Cloyster -> Gastly Theory?

(Note: Sorry if it's been done before, new to this subreddit)

Anyone else think Cloyster dies and becomes Gastly?

I know there are probably a ton of theories regarding the Ghost Pokemon but this is one I've believed since I first bought the original 150 Pokemon guide so I figured I'd share it here . . .

EDIT: Found these diagrams which help support my theory or at least some elements of it