Alright actually you can get Cranidos in pearl. All you need is the national pokedex. Go back to the underground until you find the fossil but you do need the national pokedex to find it.
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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.
diamond- skull fossil- cranidos-rampardos pearl-armor fossil- sheldon-bastiodon
To get Cranidos, dig up Skull Fossil while you are in Sinnoh Underground and take to the Oreburgh Mining Meseum to have it revived into a Cranidos. To get Shieldon, well, sorry, he's exclusive to Pearl. But of course, I can trade you my Cranidos to get Shieldon.
cranidos talk to a scientist in ore burg and he will give you one
There's one simple answer: trade! Cranidos is to Diamond as Shieldon is to Pearl, so find a friend who has Pearl and trade with them.