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What Causes Fossils?

Fossils, Latin for "having been dug up," represent the remains and traces of plants and animals. These remains can be preserved within a number of materials, including rock, ice, tar and amber. Fossils can also be preserved in a variety of ways, including mummification, skeletons and shells, petrifaction and replacement, impressions and traces, and casts and molds. Each of these five fossilization types is produced by a unique cause.
  1. Mummification

    • One process by which fossils are formed is called mummification. This process preserves complete pieces of the original animal. In mummification, both hard and soft components are preserved. Two primary ways this occurs is when animals die and their bodies become sealed in ice or tar, preventing any decay. Small insects, plants and animals are also mummified when they become sealed in amber. This sticky resin possesses an antibiotic nature that limits decay.

    Skeletons and Shells

    • The more familiar method of fossilization preserves only the hard parts of an animal. This occurs when the soft portions of a dead animal are decayed away, leaving the bones and shells to be fossilized. This decay can take place either before being covered or after. In warm, moist climates, even large animals can be reduced to bones in only 10 days. However, the hard parts of the animal must be covered quickly to avoid being destroyed by weathering, or destroyed by scavengers. Prime locations for stable, secure burial include the soft sediments found on seafloors and river beds.

    Petrifaction and Replacement

    • Another method by which fossils are formed is called petrifaction, or petrification. This occurs when minerals replace hard materials, such as bone, teeth and claws. As water filters down through the sedimentary layers, it is absorbed into these parts. This water contains minerals that replace the original hard material, one cell at a time. The same process occurs with wood. Even though the soft materials decay away, the hard remains must be covered quickly by sand, soil, ash or mud, in order to avoid being destroyed by weathering or scavengers. This method of fossil formation requires mineral-rich soil and rain water that can filter down to the remains.

    Impressions and Traces

    • Some types of fossils, called impressions, are completely devoid of any actual remains. Impressions are formed when the covered remains of small plants or animals are completely dissolved or destroyed. This process is carried out by acidic soils and chemicals in the ground water that are capable of dissolving even the hardest parts. If the pocket left behind becomes a permanent part of the rock, an impression is formed. Traces are simply the impressions of tracks, teeth marks, droppings and burrows that are permanently preserved in rock.

    Casts and Molds

    • Molds, another type of fossil, are formed the same way as impressions; however, molds are made by larger animals. These large molds become filled with loose sediment. As additional layers of sediment form above, the resulting pressure compresses the sediment in the mold into rock. The resulting rock forms a cast of the original animal.


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