Fossils are traces or remains of organisms that have been preserved in rocks. They were once part of plants and animals that lived and breathed, fed and bred, moved and died. They interacted continuously with their environment and each other, and their lives were an essential prelude to the development of organisms living today.
Fossils furnish information about the kinds of plants and animals that existed, when they appeared and vanished, where and how they lived, and the type of environments they preferred. Fossils help us to learn how species evolved, how some descended from others, and how groups of organisms are related.
Although fossils are abundant in some places and absent in others, they are most often found in marine sedimentary rocks such as limestone, sandstone, and shale. Some fossils are also found in rocks of nonmarine origin such as sand dunes, volcanic ash, lake sediments, and ice. The characteristics of the enclosing rock reflect the environment in which organisms lived and were buried. Fossils are common only when the conditions for burial were favorable for the preservation of the organisms’ remains. Bones, teeth, shells, wood, burrows, worm trails, and impressions (for example, leaf imprints) are a few remains that are commonly preserved as fossils.
How Organisms Become Fossils
To be part of the fossil record, all or part of an organism or some trace of its activity must be preserved in the rock. Not all plants and animals however, have an equal chance of being preserved and almost no organisms are preserved in their entirety. Also, all geologic environments are not equally favorable for preservation. Hard parts of plants and animals, such as skeletons and shells, are most likely to be preserved as fossils. Their preservation requires rapid burial in a substance that protects them from scavengers and other predators such as bacteria, and from physical and chemical changes in the environment, such as weathering.
Direct and indirect evidence are categories used for preservation classification.
Preservation of the entire organism is rare. Remains of mammoths and rhinoceroses preserved in the frozen tundra of Siberia and the entrapment of organisms in resin or oil seeps are examples of preservation without alteration.
Remains of unaltered hard parts such as shells, teeth, and bones are more frequently fossilized than the soft parts. Many shells and skeletons are preserved in the rocks with no recognizable change except for the removal of the less stable organic matter. The hard parts of most invertebrates are composed of calcium carbonate in the form of calcite (corals, mollusks) or aragonite (brachiopods, echinoderms, forams, sponges, bryozoans), calcium phosphate (brachiopods, arthropods, vertebrates, conodonts), silica in the hydrated opaline form (sponges, radiolarians), complex organic compounds (complex molecules of carbon, hydrogen, oxygen, and other elements such as those found in chitin in arthropods, graptolites, and some other invertebrates), or a combination of these.
When solution and other types of chemical action underwater transform the composition of plant and animal tissues to a thin film of carbon, the organic remains are carbonized. During carbonization, the volatile substances of the original organism -- including hydrogen, nitrogen, and oxygen -- are driven off leaving a thin film of carbonaceous material to show the outline of the original organism. Graptolites, arthropods, fish, and plants are commonly preserved this way.
Permineralization or petrifaction
Shells, bones, and plant material that are porous and buried in the earth are sometimes affected by underground solutions. When the solutions come into contact with the air-filled pores of the organisms, the solutions evaporate leaving a residue of dissolved mineral matter -- commonly calcium carbonate or silica -- in these spaces.
Solution and precipitation can change the internal physical structure of some shells so the original microstructure is blurred or lost and the shell is converted into a mosaic of interlocking crystals. Recrystallization commonly maintains the original mineral composition but it sometimes changes one mineral to another of similar chemical composition. For example, many Mesozoic and Cenozoic shells originally made of aragonite have recrystallized into the more stable form of calcium carbonate, calcite.
Replacement involves the complete or almost complete removal of the original hard structure by solutions and the deposition of new mineral substances. The most common replacing minerals are calcium carbonate, silica, and pyrite.
Preservation as molds and casts
The most common indirect methods of preservation are as molds and casts. After an organism has been trapped in sediment that has hardened around the fossil, later solutions may remove the fossil leaving a cavity in the rock known as a mold. If the shape of the outer side of the original organism is shown, the cavity constitutes an external mold. If the cavity reveals the form and markings of the inner surfaces of the organism, it is termed an internal mold or steinkern. Many times an external mold is later filled with mineral matter and forms a cast. It is sometimes difficult to distinguish between an internal mold and a cast, but on the surface of a cast the external part of the original animal is represented while on the surface of an internal mold the internal surface of the shell is represented.
Tracks and trails
Tracks and trails made by the feet, tails, and other portions of animals are abundantly preserved in mudstones at certain places. (They were originally made in mud and then solidified in mudstone.) Impressions made by jellyfish and other delicate organisms are also found in some fine-grained sediments.
Burrows and borings
Burrows or tubes made by worms and other animals such as clams are sometimes preserved as molds and casts. Round holes in clam and other shells indicate that predator snails once sought the soft part of the animal as food.
Coprolites are fossilized animal excrement. They are important because they may show the structure of the animal’s gut as well as indicate its diet.
Source: CGS Note 51