The stapes of Massetognathus pascuali Romer, 1967 (Eucynodontia, Traversodontidae) from the Middle Triassic of Argentina

GAETANO, LEANDRO C.; ABDALA, NESTOR F.

Jornada; XXVII Jornadas Argentinas de Paleontología de Vertebrados; 2013

Several Massetognathus pascuali Romer, 1967 specimens are known, although only in a few of them the stapes is preserved. We provide here a detailed description of the stapes of M. pascuali and account for the intra-specific variation of this bone for the first time. In the present study, three well preserved M. pascuali specimens were analyzed (CRILAR - PV414; Instituto ?Miguel Lillo? - PVL4727; and the Bernard Price Institute, University of the Witwatersrand, Johannesburg - BP/1/4245). In general, the stapes of Massetognathus is a slender bone with two crurae defining a relatively large ellipsoidal stapedial foramen that faces dorso-ventrally. The close contact between the stapes and the fenestra ovalis points to the absence of a well developed cartilaginous cap proximally as was suggested for other non-mammalian cynodonts. Distally, the stapes and the quadrate bones are in contact. There is no evidence of a contact between the stapes and the paroccipital process. The cranio-dental analysis of the three Massetognathus specimens studied suggests that they are co-specific. Nevertheless, several differences among their stapes are recognized, including the general proportions of the bone, the relative size of the crurae, the extension of the proximal and distal plates, the morphology of the stapedial footplate, and the presence of a dorsal flange. Some of these discrepancies might be explained by size differences between the specimens analyzed (CRILAR-PV414, skull length 135mm, vs BP/1/4245 and PVL4727, skull length 90mm) whereas others highlight the morphological plasticity of the stapes even among representatives of the same taxon.