Postorbital bar

The postorbital bar (or postorbital bone) is a bony arched structure that connects the frontal bone of the skull to the zygomatic arch, which runs laterally around the eye socket. It is a trait that only occurs in mammalian taxa, such as most strepsirrhine primates^[1] and the hyrax,^[2] while haplorhine primates have evolved fully enclosed sockets.^[1] 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.^[1]

Postorbital bars evolved several times independently during mammalian evolution^[2] and the evolutionary histories of several other clades. Some species, such as tarsiers, have a postorbital septum.^[3] This septum can be considered as joined processes with a small articulation between the frontal bone, the zygomatic bone and the alisphenoid bone and is therefore different from the postorbital bar, while it forms a composite structure together with the postorbital bar. Other species such as dermopterans have postorbital processes, which is a more primitive incomplete stage of the postorbital bar.^{[citation needed]}