As a classic reference to the original "Pokemon Red" and "Pokemon Blue" games, players are forced to make a choice between the Dome Fossil and the Helix Fossil in "Pokemon LeafGreen" for the Game Boy Advance remake. These fossils are important, since each contain DNA used to bring back long-extinct Pokemon. Kabuto and Omanyte, as well as their evolved forms Kabutops and Omastar, can only be obtained through the revival process that starts with their fossilised forms. With some planning, anyone can get both sets of fossilised Rock and Water Pokemon in "Pokemon LeafGreen."
Go to the second basement level of Mt. Moon and defeat the Super Nerd's Grimer, Voltorb and Koffing.
Choose either the Dome Fossil or the Helix Fossil. The Super Nerd will take the other fossil.
Travel to Cinnabar Island and give the fossil you picked to a scientist inside the laboratory. A Dome Fossil will be revived into a Kabuto, and a Helix Fossil will be transformed into an Omanyte.
Trade for the other regenerated fossil Pokemon from either "Pokemon FireRed" or "Pokemon LeafGreen" to your copy of "Pokemon LeafGreen."
Sweeten the trade for the person you trade with by breeding your Kabuto or Omanyte with a Ditto to make a copy of the creature. Leave a Ditto and the fossil Pokemon with the Day Care Man on Route 5 for a while and return to pick up an egg containing a young copy of the fossil Pokemon. Whoever is trading with you will likely be thankful to own the creature they had to abandon in Mt. Moon.
If you have obtained the "Old Amber" item from Pewter City's Museum, you can also bring it to Cinnabar City's lab to revive an ancient Pokemon. This is the only way to get an Aerodactyl in "Pokemon LeafGreen."