The term fossil means something dug up. Fossils are the remains of ancient organisms preserved close to their original shape. It is unusual to find a completely preserved organism. Usually, the hard parts survive, such as bones, teeth, and shells of animals, and woody parts or seeds of plants. These are also often replaced by mineralization, such as when the shell of a nautilus is replaced by the mineral calcite. Other fossils include the molds, casts, or imprints of an animal or plant.