u need to go underground by going into a house and talk to the old man in Eterna city right near the Pokemon center
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You can get all the fossils underground, except for shieldon or cranidos. You find shieldon if your ID is even and Cranidos is your ID is odd.
you'll get a cranidos in the under ground in Pokemon platinum. just keep on levelling it up till it learns headbutt.
You can't catch Cranidos in the wild or from a special event. You'll need to trade for it from Pokemon Diamond, Pearl, or Platinum.
Must trade Cranidos appears in Diamond/Platinum (odd ID number) - Revive Skull Fossil
It takes 7,680 Steps to hatch a Cranidos. And it hatches faster when you have a Pokemon with the ability Flame Body