The highlight makes it seem like plastic, I'm pretty sure skulls aren't that reflective. The highlight needs to be dispersed to make it like bone. Perhaps give the highlight a little more yellow, it looks kind of clean. You have a lot of unnecessary AA.
Just a quick edit to the lighting giving more emphasis to the eyebrow ridge, cheek bones, bridge of the nose and the sides of the forehead which flatten out with the temporal bone on the side. I didn't mess with the jaw and teeth much but right now they're almost invisible.

Oh, and the cheek bones are too round, the actual bones, if you look at references, are angular and square.