A whole generation of prehistoric Pokémon. However they are not fossils, and are therefore not part rock. The game takes place 100,000,000 years in the past after the player goes to study prehistoric Pokémon.

Existing fossil-mons are all there (also no longer part rock), alongside other Pokémon based on prehistoric creatures like the Venipede line (if Scolipede isn't an arthropleura then I don't know what it is) and the Yanma line, who no longer needs Ancientpower to evolve to Yanmega. There are also other large insects alongside other small mammalian Pokémon, such as a Cimolestes-esque local rodent.

At the end of the game, the player's starter Pokémon gets left behind when you return to the present. You must then prowl around the endgame dungeon to find its fossil, which will resurrect into your old Pokémon, who still remembers you (though is now part rock)

I tried to find the idea guy to present this thread, but I couldn't find him, so here's a Scolipede instead.