holy cow , that is stunningly fantastic. I wish you had these available for sale, there are so many things you could make with this. I was looking for a great big skull like that for my mud monster and finally found one at anatomical company, but i think yours looks a lot better. If you ever decide to make any to sell, please drop me a pm.
I started out with a chunk of hard foam, which I then cut/ground to general proportions- I like using foam because it lets me work pretty fast on getting to general size/shape. I then applied a thin coat of bondo to seal it, and then started sculpting the detail work in Aves epoxy putty. The small skull is a casting of the Revell visible skull, which I made to avoid the seam lines on the original, and so I could have a readily available life-size skull for reference/sculpting. I'm almost done with the pattern finishing, after which I'm going to mold it and rotational cast it, !: in order to sell copies, 2: to have a large armature to make other giant skulls (cyclops, etc.). I have to look at the forum rules re: selling, because I think I'm too new a member to sell anything yet.