Shell + Fragments in mystery rock

[kipper327]

A friend found this stone west of the Missouri in SD. It's now in my possession. There's one very visible spiral shell and what I believe are fragments of more scattered throughout, plus some bonus crystalline spots and something funky going on next to them (pic 4). I have no idea when it dates to, but another rock found nearby is probably Ordovician. However, according to the friend, they were all deposited there by the river more recently and were near the surface, so context is pretty lacking. As you can see, the rock itself is pretty weird too, especially on the back. Answers to any of my questions (what the fossils are, how old it is, what is going on with the rock) would be greatly appreciated. Thanks!

[Fossildude19]

Looks like a gastropod.