Just as in Pearl, you've to dig up an Armor Fossil from underground, but because this is the third game, it doesn't depend on which version you get (In Diamond, you rather dig up a Skull Fossil, of a Cranidos).
It's depends on your ID number now. When you get an odd number (ends with 1, 3, 5, 7 or 9), you can ONLY dig up Skull Fossils. When you have an even number (ends with 0, 2, 4, 6 or 8), you'll dig up Armor Fossils instead.
If you get a fossil, you know the story. Go to the Oreburgh Museum, talk to the righter guy in the lefter-upper corner at the bar, that accepts the fossil, then go in and out, and accept the Pokémon. At least, when you have enough space in your Party.
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Shieldon evolves at Lv30
when i play Pokemon platinum theres no one that want to trade Shieldon for strong Pokemon i hope you understand it
i think you can if you can;t just use a cheat
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.
I can sure give you a cranidos for a shieldon if you like.