I know that you get it in Twist Mountain, but what is the chance of getting it?
I've had almost all fossils but Cover Fossil, so I'm kind of mad at fossils now.
Actually, you get that in Relic Castle (First Floor). A Backpacker ask's you to choose one, and you can choose the cover fossil. (The other option is the Plume Fossil.) Also, once you choose the fossil, you can't get the other one in your game.
<- A backpacker.
Hope This Helped :D