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How do fossils form?
When an organism dies and is buried by sediments, there is a chance that it may be preserved as a fossil. In most cases, this does not happen, and the remains will be eaten by natural scavengers and decay processes. However, due to specific environmental factors such as rapid burial or in extremely low oxygen conditions, the natural scavengers cannot reach the remains and an organism can be preserved as a fossil. Due to the forming of new layers of sediment on top, the pressure increases in the layer in which the organism or the residues are burried. This starts the fossilization processes, by which the original organic material is replaced by mineral components: the remains are 'fossilized'. When, in the course of time by tectonic movements and erosion these layers emerge on the surface, the fossil can be discovered and excavated.
In these arid conditions the chance of fossilisation is limited, unless the remains are burried very fast
Good conditions for fossilization are very rare, and usually disruption of the remains occurs to some extent, so especially the hard parts of organisms (shell, bones and teeth) are preserved. Since sedimentation is more common in river and marine environments, fossils of aquatic organisms are much more numerous than fossils of land organisms. The latter are mainly found in deposits of catastrophic events such as volcanic eruptions, landslides or floods. The best preserved fossils occur where a quickly burial of the remains with fine-grained sediments is followed by very low oxygen levels. In such conditions, normal decay processes have no chance and even soft tissues can be preserved, sometimes to the finest details.
Nonetheless, much of the eventual preservation still depends on the chemical composition of the deposits in which the fossil remains are embedded. Within the deposits, the fossil can be partly converted into other minerals by the influence of chemical processes. In that case, the internal structure of the fossil remains well preserved. It is also possible that the original fossil completely dissolves after deposition. The remaining cavity can then later be filled by other minerals, but again you will only find a "print". The internal structure of the fossil will be lost. Even after the fossilization the fossil may be damaged or get dissolved, for example by percolation of water through the rock, or by differential weathering processes when the fossil is close to the Earth's surface. The latter you can often observe in the field yourself. When a fossil-bearing layer is exposed by natural erosion, fossils that are several feet deeper in the layer are sometimes better preserved than those located on or near the weathering plane.
Deposits with a very special fossil content are known in science as Lagerstätten. There is a distinction between the so-called 'Konservat-lagerstätten" in which the fossils are preserved in an exceptional way, and 'Konzentrat-Lägerstatten', in which excessive amounts or concentrations of fossils are found. These lagerstätten are without exception of great scientific significance. Konservat-lagerstätten are often the result of very specific chemical conditions, in which the fossils have been through a conversion to phosphate, silica and pyrite, like carbon film, or like imprint in another medium (e.g. microbial mats on the sea floor).
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