From Bulbapedia, the community-driven Pokémon encyclopedia.
The Claw Fossil (Japanese: ツメのカセキ Claw Fossil) and Root Fossil (ねっこのカセキ Root Fossil) are fossils from the Generation III games. They are found in Hoenn's Desert on Route 111. The fossils can be given to a scientist at Devon Corporation, who will revive the Claw Fossil into an Anorith or a Root Fossil into a Lileep. Both fossils are key items.
In Pokémon Ruby and Sapphire, there is only a chance to acquire one of the fossils, as when the chosen one is picked, the other fossil will sink into the sand forever. In Pokémon Emerald, one can be selected in Mirage Tower, and after defeating the Elite Four, the other one can be found deep in the Desert Underpass. It is likely that the fossil in the Desert Underpass is not the same fossil in Mirage Tower, indicating that more fossils exist than are actually shown in the game.
As is the case with the Dome and Helix Fossils, Claw Fossils and Root Fossils are available in Sinnoh's Underground. Claw Fossils are more common in Pokémon Diamond while Root Fossils are more common in Pokémon Pearl. These fossils can be revived at the Mining Museum. Unlike the fossils in Generation III, these fossils are not Key items.
In HeartGold and SoulSilver, the Root and Claw fossils can be found randomly by smashing rocks in the Cliff Cave located on Route 47, and can be revived at the Pewter Museum of Science. The Root Fossil is exclusive to the SoulSilver version while the Claw Fossil is exclusive to the HeartGold version.