Fossil

From Free net encyclopedia

For other uses of the term, see Fossil (disambiguation)

Image:Igammonite.jpg Fossils (from Latin fossus, literally "having been dug up") are the mineralized or otherwise preserved remains or traces (such as footprints) of animals, plants, and other organisms. The totality of fossils and their placement in fossiliferous (fossil-containing) rock formations and sedimentary layers (strata) is known as the fossil record. The study of fossils is called paleontology. Because fossils are by their nature old, the word has also entered the modern vernacular as a derogatory term for an elderly person.

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Fossilization is actually a rare occurrence because most components of formerly-living things tend to decompose relatively quickly following death. In order for an organism to be fossilized, the remains normally need to be covered by sediment as soon as possible. However there are exceptions to this, such as if an organism becomes petrified or comes to rest in an anoxic environment such as at the bottom of a lake. There are several different types of fossils and fossilization processes.

Fossils usually consist of traces of the remains of the organism itself. However, fossils may also consist of the marks left behind by the organism while it was alive, such as the footprint or feces of a dinosaur or reptile. These types of fossil are called trace fossils, as opposed to body fossils. Finally, past life leaves some evidences that cannot be seen but can be detected in the form of chemical signals; these are known as chemical fossils (for lack of a better term).

The oldest known structured fossils are most likely stromatolites. Now understood to be formed by the entrapment of minerals by mucous-like sheets of cyanobacteria, the oldest of these formations dates from 3.5 billion years ago. Even older deposits (3.8 billion years old) of heavy carbon that are indicative of even earlier life are currently proposed as the remains of the earliest known life on Earth.

1 Rarity of fossils
2 Permineralization
3 Replacement and compression fossils
4 Trace fossils
5 Microfossils
6 Resin fossils
7 Pseudofossils
8 Living fossils
9 See also
10 External links

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Rarity of fossils

Because fossilization requires the relatively quick preservation of bodily remains, fossils are relatively rare. Fossilization favors big organisms with hard body parts, organisms that were widespread, and organisms that lived for a long time. On the other hand, it is almost impossible to find fossils of small, soft bodied organisms living in small areas that did not live for a long time, either because they became extinct, or because they evolved quickly (for a better explanation, see article on punctuated equilibrium). Geological processes such as erosion and subduction also destroy many fossils. For all these reasons, fossils are somewhat rare.

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Permineralization

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Permineralization consists of organic remains being to some degree impregnated by minerals derived from the surrounding sediments or waters. For permineralization to occur, the organism must become covered by sediment soon after death or soon after the initial decaying process. The degree to which the remains are decayed when covered determines the later details of the fossil. Some fossils consist only of skeletal remains or teeth; other fossils contain traces of skin, feathers or even soft tissues. Once covered with sediment, these layers slowly become compacted and cemented into rock, and the organic remains are slowly replaced with hard minerals.

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Replacement and compression fossils

In some cases the original remains of the organism have been completely dissolved or otherwise destroyed. When all that is left is an organism-shaped hole in the rock, it is called a mould fossil or typolite. If this hole is later filled with other minerals, it is called a cast fossil and is considered a replacement fossil since the original materials have been completely replaced by new, unrelated ones. In some cases replacement occurs so gradually and at such fine scales that no "hole" in the rock can ever be discerned and microstructural features are preserved despite the total loss of original material.

Compression fossils such as those of fossil ferns are the result of chemical reduction of the complex organic molecules composing the organism's tissues. In this case the fossil consists of original material, albeit in a geochemically altered state.

To sum up, fossilization processes proceed differently for different kinds of tissues and under different kinds of conditions.

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Trace fossils

Template:Main Trace fossils are the remains of trackways, burrows, footprints, eggs and eggshells, nests, droppings and other types of impressions. Fossilized droppings, called coprolites, can give insight into the feeding behavior of animals and can therefore be of great importance.

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Microfossils

Template:Main 'Microfossil' is a descriptive term applied to fossilised plants and animals whose size is at or below the level at which the can be resolved by the naked eye (a commonly applied cut-off point between micro and macro fossils is 1mm, although this is only an approximate guide). Microfossils may either be complete (or near complete) organisms in themselves (such as the marine plankters foraminifera and coccolithophores) or component parts (such as teeth or spores) of larger animals or plants. Microfossils are of critical importance as a reservoir of paleoclimate information, and are also commonly used by biostratigraphers to assist in the correlation of rock units.

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Resin fossils

Smaller animals, such as insects, spiders and small lizards, can be trapped in resin (amber), which is secreted from trees. These fossils can be found in sandstones or mudstones or washed up on beaches like those around the Baltic Sea.

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Pseudofossils

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Pseudofossils are visual patterns in rocks that are produced by naturally occurring geologic processes rather than biologic processes. They can easily be mistaken for real fossils. Some pseudofossils, such as dendrites, are formed by naturally occurring fissures in the rock that get filled up by percolating minerals. Other types of pseudofossils are kidney ore (round shapes in iron ore) and moss agates, which look like moss or plant leaves. Concretions, round or oval-shaped nodules found in some sedimentary strata, were once thought to be dinosaur eggs, and are often mistaken for fossils as well.

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Living fossils

Template:Main Living fossil is a term used for any living species which closely resembles a species known from fossils, i.e., as if the fossil had "come to life". This can be a species known only from fossils until living representatives were discovered, such as the coelacanth and the ginkgo tree, or a single living species with no close relatives, such as the horseshoe crab, that is the sole survivor of a once large and widespread group in the fossil record.

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