results of this analysis resolve some important polytomies and yield higher support values than recent analyses. All previous alternative hypotheses on the affinities of Gracilisuchus were found to be highly suboptimal for the new data matrix.