Re: A thought on Marowak.
Well, none of Marowak's Pokedex entries say anything about still having the skull on it's head, or about losing the skull. They all mention it collecting bones, but that's it.
Looking at the sprites, the skull on Cubone's head is cracked, but Marowak's is not. There are slight differences in it as well, the eye hole being smaller, the nose hole not being as present. Granted there is a slight height difference with the evolution (from 1′4″ to 3'3").
I just always assumed Marowak's head looked like a skull, as GyaraKKO has already said. So, when it evolves, the mother's skull is broken and Marowak moves on, with it's new head, shaped like, and having the appearance of, a skull.