Comparative study of the limb loadings in Thrinaxodon liorhhinus (Therapsida, Cynodontia) from the Early Triassic.

IQBAL, S.; CARLSON, K.J.; KIENHIFER, F.; CHOINIERE, J.N.; ABDALA, F.

Johannesburgo

Conferencia; 3rd National Conference on imaging with radiation (imgrad); 2017

Evolutionary Studies Institute

Therapsida is a lineage represented by living mammals and a series of transitional fossil groups. The fossil forms were severely affected by the Permian-Triassic mass extinction event, resulting in the survival of only a few lineages. One of these lineages, Cynodontia, includes the best known non-mammaliaform transitional fossil taxa, Thrinaxodon liorhinus. This species has been of captivating significance in the evolution of the therapsids that ultimately gave rise to mammals because of its combination of primitive and derived traits. Fossilized skeletons of Thrinaxodon are abundant in the South African Karoo Basin, which allow for inspection of their limb posture. The goal of this research is to better understand Thrinaxodon limb functional morphology. Specifically, the hypothesis to be tested is that proximal limb segment bone (e.g., humeri and femora) are better adapted structurally to resist loading associated with semi-sprawled postures rather than sprawled or parasagittal postures during gait. The present study uses Finite Element Analyses (FEA), and cross-sectional properties of humeri and femora of Thrinaxodon to assess structural competency. FEA is a computational technique that provides an approach for assessing mechanical behavior of materials undergoing loading. Thrinaxodon limb loading corroborates external evidence of semi-sprawled postures for this taxon. This would appear to indicate that Thrinaxodon retained the reptilian skeletal configuration in the forelimb, but confirms that it had adapted a posture that had begun to resemble parasagittal posture in the hindlimb. The application of FEA in similar future studies of Karoo fossil taxa may shed light on the evolution of additional prehistoric organisms.