The inner ear of Riograndia guaibensis (Cynodontia, Probainognathia).

FONSECA, PEDRO H. M.; MARTINELLI, AGUSTÍN; GILL, PAMELA; RAYFIELD, EMILY J.; SOFIA HOLPIN; SCHULTZ, CESAR L.; SOARES, MARINA BENTO

São Jao do Polesine

Simposio; PaleoRS; 2019

The small cynodont Riograndia guaibensis is a common prozostrodontian recovered fromthe top of the Candelária Sequence (Santa Maria Supersequence), Upper Triassic of RioGrande do Sul. Despite the abundance of fossils related to Riograndia, few cranial specimenspreserve the basicranium region and information about its inner ear remained unknown. Thepresent work presents the first description of the inner ear anatomy of R. guaibensis, basedon reconstruction of the inner ear of specimen UFRGS-PV-0833-T, using microCT data.Although only the left ear is preserved, it was possible to identify the cochlea in contact withthe fenestra ovalis on the lateral face, and the fenestra rotunda, on the ventral face. In addition,the three semicircular canals are positioned posteriorly to the cochlea. The anteriorsemicircular canal is the most dorsal and is the longest in length. The posterior and lateralsemicircular canals lie ventrally and laterally to the anterior canal respectively. Thepositioning of the inner ear reflects the common non-mammaliaform cynodont condition, inwhich the cochlea is turned ventromedially in the skull. Furthermore, the cochlea inRiograndia is anteroposteriorly short and sharply pointed. This differs from the elongatedcochlea present in other non-mammaliaform prozostrodontians, such Pachygenelus,tritylodontids (e.g., Yunnanodon), and Brasilodon quadrangularis, which present the derivedcondition. However, the 3D model of the semicircular canals shows some degree ofdeformation of each structure, which could be due to taphonomic distortion of the specimen.In general, the structure of the inner ear of Riograndia is similar to the other nonmammaliaform eucynodonts, but the cochlea is still reduced when compared to more derivedprozostrodontians (e.g., tritylodontids, Brasilodon, Sinocodon, Morganucodon). [CNPq]