What you have to do is when you are underground(on Pokemon diamond/ pearl) you put the fossil in your bag. Next go to the city where you get your first bag , go to the mine go inside talk to the man. He will ask you to go outside just go in and out. Talk to him again then he will give you shieldon or cranidos.
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It is the fossil of the pokemon omantye. You need to go to cinnabar island to Bring it to life then you get omantye starting at Lv.5
you revive them in the town of oreburgh in the scientist's building
Find a fossil in the ruins and take it to the guy in the far right side of pewter mueseum
Bring it to Nacrene museum
In the dessert there are 2 fossils at the very back on the right. One is a plant fossil and that will give you lileep the other will give you anorith. Take the fossil you chose back to rustboro city and talk to the guy on the second floor and he will bring it back to life.