"Fossil" (化石Kaseki) is an anime-only archetype consisting of EARTHRockFusion Monsters used by Jim Crocodile Cook in the Yu-Gi-Oh! GX anime. They focus on a unique form of Fusion Summoning via "Fossil Fusion". In the anime, they are summoned by banishing a Rock monster in your Graveyard and another Type of monster from your opponent's Graveyard. Though never released in the TCG and OCG, they are playable in the Tag Force games where they are summoned by instead removing a Rock monster in your Graveyard and a monster of a certain Level in your opponent's. "Fossil Fusion" supports the archetype, which results in several other cards that have been released being part of this archetype.
In the Japanese version, the "Fossil" Fusion Monsters have power that each correspond to their age. The older the monster, the more powerful it is. Name-wise, they have prehistoric eras in their names, which were removed in the dub and English video games.