When I first looked at the Dino boss the hole reminded me of images of dino skulls like the one that Ben posted, so I didnt really take issue with it...but now that everyone mentioned it I think we should see the upper jaw of the opposite side from where we are, and I think ben kinda missed that. Maybe you have too static a refference for the skull here, perhaps you should scout youtube for a few clips from that Ben Stiller movie where a Dino skeleton comes to life..."Night at the Museum"
I also didnt take note of the anacronic little bony hands on his chest. you might want to change the way you shaded them so as to bring them forward....perhaps simply by darkening the metal that is surrounding them