Well they're both based off of the same kind of creature (Dinosaur really, and Archen as the next step between Dinosaur and Birds, so they're really not that much different to Cranidos in terms of what they're based on.) I still think Rock seems the most common sense type for them all, since Dinosaurs don't really resemble any of the other types.
they need a dinosaur type i think the nearest you could get to dinosaur is dragon type. But that'd be a bit too much. XD I'd have to go with Ground/Dragon for Cranidos/Rampy if he weren't rock type.
@edit:; lol at the punching moves. I'd pay to see that.
http://3.bp.blogspot.com/_6heDX0kqY7...tle+arms+2.jpg