The Skull Fossil and Armor Fossil are game-specific, meaning one can be found in one game while the other can't. The Skull Fossil can only be found in Diamond and the Armor Fossil in Pearl. There are an unlimited amount of Skull Fossils Underground for Diamond players and can be traded without having to trade your only one. After unlocking the National Pokédex, players can also find the Root Fossil, Claw Fossil, Dome Fossil, Helix Fossil and the Old Amber by digging in The Underground.
Heart Gold/Soul Silver
At Twist Mountain in Black, White, Black 2 and White 2, there is a worker near the Ice Rock room that hands out a fossil from past games after the National Pokédex has been obtained. The fossil that is given out is random, and is given out once per day. To revive the Skull Fossil, bring it to the Nacrene Museum.
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