I'm not sure why you posted this picture, but this brings up a huge grand canyon like hole in your story.
Okay we have a variety of skull sizes and shapes. Woopdeedoo!
The more important question is how the hell did a non-skulled creature accidentally and gradually mutate it's way to a skull with all the right attributes...a jaw with teeth, holes for ears, 2 eye sockets, a nose...not to mention how this just happened to have a simultaneous evolution of the brain, the backbone, the nervous system, and all the attributes a creature needs to have a use for a skull in the first place.
How do you go from a non-vertebrate to a vertebrate by a gradual accidental process? I want the whole bedtime story. I don't want you to start in the middle with a fish who conveniently is a vertebrate to begin with.