I don't think you can, in emerald you need the nationaldex and POW! it's open but in ruby and sapphire you can't.
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You have to get the the National Dex (beat the Elite Four) before you can go into the Fossil Maniac's cave.
The claw fossle is better... BUT after you beat the elite four, go into the house in route 114 the fossil maniac's house and go ito the cave he busted open. at the end of the cave there is the opposite fossil of wich you got in the desert. there is also lots of dittos in there!!!
There is no jirachi's cave.
you need azelf,uxie,mesprit to open the cave after beating the Pokemon league
To get fossil pokemon go to the Nacreen Museum and talk to the people right inside the door. One of them takes you fossils and turns them back into pokemon.