Fossils are the remains or traces of ancient organisms that have been preserved, by natural processes, in the earth's crust during some past geologic time. Most fossils consist of shells, teeth or bones, but under exceptional conditions, even soft-part anatomy may be preserved. For example, several woolly mammoths have been discovered in Alaska and Siberia with hair, flesh and internal organs intact. The remains of these organisms are referred to as body fossils to distinguish them from trace fossils such as tracks, trails and burrows, which are indications ofancient organic activity.

For any potential fossil to be preserved, it must escape the ravages of destructive processes as running water, waves, scavengers, exposure to the atmosphere and bacterial decay. Obviously, the soft parts of organisms may decompose or be devoured rapidly, but even the hard skeletal parts will be destroyed if they are not quickly buried and protected by mud, sand or volcanic ash. Even if buried, bones and shells may be dissolved in groundwater or destroyed by alteration of the host rock during metamorphism. Nevertheless, fossils are quite common. The remains of microscopic plants and animals are the most commo n followed by shells of marine animals, but even the bones and teeth of dinosaurs are much more common than most people realize.

Some fossils retain their original composition and structure and are preserved as unaltered remains, but many have been altered in some way. Dissolved minerals can be precipitated in the pores of bones, teeth and shells or can fill the spaces within cells of wood. Wood may be preserved by sihca replacing woody tissues; it is then referred to as petrified, a term that means "to become stone". Silicon dioxide (SiO2) or iron sulfide (FeS2) can commonly replace the calcium carbonate (CaCO3) shells of marine animals. Insects and the leaves, stems and roots of plants are commonly preserved as thin carbon films that show the details of the original organism. Shells in sediment may be dissolved leaving a cavity called a mold that is shaped Eke the shell. If the mold is filled in, it becomes a cast.

If it were not for fossils, we would have no knowledge of trilobites, dinosaurs and other extinct organisms. Thus fossils constitute our only record of ancient life.