In Pokémon Ruby and Sapphire, the Claw Fossil can only be found once and you have a choice between it or the Root Fossil. The one that you didn't pick wil sink into the ground and you won't have a chance to obtain it.
In all 3 games, the Claw Fossil can be revived at Devon Corp, located in Rustboro City.
Fossils can be obtained at Twist Mountain after the National Pokédex has been obtained. A fossil can be picked up from the same individual everyday; the Claw Fossil is one of the randomly obtainable fossils.
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