Limb loading of thrinaxodon liorhinus: postural inference of non-mammaliaform cynodonts

Iqbal, Safiyyah

Limb loading of thrinaxodon liorhinus: postural inference of non-mammaliaform cynodonts

dc.contributor.author	Iqbal, Safiyyah
dc.date.accessioned	2019-08-29T12:06:29Z
dc.date.available	2019-08-29T12:06:29Z
dc.date.issued	2019
dc.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	en_ZA
dc.description.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.	en_ZA
dc.description.librarian	MT 2019	en_ZA
dc.identifier.uri	https://hdl.handle.net/10539/27928
dc.language.iso	en	en_ZA
dc.phd.title	PHD	en_ZA
dc.title	Limb loading of thrinaxodon liorhinus: postural inference of non-mammaliaform cynodonts	en_ZA
dc.type	Thesis	en_ZA