From mauville go north towards the desert, when you reach the desert go in (you need go-go goggles) and follow the path (the smooth sand) you should reach a tall building that fades, go in and follow the stairs upwards until you find cracked floor (you need a mach bike to get across). Once you are across you will find one more floor get through there and you will be in the top floor, there should be two fossils, like the cave under the house you can only choose one so choose the fossil you dont have.