Play Root Fossil as if it were a Basic Pokémon. While in play, Root Fossil counts as a Colorless Pokémon (instead of a Trainer card).  Root Fossil has no attacks of its own can’t retreat and can’t be affected by any Special Conditions.  If Root Fossil is Knocked Out it doesn’t count as a Knocked Out Pokémon.  (Discard it anyway.)  At any time during your turn before your attack you may discard Root Fossil from play.

Spongy Stone:

At any time between turns, remove 1 damage counter from Root Fossil.