Foot and ankle functional morphology in anthropoid primates and Miocene hominoids
Authors:   , Kuo Sharon 
Published: December 2023

Locomotion is essential for survival in many taxa. It also varies greatly among organisms, including primates. Studying locomotor diversity in extant and fossil primates requires an understanding of form-function relationships. This is particularly true in the foot and ankle, as the foot directly contacts the substrate and tarsals are well-represented in the fossil record. Morphological differences alone provide limited aid when inferring locomotion from fossil tarsals in the absence of in vivo biomechanical consideration. This dissertation takes a three-step approach to analyze both in vivo rotations in the foot and ankle as well as morphological variation in tarsal form in extant anthropoid primates and Miocene hominoids and will provide important new data from a poorly understood anatomical region. The amount of talocrural, subtalar, and transverse tarsal rotations among the tibia, calcaneus, and navicular were visualized and quantified during the gait cycles using biplanar fluoroscopy and 3D scans of marked bones, a method known as x-ray reconstruction of moving morphology (XROMM) in rhesus macaques (Macaca mulatta). This study supported previous hypotheses that the midfoot break occurs distal to the cuboid, demonstrated the predominance of plantarflexion/dorsiflexion at the talocrural joint on a flat surface, quantified conjunct rotation at the subtalar joint, showed evidence that the transverse tarsal joint does not function as a single joint complex. Geometric morphometric techniques were used to describe and quantify shape differences in isolated tarsals of extant anthropoid primates. PCA and M/ANOVA analyses were run on a Procrustes-fit landmarks taken on broad range of anthropoid tali (n = 241), calcanei (n = 230), cuboids (n = 282), and naviculars (n = 254). In addition to the typical geometric morphometric techniques, the interlandmark distances that accounted for the greatest amount of variation in this sample were isolated and plotted against centroid size. Phylogenetically controlled generalized least squares analysis revealed which of these measurements were related to locomotion. The relative orientation of the posterior subtalar facet on the talus, talar neck length, calcaneal tuber height, calcaneal anterior length, cuboid length, and navicular anteroposterior length were the morphologies that best separated based on differences in locomotion. The same landmarks were taken on 16 Miocene hominoid tarsals in order to infer foot function based on tarsal form. The geometric morphometric technique of the extant sample allowed for subsetted analyses for incomplete fossils. Early Miocene taxa Ekembo, Proconsul, and Rangwapithecus shared common bony features that suggest that they were generally above branch quadrupeds. Nacholapithecus showed a mixed or varied locomotor behavior. Oreopithecus was shown to not be bipedal, as previously hypothesized, but rather was suspensory. This dissertation provided the first ever quantification of intertarsal and talocrural rotations in anthropoid primate feet and ankles and an analysis of how rotations within and among joints are related. It also provided a quantification of shape differences in tarsals of extant anthropoid primates and fossil Miocene hominoids. Together, the in vivo biomechanics and morphometrics provide insight into form function relationships as well as a foundation for future studies of primate locomotor diversity.