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.
that Pokemon is in the area before the cave
The ruin maniac is under veilstone city in the cave on the left.
Ruin Maniac Cave
in the cave of origin in sootopolis
In Pokemon Ruby, Sapphire, and Emerald, the Fossil Maniac is in Fallarbor town. His house has a large hole on the inside. When you go through the hole, he is all the way in the back of his homeade cave. NOTE: You get dig in this house. Talk the the Fossil Maniac's son before going into the Fossil Maniac's cave.
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.
ditto can be found in sapphire, you need to go to the fossil maniac's cave to look around to get ditto. Or you could just get it in a trade. Ditto is pretty common
in the cave where the unknown are
talk to the kid next to the cave in the fossil maniacs house in fallabor
You can get ditto in Pokemon ruby by beating the Pokemon league then going to the cave the fossil maniac is digging he will be finished digging and he will say to be careful that there are Pokemon in there the Pokemon in there are ditto.
In Pokemon Emerald, the fossil maniac can be found in a house outside of Fallarbor Town. I'm not exactly sure when (Possibly after you get the fossil out of Mirrage Tower), he will not be in front of the hole in his house. Enter the hole, and you can find the other fossil, plus the cave has ditto in it, an excellent bonus. You can get there after you beat the elite 4
To the east of Fallarbor Town on Route 114, there is a house (Fossil Maniac's house). Inside, just before the cave, is fossil maniac's little brother, who will give you the TM dig. (After defeating the Elite Four, you may also find the fossil that you didn't pick up in Mirage Tower. It is in the cave behind the Fossil Maniac, which is called Desert Underpass. In this cave is also Ditto, Whismur, and Loudred. [That is in order of least rare to most rare.])
What cave?
You can't. After, however, go to Fallabaror town and go to the fossil maniac's house. Inside is a cave with ditto.
In Pokemon Ruby and Sapphire you will be given a chance to choose EITHER a Claw Fossil OR a Root Fossil in the Hoenn Desert along Rt 111. The fossils can be given to a scientist at the Devon Corporation and he will revive it into a living Pokemon. The Claw Fossil will be revived into an Anorith and the Root Fossil will be revived into a Lileep. There is only one chance to get these fossils, and once one is chosen the other fossil will sink into the sand forever. After that, the only way to get the other Pokemon is by trading or cheating. ^^^^the above information is correct, except for the last bit. You can get the other pokemon in the fossil maniac's house after beating the elite four west of the pokemon center in fallarbor town. He'll say it's not safe to go through the cave, but go anyway. There you can catch ditto and at the end is the fossil which hasn't been retrieved yet. then when you exit you can talk to the fossil maniac's brother and he will give you DIG
I can only find one fossil, defeat the Elite four then go to the fossil maniac and go in the cave, you will find Dittos in the far right you will find a fossil Your cheater, Shadow (the Hedgehog)