Take the fossil into the Oreburgh museum
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You cannot get a Skull Fossil (Cranidos) in Pokemon HeartGold.
You revive it from a Skull Fossil, like in Diamond and Platinum.
Must trade Cranidos appears in Diamond/Platinum (odd ID number) - Revive Skull Fossil
First you have to excavate a Skull Fossil in the Underground. Transfer the fossil to your bag, and take it to the mining museum in Oreburgh City. Talk to the man behind the counter, and he will turn the fossil into Cranidos.
That's easy, it's the Skull Fossil, which then seems to evolve into Cranidos. Which is 10x better than Shieldon. (STATS)