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|See Fossil in Wikipedia, the free encyclopedia.|
A fossil is the preserved and hardened remains or traces thereof of animal or plant life from a prior geological era, preserved in rock formations of the Earth's crust. The majority of fossils are found in sedimentary rocks (formed from compressed mud or silt), with the areas of rock containing many fossils being known as LagerstÃ¤tten (German: storage places).
Fossilization, the formation of a fossil, is a rare occurrence, as dead organisms tend to quickly decompose. Generally, fossilization occurs when the organism is covered by sand or silt just after death (or as the cause of death), but can also occur if the organism dies in an extremely cold, dry or anoxic (i.e. without oxygen) environment, free from decomposition. Preserved tree resins, known as amber, may also contain fossils.
The most common fossils are, of course, those organisms that were plentiful in areas conducive to fossilization. Additionally, hard parts of the organism, such as shells and bones, are less likely to be broken or decay, so are therefore more likely to be preserved as a fossil. Soft tissues, such as skin or feathers, will only be present if the organism was buried shortly after death, and didn't decay much further.
One common form of fossilization is permineralization. This gradual process involves water containing dissolved minerals, such as calcite or silica, permeate the empty spaces of the dead organism before it decays. The minerals are deposited, thus leaving an rock matrix around the tissue of the organism. This can often reveal detailed structure, such as individual cells, within the organism.
This form of fossilization involves the organism decomposing within the rock, leaving an impression in the shape of the organism known as a mold. Sometimes, this mold can be filled with other minerals, forming a replacement fossil, known as a cast.