Instructions
Going into the desert east of Lavaridge Town and north of Mauville, the player must use the Mach Bike to enter the top of the Mirage Tower. It is not always there, so the desert may have to be searched more than once. Inside the tower, use the Mach Bike to cross any larger chasms.
Choose either the Root Fossil, which results in Lileep (who eventually evolves into Cradily), or the Claw Fossil, which gives the player Anorith (and evolves into Armaldo). You will find that the tower collapses before the second fossil can be recovered. After defeating the Elite 4, you can recover the other fossil.
Enter the cave near the Fossil Maniac's house to find the Desert Underpass if you have beaten the Elite 4. By following the short cave on the right, you will find the fossil not recovered from the Mirage Tower.
Return to the Devon Corp and find the scientist who claims he has a successful procedure. He will be able to revive the fossilized Pokemon. Both revived Pokemon start at level 5 and evolve at level 40.