The orbits may not be well defined. For example, birds have a sclerotic ring that supports the eye that would fall out once there's only bones left. Since they don't have an orbit like we do they can't move their eyes as much, and therefore birds swivel their head to look around instead.
For example, most skulls on this page (
http://www.hiltonpond.org/ThisWeek070108.html) have pretty poorly defined orbits because the sclerotic ring fell out. Half way down the page, however, the bald eagle skull still has this ring.
If it WAS a bird the lower jaw and upper beak (which has a weak hinge) as well as the sclerotic ring would be gone.
I also thought it could be a fish too, but I really am clueless on the topic.