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.
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You cannot get a Skull Fossil (Cranidos) in Pokemon HeartGold.
Cranidos is a Rock type Pokemon that can be revived from a Skull Fossil in Pokemon Diamond. It will evolve into Rampardos at level 30.30
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.
Archebash
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.