The postorbital bar is where the zygomatic bone and the frontal bones join to form a lateral strut which runs around the eye socket of most strepsirrhine primates and some other mammals, such as the hyrax. This is in contrast to the haplorhine primates, which have evolved fully enclosed sockets.
One theory for this evolutionary difference is the relative importance of vision to both orders. As haplorrhines (tarsiers and simians) tend to be diurnal, and rely heavily on visual input, many strepsirrhines are nocturnal and have a decreased reliance on visual input.
- Campbell, Bernard G., Loy, James D. (2000). Humankind Emerging (8th edition). Allyn & Bacon. p. 85.
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