Fossils, phrasally referred to as Pokémon Fossils, are physiological vestiges of prehistoric Pokémon that can be biogenerated into fossilized Pokémon.
Fossilized Pokémon tend to be scarcer than more organic Pokémon, as they are only originatively obtainable by obtaining a Fossil—a rarity in its own right—and reanimated into the living version of the Pokémon.
They are most commonly obtained by unearthing them; the two systems available for doing so are diggable patches and Excavation Sites. Additionally, all fossils but the Jaw Fossil and Sail Fossil are subsumed under Brock's prizeset.
Fossils can be reanimated by turning them in to bioscentists; one cadre of them is stationed in each region, as explained below.
All fossilized Pokémon that are reanimated in the labs will be initialized at level 25, and there is a 1/1,000 chance of shininess. Those that are reanimated by bioscientists in Excavation Sites will be initialized at level 10; and as is the case with all NPC-initiated wild-Pokémon battles, they have no shiny chance.
The Dome Fossil, Helix Fossil, Old Amber, Sail Fossil, and Jaw Fossil can be reanimated in Cinnabar Island's lab in room 3 of the lab; this also requires that you have defeated Blaine in Cinnabar Gym, first, since the room will be inaccessible before then.
Contingent that the evolved-data entries of Omastar and Kabutops and the caught-data entry of Aerodactyl has been registered in your Pokédex, the bioscientist in the Ruins of Alph research center will reanimate any Claw Fossil and Root Fossil turned in to him.
Upon completing the Devon Corp quest, one of the employees in Devon Corporation 2F (Rustboro City) will offer to the Dome Fossil, Helix Fossil, Old Amber, Root Fossil, and Claw Fossil.
With an accrual of at least 6,000 excavatory points, all-seven of the available fossils can be reanimated by Gingery Jones in the Historial Site.
While this also stipulates a fee of 10,000 to do so, you will also have a wild encounter of the fossilized Pokémon; this allows you to synchronize your Pokémon's Nature with the fossilized Pokémon upon catching it if your lead Pokémon possesses that ability.
The Armor Fossil and Skull Fossil—Sinnoh's generationally respective subset of fossils—are reanimable at Oreburgh Mining Museum, stipulating that Cradily and Armaldo's evolved-data entries are registered on your Pokédex—the fully evolved biogenerations of the Root Fossil and Claw Fossil, respectively.
|Helix Fossil||Reanimates into →|
|Dome Fossil||Reanimates into →|
|Old Amber||Reanimates into →|
|Root Fossil||Reanimates into →|
|Claw Fossil||Reanimates into →|
|Jaw Fossil||Reanimates into →|
|Sail Fossil||Reanimates into →|
|Armor Fossil||Reanimates into →|
|Skull Fossil||Reanimates into →|
They can also be donated to Pewter Museum; after doing so, you will see the Fossil visibly on the pedestal north of the Curator.
After donating three discrete Fossils, you will have access to the Ancient Power-tutor in Ruins of Alph Research Center for 3,500 per tutee.