Revisiting the anatomy of Riograndia guaibensis (Cynodontia; Probainognathia): new perspectives from new techniques

PEDRO MORAIS FONSECA; RAYFIELD, EMILY J.; MARINA BENTO SOARES; AGUSTIN MARTINELLI; SOFIA HOLPIN; GILL, PAMELA; CESAR SCHULTZ

Congreso; XXVI Congresso Brasileiro de Paleontologia; 2019

The small non-mammaliaform prozostrodont Riograndia guaibensis is a common taxon recovered from two outcrops assigned to the top of the Candelaria Sequence (Santa Maria Supersequence) of the Upper Triassic of Rio Grande do Sul, Brazil. Despite the abundance of fossils referred to this species, only three studies have described its cranium osteology: the first description of the holotype (MCN-PV 2264) and hypodigm materials; (2) an overall description of the cranial anatomy based on new findings (including UFRGS-PV-0596-T); (3) the description of the encephalon of UFRGS-PV-0596-T using µCT scan. In the last years 14 specimens of Riograndia were scanned at a high resolution µCt machine, which are the base to improve the cranial anatomy of this taxon. Although the first approaches on Riograndia, the new data provides new clues on the internal structures of the skull, like the inner ear and nasal cavity. Furthermore, it is possible to obtain a more detailed osteology with the µCT data, like the inner contact among bones and their internal surfaces. For example, the septomaxilla in lateral view, the small posterior process of the septomaxilla is interposed between the nasal and the premaxilla, participating of the posterior margin of the external naris. In the µCT images, by removing the maxilla, it is possible to determinate that the septomaxilla of UFRGS-PV-0596-T is a more developed element than thought, extending beyond the anterior limit of the second postcanine, and not only until the first one as previously described. In addition, the contact between septomaxilla and nasal is widest than previously described. The posterior projection of the septomaxilla contacts the anterior border of a pronunciated ventral projection of the nasal, which is also hidden by the maxilla. Like the septomaxilla and nasal, other internal contacts between bones remain unknown in Riograndia. In addition, a more detailed study of the basicranium, changes in the encephalon cavity along individual of different sizes, the orbital medial wall and the structures of the auditory apparatus are being addressed in Riograndia, with the new obtained data, and are relevant features to understand the changes occurred previous to the origin of mammaliaforms.