HOW ARE FOSSILS FORMED?
When an organism dies, it generally decays away completely, but sometimes leaves a record of its existence. Those that leave a record do so as a result of various physical and chemical processes. Fossils can be divided into two main classes unaltered
remains and altered remains.

Unaltered Remains
Unaltered remains are fossils of organisms or parts of organisms that are preserved with little or no change.

  1. freezing - Ancient organisms are sometimes frozen in ice or permafrost (frozen soil) at high latitudes and /or high altitudes. If this occurs, the organism may be preserved for a long time. In Siberia, whole mammoths are found in a near-perfect state of preservation. Meat from one these extinct animals was even eaten at a National Geographic Society meeting in the 1930s.
  2. drying - In desert environments, animals and plants have been found in a mummified condition. The lack of moisture in these areas lessens or eliminates decay.
  3. encasement - Organisms that are trapped in tar or tree resin area often protected from air and moisture, both of which are agents of decay. Bones of many extinct animals have been found preserved in this manner at the La Brea Tar Pits in California. The forms of some insects have lasted for millions of years in resin that has hardened into amber.

Altered remains
Plants and animals may become fossilized when they are changed by various means.

  1. carbonization - Organic remains can be altered by heat and pressure. These agents force the soft tissues of the organism to give off hydrogen and oxygen, and leave a residue of carbon that outlines the shape of the organism. Delicate black outlines of leaves are commonly preserved in this manner in the Middle and Upper Devonian rocks of the Catskill Mountains of New York, and in the Carboniferous age coal deposits in Pennsylvania.
  2. permineralization (also called petrification) As water seeps through the hard parts of organisms, minerals dissolved in the water may be deposited in the spaces within bones and wood. These minerals are added to the bone or woody tissue of the fossil. Thus, petrified wood and dinosaur bones are still composed of wood and bone, respectively, but are much heavier because of the added minerals.

  3. replacement - In this type of fossilization, the original hard parts are completely dissolved by water. The cavity or outline of the shell is then filled with minerals such as chert or quartz (silicon dioxide or silica, quartz [the common mineral in many beach sands) is coarsely crystalline silica, chert is micro-crystalline silica), hematite (an iron oxide, one type of iron ore), calcite (calcium carbonate, or lime), or pyrite (iron sulphide, often called "fool's gold").
  4. molds and casts - An external mold is the imprint of the exterior form of an organism's shape or shell. An internal mold is composed of material that fills a cavity (often the hollow space under a shell or between two shells) . If the shell is dissolved away, the internal mold may remain. A cast is made either by filling the impression of an external mold or by removing the sediment or rock that composes an internal mold. In both molds and casts, the original soft tissue of the organism or its shell are destroyed.

Trace fossils
Trace fossils are fossilized evidence of behavior. They include tracks, trails, burrows, and borings made by animals and the hollows made by plant roots. Tracks are single impressions of feet or foot- like structures. Trails or trackways are lines of tracks or drag marks made by an animal that moved over a soft surface, usually mud. These evidences of locomotion are preserved if the mud is hardened in to rock. Burrows are the tunnels
made by animals in mud or sand that is later hardened. Fossil evidence of soft-bodied organisms, such as worms, is provided mostly by burrow fossils. Complete burrow systems made by ants and termites in soil are known in sandstones in the western United States. Borings are tunnels cut by organisms into hard substances such as wood (termite borings), shells or coral heads, or even in to hardened rock. Interestingly, a number
of marine (salt water) clams can burrow through wood (these clams are called "ship worms"), living coral heads, and even through such hard rocks as granite. These clams live in the burrows, and use them for protection.

Chemical fossils
Chemical fossils are carbon-based chemical remains of organisms. Petroleum and natural gas are produced from fatty chemicals derived from dead organisms. Amino acids de rived from the break-down of protein are known in ancient rocks. The oldest evidence of life on earth is indicated by chemical isotopes of carbon in 3.8 billion year-old
rocks found in northern Canada.


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