The Upright Battle: Morphological Trends of the Bipedal Pelvis

Abstract

The shift to bipedal locomotion is a distinguishing feature of the human lineage that required substantial remodeling of the postcranium in hominins. The pelvis, due to its important functional role as a stabilizing and weight-bearing structure, has undergone one of the most drastic transformations in the skeleton to accommodate obligate bipedalism, thus making it a valuable region for studying locomotor behavior within the fossil record. Although bipedalism occurs in several mammalian groups, it is rare within primates and the ability to utilize a striding gait with an erect, or orthograde, posture remains unique to hominins. Orthograde posture in this context is therefore an equally important consideration, as it reorients the trunk into a vertical stance that undoubtedly contributes to the specialized pelvic anatomy found in hominin taxa. Given that all extant hominoids partake in varying degrees of orthogrady, it is argued that this change in posture preceded, and possibly even preadapted hominoids for, the eventual shift to terrestrial bipedalism. Therefore, this dissertation seeks to isolate features relating explicitly to bipedality from those required for orthograde posture to aid in understanding the functional roles of both in the evolution of hominin bipedalism. This dissertation utilizes a comparative sample that consists of extant primates and non-primate bipedal mammals that differ in body size, yet converge on aspects of their locomotor and postural repertoires, providing a comprehensive assessment of pelvic skeletal anatomy that lends novel insights into the functional interpretations that can be derived from this region. Both the trabecular microarchitecture and the overall dimensions of the pelvis are considered collectively in a phylogenetic context to isolate morphology that is informative for accurate reconstruction of locomotor behavior. Building on these insights, the first finite element analysis (FEA) validation study is performed on a non-human primate pelvis using previously collected strain gauge data to establish accurate parameters that will enhance future modeling capabilities thereby refining methods available for inferring locomotor behavior within the fossil record. Results indicate a corroboration of functional skeletal attributes at different levels throughout the pelvis that accommodate loading sustained at the hip joint during locomotion. Findings are complementary in nature, suggesting a complex interplay between body size, phylogeny and function that reinforce the importance of using a holistic approach to adequately evaluate pelvic skeletal morphology. Additionally, finite element modeling demonstrates that the interaction between the external and internal skeletal attributes are important for recreating realistic strain environments within the pelvis. Validation of the primate pelvis FEA model was partially obtained as congruence at strain gauge sites was observed for several regions. However, the gauges near the loading surface and those surrounding the pubic symphysis were inaccurate regardless of modifications to boundary conditions. The phylogenetic modeling component of the study elicits better model fit when an Ornstein–Uhlenbeck approach is implemented and when acetabulum height is used as a predictor variable, thereby elucidating important relationships between pelvic traits and body size over time that can be used for evaluating pelvic morphology encountered in the fossil record