The ‘Sprawling-to-Parasagittal’ Transition: Evolution, Function, and Development of the Mammalian Hip Joint

Abstract

Mammals are an ecologically diverse group of animals, yet nearly every extant species has limbs adducted underneath their body in a ‘parasagittal’ posture. In contrast, mammals evolved from ‘sprawling’ synapsid ancestors with limbs splayed out to the side. Despite more than a century’s worth of research, the precise timing and acquisition of mammalian posture along the ‘sprawling-to-parasagittal’ transition remains elusive. Modern technological advancements provide paleobiologists with a new toolkit for revisiting old questions, but the hindlimb has not yet been comprehensively investigated. In this dissertation, I use a multi-disciplinary approach to examine the evolution, function, and development of mammalian parasagittal posture, focusing on the hip joint and associated bones (i.e., pelvis and femur). In Chapter 1, I focus on direct evidence from the extensive synapsid fossil record by investigating macroevolutionary patterns of pelvis and femur morphology. The evolution of synapsid pelvis and femur morphology included multiple, semi-independent adaptive optima with the largest-in-magnitude change for both bones occurring within prozostrodontian cynodonts and a final pulse of morphological change for the femur among stem therians – the latter of which aligns with evidence from musculoskeletal modeling of the forelimb and hindlimb in a limited number of taxa. The cranially-extended and elongate ilium, reduced pubo-ischiadic plate, and medially offset femoral head show the strongest patterns of change across synapsid evolution – all features associated with a more adducted limb. In Chapter 2, I explore how rarely-fossilized soft tissues (e.g., integument, muscles) constrain hip joint mobility in the extant sprawling tegu and the extant parasagittal Virginia opossum. I found that the integument keeps the distal femur elevated in tegu, preventing more adducted poses, while extrinsic musculature in the opossum prevents the femur from retracting and depressing beyond the extent of poses used in vivo during the stance phase of walking. While ‘deep thigh’ musculature is ancestral for synapsids, mammalian integument evolved at least by Mammaliaformes, if not earlier, and may have permitted more adducted limb poses. In Chapter 3, I explore the evolution of morphogenetic development for the pelvis and femur in the parasagittal mouse compared to the sprawling brown anole and axolotl, and I also compare it to macroevolutionary patterns of morphological evolution in synapsids. Broadly, I found that mouse pelvis and femur morphology is already distinct from anole and axolotl at the earliest cartilaginous stage of development (E13.5) and continues to proceed along a unique morphogenetic trajectory. Further, early stage mouse pelves and femora, potentially as a result of heterotopic change to cranially direct the ilium and medially offset the femoral head, are most similar in shape to eucynodonts and mammaliaforms, respectively, among synapsid subclades.