Postorbital Bar

The postorbital bar is where the zygomatic bone and the frontal bones join to form a lateral strut which, with the exception of Tarsiers runs around the eye socket of most Prosimians and some other mammals, such as the hyracoids. This is in contrast to the higher primates, Anthropoids, who have evolved fully enclosed sockets to protect their eyes.

One theory for this evolutionary difference is the relative importance of vision to both orders. As anthropoids tend to be diurnal, and rely heavily on visual input, prosimians are mainly nocturnal and have a decreased reliance on visual input.

References

Campbell, Bernard G.; & Loy, James D.; (2000). Humankind Emerging (8th ed.). Allyn & Bacon. p 85.

Vaughan, T.A. et al. Mammalogy 5th edition, Jones and Barlett Publishers, 2011

Feagle, John G. (1999). Primate Adaptation and Evolution (2nd ed.). Academic Press. San Diego. (pg 25)