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Fossil Taxson A fossil taxon is a pointer to the large number of complex fossil species that have formed on earth. Fossil taxon refers to the classification of all kinds of fossils and sets one or more different taxon according to different criteria (Amler, 1999). Taxon is a specialized term in biology, usually used to distinguish the categories or groups of organisms (Meyer, 1926). Fossils are any remains of ancient organisms preserved in geological strata by various processes of nature (Teichert, 1956). There are many fossils on Earth, examples include microbes of a few microns, tens of meters of dinosaur bones, shells, footprints, hair, faeces, eggs, oil, coal, and more.

= Description = By classifying fossils of the same type with similar characteristics into the same fossil, a catalogue can be created for a large number of fossils, so that people can identify the fossils around them or find their families for the newly discovered fossils in a relatively convenient way. Fossil taxon can be different according to different criteria. According to the characteristics of preservation, fossils can be divided into: solid fossil, mold - casting fossil, trace fossil, and chemical fossil (Simpson, 1975). Fossils can be divided into Archean, Proterozoic, Paleozoic, Mesozoic and Cenozoic fossils according to the age of the strata where the fossils are located (Hug, & Roger, 2007). According to the size of fossils, fossils can be divided into macrofossils, microfossils, ultra-microfossils (Birks, 2007). If macrofossils are visible to the naked eye, microfossils refer to a class of tiny fossils observed with an optical microscope, and ultra-microfossils refer to fossils that can only be seen with an electron microscope (REGION et al., 1996).

= Main types =

Solid fossil
Solid fossils refer to the fossils formed by the preservation of almost all or part of the remains of ancient organisms (Teichert, 1956). Under particularly favorable conditions, the original organism avoided the oxidation of air and the corrosion of bacteria, and some of its hard or soft parts could be relatively intact without obvious changes (Bartuska et al., 1977). The number of solid fossils is the largest, there are animal bones and teeth, crustaceans, shells, as well as plant stems, leaves, flowers, fruit and so on. Among them, the mammoth and the coal formed by plants are well known to the public.

Mold - casting fossil
Impressions or castings left in rock by the remains of ancient animals are called molded fossils (Amler, 1999). Bivalves, for example, the two shells are often scattered save, when they are buried by sediment, sediment diagenetic process consolidation became the rock, and shell sometimes is dissolved by water, but in contact with the shell of the outer surface of surrounding rock under the seal the outer mold, at the same time on the inner surface of the surrounding rock and shell contact surface under the seal of the internal model (Cameron, 1969).

Trace fossil
From the perspective of sedimentology, trace fossils can also be said to be sedimentary structures of various biological origin, such as various biological disturbances, footprints, migration, burrows, coprolites, etc. And biological erosion structures, such as drilling holes (Simpson, 1975). Trace fossils do not include physical fossils transformed from living organisms, let alone inorganic sedimentary structures formed by various natural stresses (physical and chemical). The behavioral classification of trace fossils proposed by Seilacher is a relatively widely used classification of trace fossils. As relics are evidence of biological activities, each relic reflects certain behavioral habits, so corresponding ecological habits can be judged according to morphological characteristics (Seilacher, 1967).

Chemical fossil
Chemical fossils, also known as molecular fossils, are organic molecules that exist in rock formations in the form of amino acids or proteins. The organisms that can only be accurately observed and studied on the high-tech equipment come from the living geological bodies. Although they have undergone certain late changes (diagenesis, soil formation, etc.), they have basically maintained the basic carbon skeleton of the original biological and biochemical components, with clear life significance (Peters & Moldowan, 1993).

Other fossils
There are other fossils, such as subfossils that were not fully fossilized or formed too recently. And there are transitional fossils, and pseudofossils.

= Dating =

Biostratigraphy
Biostratigraphy is a subject that mainly studies the spatial and temporal distribution of biological fossils, formation and development rules of strata, and determines the relative ages of strata. Biostratigraphy is a branch of stratigraphy, which focuses on the correlation and division of relative ages of strata by using fossil assemblages contained in strata (Lister, 1992).

Biological fossils show different specific features in different geological ages, while fossils in the same geological age have roughly the same features. The stage development of this kind of organism is closely combined with the stage of geological history, so it is possible to name the big geological age with the biological development, such as Paleozoic, Mesozoic and Cenozoic and so on (Lister, 1992).

Estimating dates
Although there are some advanced dating techniques, the main dating technique is still radioactive dating. It is based on the principle of roughly dating the decay of naturally occurring radioactive isotopes in fossils, and locating the age of the organisms behind them (Boltwood, 1907).

According to the measurement and calculation of instruments, for different fossils from different rock layers of different ages, fossils can be divided into Archean, Proterozoic, Paleozoic, Mesozoic and Cenozoic fossils according to the age of the strata where the fossils are located (Hug, & Roger, 2007).

= Size =

Macrofossil
Macrofossils, also known as megafossils, are those that can be viewed with the naked eye without any microscope. So the specimens that you see in the museum are basically Macrofossils. There are two types of macrofossils: plant macrofossils and animal macrofossils (Birks, 2007). Some of them are quite famous, such as algae fossils, dinosaur fossils and so on.

Microfossil
Microfossils are usually the study of micropaleontology, those fossils that require a microscope to see organisms and their forms and details in fossils. Generally speaking, the study of microfossils requires the use of optical or electron microscopes. Microfossils are common in Marine sediments, such as Bryozoa, Sarcodina, and the spores of vascular plants (Drewes, 2005).

Ultra-microfossil and so on
Ultra-microfossil are microscopic fossils that need to be studied under an electron microscope. It includes a wide variety of organisms, mainly ultrafine plankton. There is no consensus on the size range of ultra-microfossils, which are generally thought to be limited to less than 10 microns, and there are also proposals to include micro fossils with a size of less than 25 microns or 30 microns. The microscopes used in the study of ultra-microfossils are all highly technical devices, such as Petrographic microscope, scanning electron microscopes and transmission electron microscopes (REGION et al., 1996).

= References =

= Further Reading =
 * It's extremely hard to become a fossil, by Olivia Judson, The New York Times
 * Bones Are Not the Only Fossils, by Olivia Judson, The New York Times
 * Brasier, M.D. (1980), Microfossils. Chapman and Hall publishers. ISBN 0-412-44570-0