Actually, you can catch Aerodactyl in the wild in Route 111 and Mirage Tower. It is a wild encounter, it is not in fossil form, it is in the wild.
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The only way to obtain Aerodactyl in Ruby, Sapphire, or Emerald is to trade it from another game.
You can't catch Aerodactyl you have to get its fossil at the Pewter City Museum (it's called Old Amber) then bring that fossil to Cinnabar Island where they'll give you the Aerodactyl.
Im pretty sure you dont catch aerodactyl in heartgold but I think you can get him as a fossil, if you do, talk to the guy in the pewter city mueseum and he will turn it into aerodactyl for you.
You need firered or leafgreen to trade you one.
You don't, you have to trade Aerodactyl from FRLG