Figure 1. This image depicts a postorbital bar in the skull of a lemur (Lawlor, 115). This study examines the functional anatomy of the primate postorbital bar. Postorbital bars are “bony arches that encompass the lateral aspect of the eye and form part of a circular orbit,” (Heesy 2005). They are formed by the dorsal part of the frontal bone and the ventral part of the zygomatic. Postorbital bars are not unique to primates. In fact, historically, they have evolved many times in many different mammalian species. However, they are found in all primates and, thus, are considered a defining characteristic of the family. Nevertheless, while their importance is undeniable, their reason for evolving and their functional role is highly debated among scholars.
There are many different theories as to the origin of the postorbital bar in primates. Some can be easily disregarded while others bear more weight. However, there seem to be three basic hypotheses that are most seriously considered by experts in the field.
The first was put forth by Prince (1953, 1956) and Simons (1962). It suggests that the postorbital bar serves to protect against external trauma by shielding the orbit from things like lateral blows, bites, etc. While this may be true to some extent, Cartmill (1970) argues that, considering the thickness of the bar, it would not defend very well against something like a tooth puncture. Therefore, this hypothesis can be rejected.
The second popular hypothesis states that unilateral bite force without a postorbital bar present would severely increase torsional loading on the skull (Greaves 1985). This would then be “transmitted to the facial skull and probably cause the face to bend and, to some extent, twist up against its moorings,” (Rosenberg, 1986). The inference here is that the bar serves to reduce some of this torsion and subsequent twisting by stabilizing the skull. However, Greaves’ helices of tension and compression are not accurate portrayals of those in primates and anthropoid strains don’t support torsional loading (Hylander and Ross 1996).
Finally, the third hypothesis from Cartmill (1970,1972,1980) states that increasing orbital convergence shifts the anterior temporalis laterally, thereby distorting the lateral orbital wall. This distortion would then presumably disrupt oculomotor precision unless stabilized by the postorbital bar. While it may be the best prediction thus far, the problem with this is that it does not account for the evolution of bars in other non-primate mammals that lack orbital convergence.
The reason it is so hard to prove why exactly the postorbital bar evolved is because we still have not proved what exactly the postorbital bar does. What function does it serve? Why is its presence essential? It has been long suggested and agreed upon that postorbital bars function to stiffen the lateral orbit, thereby decreasing deformation and disruption to oculomotor precision. However, there is very little proof on the actual mechanical function and significance of the postorbital bar, as it has never been experimentally tested directly.