Title: The ‘Sprawling-to-Parasagittal’ Transition: Evolution, Function, and Development of the Mammalian Hip Joint
Abstract: Extant mammals navigate diverse habitats around the planet with a wide array of locomotor ecologies, yet nearly every species has limbs oriented close to the midline of the body in a ‘parasagittal’ posture. However, the precise evolutionary origins of parasagittal posture from ‘sprawling’ synapsid ancestors with limbs splayed out to the side remains elusive. In my dissertation, I take an interdisciplinary approach to address this question and focus on the hip joint as its morphology and function is central to limb posture. In Chapter 1, I investigate macroevolutionary patterns of pelvis and femur morphology across the exceptional 300+ million year old synapsid fossil record. I show that these bones evolved semi-independently with multiple shifts in adaptive optima, including as late as stem therians. In Chapter 2, I experimentally measure ex vivo hip joint mobility in two extant species that bracket this postural continuum (i.e., opossum and tegu) while sequentially removing soft tissues and compare it to in vivo mobility used during walking. I demonstrate that the integument keeps the femur elevated in a sprawling posture in the tegu, while extrinsic ‘deep thigh’ muscles, which are ancestral for synapsids, prevent the femur from retracting and depressing beyond the extent of in vivo poses in the opossum. In Chapter 3, I explore the evolution of development of the pelvis and femur by comparing morphogenesis of these bones in mouse to brown anole and axolotl as well as to the synapsid fossil record. Broadly, I find that the earliest cartilaginous stage in mouse is already distinct from anole and axolotl, while the latter two are similar, and all three species have unique morphogenetic trajectories. I also show that early stage mouse pelves and femora morphologically resemble eucynodont and mammaliaform fossils, respectively.
Committee: Stephanie Pierce (Advisor), Javier Ortega-Hernández (Chair),Andrew Biewener, Terence Capellini
LOCATION:Science Center Hall D, 1 Oxford Street STATUS:CONFIRMED DTSTART:20250402T170000Z DTEND:20250402T180000Z END:VEVENT END:VCALENDAR