Computational Relationships among Form, Function, and Phylogeny in the Catarrhine Ulnar Carpus, and the Evolutionary History of Ape and Human Locomotion

Abstract

Catarrhine wrist elements are commonly preserved in the fossil record, and the morphology of these specimens plays a prominent role in shaping our understanding of hominoid evolution. This joint complex may have special utility in understanding how morphology tends to change in association with positional behavior and in reconstructing the prevalence of these behaviors in extinct primates. However, we continue to lack a detailed understanding of the relationships among function, phylogeny and wrist morphology, including the degree to which quantifiably similar morphological variation should be expected in behaviorally convergent lineages. Quantitatively-confirmed morpho-functional links within the anthropoid wrist therefore remain exceedingly rare, hindering our ability to characterize the locomotor evolution of early catarrhine groups, including that of our own lineage.

This dissertation addresses this gap through analysis of 3D morphometrics derived from a broad sample of extant and fossil anthropoid carpals, while ameliorating the confounding influences of allometry and phylogeny through their explicit inclusion in statistical models. Several cases of morphological convergence are identified in association with each of several behavioral modes. Features determined to meet both statistical and biomechanical criteria are selected as plausible locomotor or postural adaptations, and multivariate predictive models are demonstrated to be highly accurate in predicting differing degrees of reliance on the major anthropoid positional behaviors. These predictive models and other insights are applied along with additional analytical techniques to a sample of seven morphologically diverse, formerly-undescribed catarrhine capitates from sites in the early Miocene Tinderet sequence of Western Kenya. Each specimen is formally described, and its taxonomic identity determined or constrained through estimation of body mass and analyses of taxonomic, phylogenetic, and functional affinities. The functional diversity of this fossil sample is also quantified, and found to be comparable to that of broad extant groups. Two specimens demonstrate substantial suspensory affinities, and another is uniquely great ape-like among capitates known from the early Miocene. The latter specimen is assigned to Rangwapithecus gordoni, and a new criterion by which to distinguish specimens of this taxon from Proconsul africanus is suggested. The range of variation in functionally diagnostic traits within this sample indicates a greater diversity of positional behavior among early Miocene catarrhines than what is generally recognized, and results add further support for the presence of a mid-sized, behaviorally-derived catarrhine at Songhor.

This study also reconstructs adaptive transitions during hominoid locomotor evolution, as well as the positional repertoires ancestral to each of the various nested clades within Anthropoidea. While patterns of variation are inconsistent among carpal elements, results are consistent in providing further support for the frequency of parallelism in the locomotor evolution of hominoids, particularly as it applies to suspension, with the last common ancestors of both hominoids and great apes predicted to have been more generalized than any of the clade’s extant representatives. The last common ancestor of humans and chimpanzees is also estimated to have lacked adaptations in association with knuckle-walking, providing further support for a relatively generalized ancestral morphotype at the root of the hominin clade – one that is not well modeled by any of the surviving hominoid lineages.

These results advance our understanding of the complex relationships among form, function, and phylogeny, and of the locomotor evolution of extant and fossil anthropoids, with important caveats, and provide guidance for future analyses of extant and fossil anthropoid functional morphology.