Orbital Frontalisation and Morphological Cranial Variability in Anthropoidea

Abstract

The evolution of a developed visual system in primates is the result of an integration between physiological and morphological adaptations, involving different structures and tissues. In this context, cranial variability in Platyrrhini and Catarrhini has been analysed through a multivariate approach to landmark coordinates by mean of geometric morphometric tools. The main differences are related to a lateral expansion of the skull, sagittal decreasing and separation of medial structures (nasion, glabella), enlarging of inter-orbital distances. This component is markedly size-related, and it separates Hominoidea from the other Anthropoidea. This morphological shifting leads to an increasing distance between optic foramina, with consequent alignment of the visual axes. A second pattern separates Platyrrhini from Catarrhini, in particular from Cercopithecoidea. The South American primates are characterised by a decreasing of transversal diameters and orbital lateral shifting, with consequent divergence of the visual axes. Platyrrhini then show a minimum orbital convergence due to both these components, while 'Pongo pygmaeus' stands at the positive pole of these two frontalisation vectors. In Cercopithecoidea, the genus 'Papio' seems the more homogeneous and frontalised. 'Alouatta' spp., due to the extreme ayrorhinchy, stands phenetically related to Platyrrhini but showing an unique independent pattern.