Craniofacial Covariation in Extant Great Apes: A Geometric Morphometric Study

Abstract

The face is a complex organization of bones, the morphology of which is partly influenced by the rest of the cranium (e.g., the cranial base and the neurocranium). The characterization of facial morphological variation and craniofacial covariation patterns in extant hominids is fundamental for the understanding of their evolutionary history. While facial orientation on facial shape is important, as has been suggested, few studies have considered this character. In this study, the aim is to assess the morphological relationship between facial shape and facial orientation. The morphological covariation between facial shape and facial orientation is assessed. To achieve this aim, geometric morphometrics (landmark positioning, and Procrustes superimposition) and multivariate statistics (partial least squares) are used on 3-D virtual crania of three extant hominid genera (Homo, Pan, and Gorilla). Results show a significant covariation between facial shape and facial orientation. Facial orientation and basicranial flexion are strongly related to facial shape characters such as facial enlargement, lower face reduction, nasal aperture shape, and orbit shape. This study raises important new elements of major interest in the discussion of the relationship between facial shape and facial and basicranial orientation in the development of modern human facial characteristics during evolution. This study also highlights the great variety in facial morphology present in extant hominids and their species-specific pattern of facial shape relative to facial positioning. This work notably underscores the peculiar facial shape pattern in Homo as linked to its important basicranial flexion and its short and orthognatic face.