In Pokémon Ruby and Sapphire, the Root Fossil can only be found once and you have a choice between it or the Claw 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 Root 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 Root Fossil is one of the randomly obtainable fossils.
Related ThreadsWhere do you take the claw fossil and root fossil to? - last post by @ Jan 22, 2009
Root Fossil? - last post by @ Nov 2, 2013
root fossil pokémon - last post by @ Nov 9, 2003
how do you get the 2nd root fossil - last post by @ Nov 6, 2003
Fossils? - last post by @ Oct 19, 2013