So where was I? Ah, right. Mammals lost the postorbital bar early in their evolution but many taxa have secondarily evolved them again. What’s up with that?
Any explanations regarding the reappearance of the PO bar will necessarily make an explicit functional hypotheses. Heesy (2005) cites three common ones and dismisses all but the last: that the bar is there to protect against trauma, that it reinforces the PO region against torsional loading due to mastication, and that it stiffens the PO region in mammals whose eyes have a divergent relationship with the temporal fossa (the space in mammals that houses jaw muscles). This is particularly important for mammals that need to be able to chew and see at the same time. Like, uh, primates. Or horses. Heesy’s 2005 study successfully correlated the presence of a PO bar with the degree of angular deviation between the plane of the orbit and the plane of the fossa. It is thought that when the angle is smaller, the more likely it is that the jaw muscles will deform the orbit if there is no bony support structure.
I buy it (but really, what do I know?). Yet I still wonder about some cool fossil animals with PO bars.
This is Janjucetus hunderi, a fossil baleen whale from the Oligocene with huge teeth (thanks to my advisor John Langdon for pointing it out). As you can see, he’s got a pretty big, obvious PO process, though Fitzgerald 2006 seems careful not to say it’s a PO bar. I presume that this is because the process doesn’t quite connect with the zygomatic process of the squamosal. In any case, it seems clear that things were headed in that direction. I’d really like to determine if the angle between the PO process and the temporal fossa is at all like the other mammals a la Heesy. The orbits do seem to be more facing more forwardly than in other whales, but that’s just me looking at the pictures in the publication. Though this guy probably not chewing his food with any great force, so that functional hypothesis might not work here.
Fitzgerald, E.M.G. (2006) A bizarre new toothed mysticete (Cetacea) from Australia and the early evolution of baleen whales. Proc. R. Soc. B. 273: 2955-2963.
Heesy, C.P. (2005) Function of the Mammalian Postorbital Bar. Jour. Morph. 264: 363-380
Sidor, C.A. (2001) Simplification as a trend in synapsid cranial evolution. Evolution. 55(7): 1419-1442