Hind limb osteology of Gracilisuchus stipanicicorum (Archosauria: Pseudosuchia)

Abstract

Gracilisuchus stipanicicorum Romer, 1972, from the Middle-Late Triassic of the Ischigualasto–Villa Unión Basin of Argentina, is an extinct pseudosuchian archosaur on the stem to Crocodylomorpha. The pelvic girdle and hind limb anatomy of a referred specimen of Gracilisuchus stipanicicorum is described and compared with that from a broad range of archosauriform taxa, including basal members such as crurotarsans and basal ornithodirans. The description of this specimen reveals new information on the anatomy of the pelvic girdle and hind limb of Gracilisuchus, through a detailed examination of some anatomical regions barely or not previously described, as well as reinterpretations of previous features. The phylogenetic affinities of Gracilisuchus within the Archosauria remain to be tested, but Gracilisuchus shares two putative synapomorphies with some non-crocodyliform crocodylomorphs, providing tentative support for the monophyly of Sphenosuchia (e.g., Sereno & Wild 1992; Wu & Chatterjee 1993) and the close relationship of Gracilisuchus to that clade. These characteristics are: (i) the morphology and poor development of the femoral fourth trochanter, closely resembling the condition of Pseudhesperosuchus and Trialestes; and (ii) a poor anterior development of the femoral head, shared with Pseudhesperosuchus. On the other hand there are characters that reject the inclusion of Gracilisuchus within Crocodylomorpha (Nesbitt 2011), such as the absence of an imperforated acetabulum, and that rather suggests a sister-taxon position to Crocodylomorpha.