Simple, effective, and nicely done. Love the Cambrian lore. Definitely good seeing some of those weird things that are so awesome and get so little camera.
Idea: Roll for rarity, then make the wait be tied to that. So the longer it takes to bite, you build some tension wondering what you will get. Difficulty should also be tied to rarity to increase that tension buildup. you know you're getting a good one but you'll have to fight for it.