The Fossils in Pokémon Diamond are the Helix Fossil which is the Omanyte Fossil, the Dome Fossil which is the Kabuto Fossil, the Old Amber which is the Aerodactyl Fossil, the Root Fossil which is the Lileep Fossil, the Claw Fossil which is the Anorith Fossil, the Skull Fossil which is the Cranidos Fossil and the Armor Fossil which is the Shieldon Fossil.
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Beat the Pokemon League, and you will be able to find all but one of the fossils of the ancient Pokemon. (For Diamond, all but Shieldon's fossil are available)
They can be found in the Sinnoh Underground. Root Fossils are more common in Pearl, and Claw Fossil are more common in Diamond.
Pokemon diamond
You can not catch any of them in black or white, but I think that you can catch all 3 in diamond.
The weakest Pokemon in diamond is a magikarp. It is also the weakest of all the Pokemon existed. Magikarp is weak but it will evolve to a mighty gyarados.