Orbital Frontation and Morphological Variability in Primates: Geometric Morphometric Analysis of the Upper Facial Structures in Anthropoidea

Abstract

The anatomical and physiological adaptations characterising visual capabilities of primates are considered principal variables in the evolutionary radiation of the whole order. Orbital frontation is related to the reduction of the splanchnocranium and to stereoscopy, and affects overall skull architecture. Within this context, morphological variability in the orbital region of Anthropoidea has been analysed by means of multivariate techniques and spatial coordinates (geometric morphometrics) on a sample of 87 individuals. A Principal Components Analysis reveals a 'planar' morphospace, in which the first two components equally account for morphological variability. The first component explains a morpho-structural process of frontation, with posterior cranial widening, midsagittal reduction of the upper facial structures, and divergence of the optic foramina. This component is, as always, strongly size-related, depending in particular upon orbital height and neurocranial width. All Hominoidea are large animals and, therefore, particularly frontated, with 'Hylobates' and 'Pongo' showing greater frontation than expected from orbital dimensions. Ceboidea and Cercopithecoidea overlap entirely, except for 'Leontopithecus', which shows marked lateralisation because of its small size. The second component reflects a process related to orbital verticalisation, through posterior cranial widening, nasal and interorbital reduction, lengthening and slight convergence of the orbital axes, inferior orbital shift in a caudad direction, and glabellar protrusion. Cercopithecinae and Papioninae are the most verticalised taxa, in contrast with all Ceboidea and in particular 'Alouatta', because of its strong airorynchy. Also in 'Pongo', airorynchy leads to low verticalisation. The second principal component is related to the verticalisation index and to interorbital width. Cercopithecoidea show inverse correlation between frontation and verticalisation, with Colobidae probably lying in the lower range of the low verticalisation/high frontation pattern. Platyrrhini show low values for both, and more heterogeneous phenotypic variability, most of all in such specialised taxa as 'Alouatta' (airorynchy), 'Aotus' (nocturnal) and 'Leontopithecus' (dwarfism).