When synapsids expanded their temporal fenestrae and made them very large, and lost the postorbital bar to join the orbit with this fenestra, how did they protect all that uncovered skull area? Is there any modern species with so large a skull opening? How did they protect their brain? For modern mammals with an orbit still confluent with the temporal fossa, how is the eye supported from behind? Don’t working jaw muscles damage it? Don’t movements of the head move it? Aren’t injuries more likely?

Hi Stafanos, there are a lot of questions there, so it's hard to get started! The openings may have been covered by a membrane, similar to that seen in the fontanelles of a young baby's skull. If the benefits of losing the bone in the area outweigh the costs of having less protection, then the bone will be lost - as we see.

As for the eye, it is supported by connective tissues and evidently there is no problem with damage from the jaw muscles behind, since we'd see a lot of examples if it was a problem!

Not sure if that fully answers the question, but the main take-home message is that non-bony connective tissues perform a variety of important functions that shouldn't be forgotten!