I honestly think all fossils are part rock due to being revived from a, well, rock fossil. I can definitely see Fossil Pokémon that don't really seem like Rock-types to me, like Anorith (feels more like an Aquatic Bug?) I do think that there were some fossil Pokémon that were biologically a different type before being revived.
The Kanto Fossils, though, honestly, they're definitely biologically Rock-types.
But... if you want my opinion, these are the Pokémon I think were NOT biologically Rock type when they were first alive:
Anorith, as I stated above. Most entries state it lived in warm seas, maybe Water/Bug back in the time they were alive.
Lileep, it lives in a lot of sea areas, apparently, and a lot of Pokedex entries seem to mention the sea. Water/Grass? Probably. It only learns one Rock type move by level up. It also only learns one water type move by level up, but that can easily be fixed.
Archen has always felt more Normal/Flying to me than a Rock/Flying Pokémon.
But other than that, I do honestly think they're all Rock types simply because they have been revived from a rock fossil. That's really it.