Comparative study of the forelimb of the Early Triassic cynodont Thrinaxodon liorhinus: exploring burrowing anatomyAZ

IQBAL, S.; CARLSON, K.J.; ABDALA, F.

Johannesburgo

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

Palaeontological Society of southern Africa

Therapsids were severely affected by the Permo-Triassic mass extinction and only a few lineages were able to survive. Burrowing is usually considered as one of the main behavioural responses that most likely aided therapsid survival across the extinction event. Several fossilized skeletons of the therapsid cynodont, Thrinaxodon have been found in curled up positions, presumably indicating that they died in enclosed spaces (e.g. a burrow). This circumstantial evidence has led to the idea that this taxon was perhaps a digger. To date, limb functional morphology of Thrinaxodon has not been systematically compared to that of extant burrowers. Irrespective of potential digging adaptations, the limbs of Thrinaxodon also have been described as exhibiting a transitional phase between classic sprawled limb postures of reptiles and mammalian parasagittal postures. The present study attempts to address both of these issues by quantitatively comparing forelimb structure (humerus, radius and ulna) of Thrinaxodon liorhinus with forelimb structure of a parasagittal cursorial marsupial (Thylacinus cynocephalus), parasagittal digging marsupials (Vombatus ursinus and Lasiorhinus keffti), and sprawled limb reptile form (Varanus niloticus). A geometric morphometric approach featuring a landmark-based analysis was completed in order to quantify the extent of similarity and difference between Thrinaxodon forelimb structure and forelimb structure in comparative groups. This series of nested comparisons permitted interpretation of the functional morphology of the fossil Thrinaxodon as having mixed functional inputs. Humeral morphology was observed to be structurally closest to that of fossorial marsupials (Vombatus and Lasiorhinus); however, radial morphology was closest to that of the sprawled reptile (Varanus). These results advance present understanding of Thrinaxodon limb structure, mobility, palaeobiology, and cynodonts in general