Cranial anatomy of Gracilisuchus stipanicicorum and its evidence for maturity

LECUONA, A.; DESOJO, J.B.; POL, D.

La Plata, Buenos Aires

Jornada; Reunión de comunicaciones de la Asociación Paleontológica Argentina; 2019

Gracilisuchus stipanicicorum Romer, 1972, a basal suchian from the upper Carnianlevels of the Chañares Formation (Upper Triassic, Ischigualasto-Villa UniónBasin), is nested within Gracilisuchidae along with Turfanosuchus dabanensis Young, 1973 and Yonghesuchus sangbiensis Wu, Liu, and Li, 2001. A recent study described in detail its postcranialanatomy and phylogenetic relationships, but a thorough revision of its cranial anatomyis still missing. Several diagnostic cranial characters are present, and some differences among the hypodigm canbe interpreted as ontogenetic variation. Gracilisuchus has large orbit openings (35?42% of the skull table),comparable with Euparkeria capensis Ewer, 1965 (35%), but larger than Turfanosuchus(30%), Yonghesuchus (25%) and paracrocodylomorphs. The supratemporalfenestra is quadrangular and widerthan long, contrasting with most archosauriforms with longer than wide orsubequal fenestrae. A small triangular postparietal is present in one specimen ofGracilisuchus (MCZVP 4117), as in basal archosauriforms (e.g., Euparkeria, Erythrosuchus africanusBroom, 1905). Some cranial characters, along with postcranial features, suggesta probably younger ontogenetic stage for the holotype specimen (PULR 08), includingits skull length (73mm) that represents 80?85% of the skull length of the referredspecimens, an interparietal suture visible along the entire interparietal contact(fused posteriorly in the referred specimens). However, the anteriorly open interparietalsuture, the unpreserved occipital condyles, and the large orbits in everyspecimen, suggest that probably none of them reached somaticmaturity. Skeletochronological studies of the available specimens would beimportant to test this hypothesis.