The axial skeleton of Gracilisuchus stipanicicorum Romer: autapomorphic characters and its phylogenetic information within the context of Crurotarsi.

LECUONA, A.

Pittsburgh

Congreso; Annual Meeting of the Society of Vertebrate Paleontology; 2010

Society of Vertebrate Paleontology

Gracilisuchus stipanicicorum is a small suchian archosaur from the Middle Triassic Chañares Formation (NW Argentina), known from well preserved specimens but poorly studied anatomically. Since its description it has been related to different lineages of Archosauria and recently considered closely related to Crocodylomorpha, although a consensus is lacking about its relationships within Crurotarsi. The present study was performed on the basis of the axial skeleton of the holotype (PULR 08) and one referred specimen (PVL 4597). As a result of this study a unique combination of characteristics present in the axial skeleton of Gracilisuchus is identified (autapomorphies marked with a *): an horizontal postzygapophysial facet of the axis, posteriorly directed, and facing ventrally*; a high and vertical anterior border of the axial neural spine*; a concavoconvex posterior articular surface of the cervical vertebrae*; cervical vertebrae with a circular depression on the mid-dorsal region of the neural arch; absence of spine table in posterior dorsal vertebrae; two paramedian osteoderms per vertebra. Several axial characters support a relationship with Crocoylomorpha, such as the presence of a presacral vertebral count of at least 24, similar to some basal crocodylomorphs and crocodyliforms. A derived character state shared with the crocodylomorph Hesperosuchus is the presence of poorly developed ventral keels in the postaxial cervical vertebrae. Probable plesiomorphic features are the absence of a developed postzygapophysial process in the axis, shared with basal archosauriforms and Parasuchus; the presence of a ventral keel in the axis, shared with members of the archosauriform clade, whereas the absence of such a keel is found in some basal crocodyliformes (Sphenosuchus, Junggarsuchus). This information is added to a cladistic analysis within the context of Archosauriformes to test its phylogenetic affinities with the addition of the new information of the axial skeleton.