Go to the Desert. In the Desert, there is a tower that appears every once in a while. Get the Mach Bike and go to the top of the tower. There will be two fossils. The Root Fossil is for Lileep, the Claw Fossil is for Anorith. Choose one and the tower will collapse, making the other fossil disappear. To get the fossil that you didn't pick, you have to trade with a friend.