The fossils only spawn within ~150 m (not a confirmed exact amount) around the spawn of the mining world.
To a point. I know the spawn frequently in the ice-wasteland near the mining world spawn, but other then that, no one else has yet to confirm otherwise.
its really hard to predict but from what ive seen the max ive found fossils away from spawn in creative and single player is 350. they become less common after 250.
