Allometric and functional influences on primate orbit orientation and the origins of the Anthropoidea

Abstract

Abstract Anthropoids are distinguished from other mammals by their forward-facing orbital apertures. Hypotheses regarding the functional significance of this feature have not been evaluated previously. Orbit orientation in most genera of extant primates, as well as several fossil taxa, was measured in terms of frontation (verticality) and convergence (the degree to which the orbits face in the same direction) using methods developed by Cartmill (1970). Hypotheses regarding the allometric and functional determinants of orbit orientation were assessed using bivariate correlation techniques. As predicted by Cartmill (1970), decreases in relative orbit diameter are correlated with increases in orbital convergence and animals with relatively large orbits in diverse lineages have intermediate values of frontation. When allometry is taken into account, anthropoids, tarsiers and lorises have high degrees of orbital convergence and anthropoids have highly frontated orbits. The high degree of convergence in tarsiers and lorises is argued to be an adaptation to nocturnal visual predation. The high degrees of convergence in anthropoids are argued to be the result of cumulative allometric increases in convergence associated with a shift to diurnality at small body size followed by a subsequent increase in body size. The highly frontated anthropoid orbits are an effect either of increases in frontal lobe size or increased basicranial flexion in a lineage of animals with orbits closely approximated below the olfactory tract. The combination of extreme convergence and frontation necessitates the presence of a postorbital septum to insulate the eye against rostrally displaced anterior temporal muscles. The orbital morphology of anthropoids is argued to have arisen most probably in a lineage of omomyid-like animals: small nocturnal animals with moderate degrees of orbital convergence and frontation and orbits closely approximated below the olfactory tract. The divergence of the anthropoid stem lineage was associated with a shift to diurnality at small body size.