Cranial ontogeny of basal non-mammaliaform cynodonts from the Karoo Basin, South Africa

JASINOSKI, S. C.; ABDALA, F.; FERNANDEZ, V

Johannesburgo

Conferencia; 18th Biennial Conference of the Palaeontological Society of Southern Africad; 2014

Palaeontological Society of southern Africa

Thrinaxodon liorhinus was a basal cynodont from the Early Triassic of Gondwana. Specimens of Thrinaxodon are common and tend to be well preserved, allowing a comprehensive examination of cranial variation within the species. Previous studies that surveyed a small sample of individuals proposed only a few cranial features that could separate immature and mature specimens of Thrinaxodon. Here we re-evaluate these proposed ontogenetic features using both qualitative and quantitative analyses based on a larger sample of individuals ranging in skull length from approximately 30?96 mm. Seven specimens were also analysed using micro-computed tomography (micro-CT), allowing the internal structure of the skull to be thoroughly investigated. Preliminary results of our comprehensive survey corroborated some of the previously proposed ontogenetic features, but our study also indicated that two features in the skull roof posterior to the pineal foramen start to manifest during the early juvenile stage: 1) the development of the sagittal crest; and 2) the progressive fusion of the parietal-parietal suture. The latter feature was not recognized in previous ontogenetic studies and was documented here for the first time using micro-CT. In addition, our bivariate analysis indicated that the length of the temporal region and height of the occipital plate shows positive allometry. This is consistent with our qualitative observations and suggests that during ontogeny the size of the sagittal crest increased at a faster rate than the rest of the skull. In order to determine the functional significance of these ontogenetic differences, our observations must be placed within a broader phylogenetic context. For this, we propose comparing our results of the ontogenetic study of Thrinaxodon with two other basal cynodonts, Galesaurus and Procynosuchus. This forthcoming contribution will document the differences in skull morphology between these taxa using micro-CT, and in conjunction with other lines of evidence such as dental morphology, will determine if the differences indicate changes in skull function and diet.