Muscular and osseous anatomy of the primate anterior temporal fossa and the functions of the postorbital septum

Abstract

Many adaptive explanations for anthropoid origins incorporate hypotheses regarding the function of the postorbital septum. Two hypotheses are evaluated here: Cachel's ([1979b] Am. J. Phys. Anthropol. 50:1-18) hypothesis that the anthropoid postorbital septum evolved to augment muscle attachment area in the anterior temporal fossa and Cartmill's ([1980] in RL Ciochon and AB Chiarelli (eds.): Evolutionary Biology of the New World Monkeys and Continental Drift. New York: Plenum, pp. 243-274.) hypothesis that the septum evolved to insulate the foveate eye of haplorhines from movements in the temporal fossa during mastication. Dissections of the masticatory muscles of 55 species of primates, with emphasis on the anatomy of the anterior temporal fossa, reveal that in all anthropoids the temporal muscles take origin from the portion of the septum formed by the frontal bone. In some platyrrhines this muscle is anterior temporalis, and in others it is zygomatico-mandibularis. In tarsiers and most platyrrhines, muscle attachment to the zygomatic portion of the postorbital septum is very restricted (and of possibly varying homologies), whereas in catarrhines the zygomatico-mandibularis arises from the postorbital ridge on the zygomatic portion of the septum. This suggests that, contrary to Cachel's hypothesis, the earliest anthropoids did not have extensive areas of muscle attachment on the postorbital septum, a suggestion supported by the bony morphology of Catopithecus browni. Dissections also indicate that in all haplorhines the anteriormost temporal fibers curve around the postorbital septum between origin and insertion, implying that, were the septum not present, the anterior temporal muscles would disturb the orbital contents when contracting. This suggests that insulation may have been the septum's original function, even in the absence of a retinal fovea. In anthropoids, the rostral migration of the line of action of the anterior temporal muscles relative to the eye is attributed to their possession of extreme degrees of both orbital frontation and convergence; in tarsiers it is attributed to their possession of both massively hypertrophied eyes and moderately convergent and frontated orbits. It is argued that the postorbital septum is most likely to have evolved in a morphological context similar to that exhibited by omomyids.