The Skull Fossil is the fossil of Cranidos. And there is no definitive place for the Skull Fossil. Just so long as it's Diamond we're talking about, dig in to the wall for treasure when there's an...
You just find out what ancient Pokemon they belong to and after you beat the Elite 4 you can go to Orebergh city (sorry if there is spelling error) and enter the fossils museum to resurrect them....
There are many places to find fossils but in MN there is a great place called lillydale. You can only find fossils in places where the temprament is just perfect for letting fossils come back up to...