Finding Fossils in Pokémon Brilliant Diamond and Shining Pearl will, like in previous games, allow you to reanimate certain ancient Pokémon.
Two new Fossils were added for Gen IV -- the Skull Fossil is exclusive to Diamond, and the Armor Fossil exclusive to Pearl. These join the Dome, Helix, Root and Claw Fossils, and the Old Amber.
How to get Fossils in Brilliant Diamond & Shining Pearl
In order to access Fossils, you need to have unlocked the Grand Underground.
To do this, speak to the Underground Man in Eterna City. He'll guide you through the process and explain digging for treasures.
Then, just keep traversing the Grand Underground, digging for treasures, and you'll come across Fossils.
They're usually pretty large, so make sure you uncover them fully before the walls collapse!
How to turn Fossils into Pokémon
Once you have a Fossil, head to Oreburgh City and visit the Museum.
Talk to the Scientist at the counter - the man in the white coat - and he'll take your Fossil.
Leave the building and reenter, talk to him again, and he'll hand over the revived Pokémon.
Here is the list of Pokémon you can obtain from Fossils.