Fossil Pokémon are Pokémon which can be obtained from specific fossils.
All of these Pokémon have quite high values of Defense and Special Defense. They all are of dual -Type, except for Cranidos. They all evolve at Level 40 in exception of Cranidos and Shieldon which evolve at Level 30, then Archen and Tirtouga which evolve at Level 37 and Aerodactyl does not evolve at all. Any Fossil can be also found in the Underground in 4th Generation games.
Generation I Fossil Pokémon
Aerodactyl is a fossil and -Type Pokémon revived from Old Amber. Aerodactyl is the only Fossil Pokémon that doesn't evolve. Its name and design the prehistoric animal, Pterodactyls. Aerodactyl is the only Flying/Rock Type Pokémon until Generation V. In the game and manga it is revived in Cinnabar Island
Generation III Fossil Pokémon
These Fossils can be found in the desert area east of Lavaridge Town in Pokémon Ruby and Sapphire. They are in the Crumbling Tower which requires a Mach Bike to get to the top. They can be brought back to life by a Devon worker in Rustboro City. You can get the fossil you did not choose after beating the Elite 4 and going to the fossil maniac's house.
Generation IV Fossil Pokémon
Cranidos is a head butt -Type Pokémon revived from a Skull Fossil. It evolves into Rampardos. Its fossil can only be obtained in Pokémon Diamond or in Pokémon Platinum if the last digit of your trainer ID is an odd number. It is the only Fossil Pokémon who only has one type. It is based on the dinosaur Pachycephalosaurus.
Shieldon is a shield and -Type Pokémon revived from an Armor Fossil. It evolves into Bastiodon. Its fossil can only be obtained in Pokémon Pearl, Pokémon Platinum, and Pokemon Black & White if the last digit of your trainer ID is an even number. It is based on a ceratopsian dinosaur, like Triceratops.
Generation V Fossil Pokémon
These fossil are found inside the Relic Castle the player can bring these Pokémon back to life at the Nacrene Museum.