Oculomotor stability and the functions of the postorbital bar and septum

Abstract

The postorbital bar and septum are circumorbital structures that are important to adaptive hypotheses for the origins of primates and haplorhines, respectively. All primates possess complete postorbital bars, bony arches formed by processes of the frontal and zygomatic bones that encompass the lateral aspect of the eye. Postorbital septa, bony walls formed by the frontal, zygomatic and alisphenoid bones, walling off the orbit from the anterior temporal fossa, are limited to tarsiers and anthropoids. Numerous functional hypotheses have been advanced for postorbital bars and septa. Many of these hypotheses can easily be rejected (Cartmill, 1970, 1972, 1980; Ravosa, 1991a,b; Ravosa et al., 2000a,b; Ross, 1994, 1995a,b, 1996, 2000, 2001; Ross and Hylander, 1996; see Heesy, 2003). Cartmill (1970, 1972, 1980; see also Collins, 1921) suggested that in therian mammals with large eyes, relatively small temporal fossae, and derived orbit convergence (orbits facing in the same direction), the plane of the bony orbit would deviate from the "plane" of the temporal fossa. Cartmill proposed that increasing orbital convergence "drags" the anterior temporalis muscle and temporalis fascia from a posterior position to a lateral position (Figure 1). In such taxa, including Primates, Cartmill suggested that contractions of the masticatory musculature, particularly the anterior temporalis muscle, would be likely to distort the lateral orbital margin, potentially disrupting oculomotor precision. Replacement of the postorbital ligament with an osseous postorbital bar should stiffen the lateral bony orbit and prevent oculomotor disruption. © Springer Science+Business Media, LLC 2007.