The fossils just come out randomly.I got a cranidos.they do not come out the first time you look for them but don't give up they are there somewhere.
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go underground with a explorer kit and dig until you find one
Find a fossil underground. I did.
You can get all the fossils underground, except for shieldon or cranidos. You find shieldon if your ID is even and Cranidos is your ID is odd.
you can probably get it Underground.
you can find a thunder stone by checking the UNDERGROUND