You can breed fossil Pokemon.
Location of fossils in Gen 3:
In Ruby and Sapphire, the player can choose one of either the Root or Claw Fossils at Route 111, and upon choosing, the other will sink away into the sand. In Emerald, however, while the player can only choose one initially at the Mirage Tower, they can retrieve the other later at the end of the Desert Underpass. In the Generation I remakes, Pokémon FireRed and LeafGreen, the player can obtain the same Fossils as in the original games: either the Helix or the Dome Fossil, and the Old Amber.
Location of fossil in Gen 4
In Diamond and Pearl, for the first time in the series, the Fossils the player is able to receive are dependent on the version played: Skull Fossils are available in Diamond only and Armor Fossils in Pearl only. In Platinum, whichever Fossil a player digs up will depend on whether the Trainer ID is even or odd. If the Trainer ID is even, the player will dig up Armor Fossils only, and if the Trainer ID is odd, the player will dig up Skull Fossils only.
In Johto's Generation IV games, the DomeSS and HelixHG Fossils as well as the Old Amber can be found randomly by smashing rocks outside Ruins of Alph. RootSS and ClawHG Fossils can be found randomly by smashing rocks in the Cliff Cave after entering the Hall of Fame. Fossils found in Pokémon HeartGold and SoulSilver can be revived at the Pewter Museum of Science. The Skull and Armor Fossils are absent from both games, so either they, or the Pokémon revived from them, must be transferred from Diamond, Pearl, or Platinum.
Root Fossil: Lileep
Claw Fossil: Anorith
Helix Fossil: Omanyte
Dome Fossil: Kabuto
Skull Fossil: Craniados
Armor Fossil: Bastioodon (probably I am wrong)
Old Amber: Aerodactyl!
Trading: 1. You cannot trade a Pokemon to a previous generation.
2. You can trade a Pokemon to newer generation.
3. You can trade a Pokemon from the same generation's game to the same generation's game.
Example: You cannot trade a Anorith from Platinum over to Leafgreen.
You Can trade a Pokemon from SoulSilver over to Platinum.
You can trade a Pokemon from Diamond to Heartgold.
HG stands for HeartGold
SS stands for SoulSilver.