Do you ever stop and think there might be a T. Rex under your house. Well fossils preserved remains of organisms. Fossils can be over 4,000 or a million years old. Usual the preserves are of animals. Fossils arranged from the size of a mouse to a ten story building.
This is how all those fossils are made. A dead organisms makes its body's imprinted in mud then buried. When covered a big amount of pressure is needed to help the body imprint further in the ground.The body dissolves into the ground. Then rocks are replaced in that structure.