"Fossil" (化石Kaseki) is an archetype of cards used by Jim Crocodile Cook in the Yu-Gi-Oh! GX anime which focus on a unique form of Fusion Summoning with "Fossil Fusion". In the anime, they are created by removing a Rock-Typemonster 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 created by removing a Rock-Type monster in your Graveyard and a monster of certain Level in your opponents. "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 corresponding 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.