Morphology, Function, and Evolution of the Anthropoid Postorbital Septum

Abstract

Almost all the skeletal features that distinguish anthropoids from typical lower primates are features of the skull. To the untrained observer. perhaps the most obvious of these is the bony postorbital septum anthropoids, which walls off the temporal fossa from the orbit proper and so converts the orbit into a shadowy eye socket. A somewhat less complete postorbital septum is found in Tarsius. As far as I know, no other vertebrate, living or fossi1, has developed a bony partition between the temporal muscles and the periorbita. Because the postorbital septum is unique to anthropoids and Tarsius, systematists who lump tarsiers and anthropoids together as “Haplorhini” have generally regarded the septum as a sign of tarsiers’ affinities to higher primates (Pocock, 1918; Jones, 1929; Hershkovitz, 1974; Cartmill and Kay, 1978; Luckett and Szalay, 1978). Conversely, those who believe that the closest allies of tarsiers are Eocene “tarsioids” (which lack the septum), or who think that lemurs are more closely related to anthropoids than tarsiers are, or who regard Anthropoidea as a polyphyletic taxon, have sought to prove that the septum of Tarsius is not homologous with the septum of anthropoids (Simons and Russell, 1960; Gingerich, 1973; Cachel, 1976, 1979; Schwartz et al., 1978).