The fossil Pokémon in Pokémon Brilliant Diamond and Shining Pearl are ones that you can find while exploring your game. You’ll only be able to find this item in a specific location, but when you do, you can restore it to run around with a fossil Pokémon. Here’s what you need to do to find the Armor Fossil in Pokémon Brilliant Diamond and Shining Pearl.
The only way you can find the Armor Fossil is by digging at fossil sites in the Grand Underground. This location opens to you when you receive the Explorer’s Kit in Eterna City from the old man who lives next to the Pokémon Center. You’ll be able to find the digging sites by the orange dots on your map in the upper left corner. You’ll then need to dig them out by completing the minigame. You’ll know you’re about to unearth the Armr Fossil by the outline you see underneath the dirt.
Unfortunately, the Armor Fossil is only available for Pokémon Shining Pearl players. It is not available for Pokémon Brilliant Diamond owners. If you want to receive the Pokémon it turns into, Shieldon, you’ll need to trade a Pokémon Shining Pearl player for it. You’ll probably want to bring your fossil Pokémon, Cranidos, to exchange with another player.