Lovely Old Vertebrae Bone with Secret Skull

I found this in a wonderful shop filled with all sorts of interesting antiques and curiosities, and though I'm not especially oriented towards bones and things, it  struck me as a very beautifully sculptural object, which becomes a different thing from every view. It was not until I was photographing it that I noticed a secret surprise: visible in the first image, in the right recess, is what looks like a tiny skull! (Maybe I'm crazy, but to me it's clear as day!)

4 1/2" w x 4 1/2" d x 2 9/16" t. I'm really not sure what kind of animal this is from. Based on the context in which I found it, surrounding by old medical equipment and preserved specimens, I believe it likely has a hundred years or so on it.