A quantitative assessment of infraorbital morphology in Homo: testing for character independence and evolutionary significance in the human midface

Features of the infraorbital region, such as infraorbital surface topography, infraorbital surface orientation, and curvature of the zygomaticoalveolar crest, have long played a prominent role in phylogenetic analyses of Homo. However, there is currently considerable debate regarding the phylogenetic reliability of infraorbital characters, as numerous researchers have questioned the degree to which these features are morphologically independent of one another and facial size. These questions largely stem from methodological limitations for accurately quantifying the curvilinear morphology of the infraorbital surface and zygomaticoalveolar crest, which have significantly impeded the ability to discern patterns of infraorbital integration and allometry. In this study, infraorbital surface and zygomaticoalveolar crest morphology are precisely assessed, through geometric morphometric methodologies well-suited for quantifying complex curvilinear structures, in a large sample of fossil (n = 71) and recent Homo (n = 303). Once quantified, measures of infraorbital surface topography, infraorbital surface orientation and zygomaticoalveolar crest curvature are further evaluated for intercorrelation and allometry in order to more fully evaluate the morphological independence of commonly cited infraorbital characters. The results of this study indicate that most aspects of infraorbital surface topography, infraorbital surface orientation and zygomaticoalveolar crest curvature are significantly correlated with facial size across Homo. Moreover, certain aspects of infraorbital shape, such the degree of infraorbital surface depression and the overall curvature of the zygomaticoalveolar crest, appear to show additional, size-independent, intercorrelations, suggesting they form a singular "infraorbital complex." In light of these results, the use of infraorbital characters as separate independent characters in phylogenetic assessments of Homo is called into question, while the importance of facial size in human craniofacial evolution is further highlighted.