The postorbital bar is a bone which, with the exception of Tarsiers runs around the eyesocket of most Prosimians. 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

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