part of part of skull fenestra antorbital fenestra antorbital fenestrae UBERON:0014465 An antorbital fenestra is an opening in the skull, in front of the eye sockets. This skull formation first appeared in archosaurs during the Triassic Period. Living birds today possess antorbital fenestrae, but the feature has been lost in modern crocodilians. In some archosaur species, the opening has closed but its location is still marked by a depression, or fossa, on the surface of the skull. This is called an antorbital fossa.