Limb loading of thrinaxodon liorhinus: postural inference of non-mammaliaform cynodonts
Date
2019
Authors
Iqbal, Safiyyah
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Abstract
Therapsids 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 one of the best known non
mammaliaform transitional fossil taxa, Thrinaxodon liorhinus. This species is significant for
understanding the evolution of the therapsids which ultimately gave rise to mammals, because of its
combination of primitive and derived traits. Complete, fossilized skeletons of Thrinaxodon are abundant
in the South African Karoo Basin, permitting accurate reconstruction of their limbs. Although much is
known about posture in extant mammal groups, fundamental questions remain about the postures of their
closest extinct relatives, including Cynodontia. The goal of this research is to better understand
Thrinaxodon limb functional morphology using a combination of limb bone cross-sectional properties
and finite element analyses (FEA) of limb bone structural competency. I hypothesized that, because of its
transitional phylogenetic position, stylopodial elements (i.e., humeri and femora) of Thrinaxodon are
better adapted structurally to resist loading associated with semi-sprawled postures rather than loading
associated with sprawled or parasagittal postures during gait. I also tested this hypothesis in Cynognathus,
Galesaurus, Tetracynodon darti, Glanosuchus macrops and Tachyglossus. The FEA results show that as
Thrinaxodon’s posture is experimentally altered from sprawled to parasagittal, stress and strain increase
in its humerus. Cross-sectional properties of limb bones revealed that the femora experienced higher
anteroposterior (AP) stress and strain. Together, these results suggest that the humerus is structurally
optimised for a sprawled to semi-sprawled posture, showing that Thrinaxodon retained a reptilian skeletal
configuration in the forelimb, paired with a more derived, incipiently parasagittal posture in the hind limb.
These Thrinaxodon limb loading and cross-sectional results corroborate other external morphological
evidence of semi-sprawled postures in the forerunners of mammals. My results do not preclude the
capacity of Thrinaxodon to dynamically adopt fully sprawling or parasagittal postures for a short of time,
but do indicate that these postures were suboptimal and likely not habitually used. This difference in
posture between the forelimbs and the hind limbs is widespread across the therapsid lineage and may
reflect shared functional demands of specific behaviours. FEA presents an independent means for
palaeontologists to conduct explicit hypothesis testing in a comparative framework. Here, I were able to
use it to test what postural transitions were occurring in some of the closest extinct relative of mammals.
Description
A thesis submitted to the Faculty of Science, University of the Witwatersrand, in fulfillment of the requirements for the degree of Doctor of philosophy
February 2019