What is a fossil?

A fossil is simply evidence of ancient life, the remains of once living plants and animals. To most geologists, this definition excludes evidence of life that has existed in the past 10,000 years. The term fossil is derived from the Latin fossilis, which simply refers to any object dug up from the ground, including minerals and archaeological artifacts. In the late eighteenth century the science of palaeontology (the study of ancient life) was born. Since then the term "fossil" has been applied solely to the remains of ancient organisms.

Fossils are important in the study of geology because they can often be used to determine the relative ages of rocks being studied. As you recall, the Principle of Faunal Succession allowed early geologists to construct the geologic time scale. Fossils are also used to correlate, or match up, rock units from different places. Finally, fossils can be used as paleoenvironmental indicators, giving us clues about the environmental conditions that existed where the fossilized organism once lived.

There are two basic types of fossils: body fossils and trace fossils. Body fossils preserve parts of the original body of an organism, like bones, teeth, and shells. Trace fossils are evidence of the life activities of ancient organisms. Examples of trace fossils include tracks, burrows, and droppings.

There are several ways that organisms can be preserved as fossils. It is important to note, however, that the vast majority of organisms will not end up being preserved. Conditions do not typically favor fossilization, and the fossil record represents only a tiny fraction of all the individual organisms that have ever lived on Earth. There are a couple factors that determine whether an organism will end up being fossilized. First, an organism's hard parts (bone, shell, etc.) are more likely to be preserved than soft tissues that will decay or be consumed by scavengers. Because of this, organisms possessing hard parts are more likely to be preserved than soft-bodied creatures (like jellyfish). Also, the more rapidly the remains are buried by sediment, the more likely they will survive to be fossilized. Rapid burial minimizes the decay, consumption, and destruction of the remains.

• Unaltered remains - Organisms can become trapped in tar (as happened in the La Brea Tar Pits of southern California) or in tree resin, which can solidify and form amber (remember Jurassic Park?). Organisms can be dried out and mummified, or even preserved by freezing. Woolly mammoths 40,000 years old have been found frozen in Siberia, virtually intact.
• Replacement - Following burial, the molecules that make up the original shell or skeleton are slowly replaced by molecules of the minerals found in the surrounding sediment.
• Recrystallization - In this process the original crystal structure of the skeletal material is changed into another form with the same composition. For instance, aragonite may be recrystallized to calcite. Only the crystal structure has changed, both aragonite and calcite are forms of calcium carbonate (CaCO₃).
• Carbonization - This is often how soft-bodied animals and plants are preserved. As the organism decays, the volatile components of the organic matter (oxygen, hydrogen, and nitrogen) are driven away, leaving behind a thin film of carbon.
• Molds and Casts - Molds are imprints made in soft sediments that are preserved when the sediments are lithified (turned to stone) and the skeletal remains have been dissolved away. A mold can later be filled in by sediment creating a cast. A cast thus forms a replica of the original remains. Casts are relatively rare in nature.

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Paleontologists make plaster molds and casts of fossils for a couple of reasons. A mold or cast can reveal fine details in the structure of the fossil that would otherwise be obscured by the rock the fossil is found in. In the case of rare fossils, a plaster replica allows more people to study the same organism.