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Functional morphology of the hindlimb during the transition from sprawling to parasagittal gaits in synapsid evolution

Wright, Mark (2018) Functional morphology of the hindlimb during the transition from sprawling to parasagittal gaits in synapsid evolution. Master's Research Project 2, Ecology and Evolution.


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Mammals are characterized by rich diversity in hindlimb morphology, locomotory specialization, and ecological habit. As mammalian species spread throughout the globe occupying novel environments, some modified their hindlimbs to reflect the challenges of these new environments. However, it is unclear whether disparate limb morphology was the driver or the result of this ecological diversification. Exploring the relationship between ecology and morphology during the transition from the earliest synapsids (mammalian ancestors) to basal mammals requires a comparative framework connecting morphology to locomotory function. This transition included a postural shift in hindlimb orientation. The earliest synapsids had limbs abducted to their sides that operated with the same sprawling movement common in many extant lizards and salamanders. However, the first mammals had limbs rotated underneath the body, upright, that operated primarily in a parasagittal plane of movement. Here, two species (the extinct sphenacodontid, Dimetrodon milleri, and the extant opossum, Monodelphis domestica) are used to represent an early and late character state, respectively, of hindlimb function during this transition. The opossum was stained using phosphomolybdic acid (PMA) for soft tissue contrast, and both specimens were scanned using microcomputed tomography (μCT). With three-dimensional modeling software, musculoskeletal models were created for each specimen to interpret functional morphology of the hindlimb at two stages during this evolutionary transition. The results here show that basal synapsids had increased mechanical advantage during sprawling movements (higher abduction-adduction moment arms) while the proximal hindlimb of ancestral therian mammals functioned as joint-stabilizers during parasagittal stances (negative sloping flexion-extension moment arms that cross through zero). Sensitivity analyses of two model- building parameters (neutral pose orientation and muscle attachment site) confirm that differences in functional interpretations are not caused by limitations during model construction.

Item Type: Thesis (Master's Research Project 2)
Supervisor name: Etienne, R.S.
Degree programme: Ecology and Evolution
Thesis type: Master's Research Project 2
Language: English
Date Deposited: 31 Aug 2018
Last Modified: 02 Oct 2018 10:25

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