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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, feces, eggs, oil, coal, and more.
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. Fossils 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).
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. Under particularly favorable conditions, which can be avoided the effects of air oxidation and bacterial corrosion, the remains of some organisms can be preserved relatively intact without significant change. One of the most shocking is that in 2012, Russian scientists found some preserved mammoth in permafrost in Siberia once live in 25000 years ago, they not only bones intact, and the skin, hair, flesh and blood, even the food in the stomach are well preserved, even more exaggerated, Russian scientists even in one of the female mammoth veins extract can still flow of blood.
Mold - casting fossil
For these casts and molds, there are mainly three kinds of conditions. Impression fossils are impressions made by the remains of living organisms (mainly soft parts) that have been trapped in fine clastic or chemical deposits. Corrosion and diagenesis have destroyed the remains themselves, but the impression has persisted, and often reflects the main features of the creature. The impressions of jellyfish, worms and plant leaves of coelenterates are fossil impressions (Chaloner & Collinson, 1975). Mold fossils include two kinds of external mold and internal mold. The external model is the impression printed on the wall rock on the surface of the hard part (such as shell) of a paleontological body, which can reflect the morphology and structural characteristics of the original biological appearance. The inner mold is the impression left by the outline structure of the inner surface of the shell, which can reflect the internal morphology and structural characteristics of the biological hardware. 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). Mold core fossils are divided into inner core and outer core. When brachiopods and certain bivalves die, their shells are often buried intact. If their internal cavities are filled with sediment, after the consolidation and dissolution of the shells, an entity is left inside called an inner core. If the shell is not filled with sediments, when the shell is dissolved, it will leave a space of the same size and shape as the shell in the surrounding rocks. If the space is filled again, it will form a solid of the same size and shape as the original shell. Such a solid is called the outer core (Amler, 1999). 
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 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. With the development of modern chemistry and the improvement of science and technology, the organic molecules of ancient organisms (refer to amino acids, etc.) can be separated from the rock layers for identification and research, and a new discipline -- Paleobiochemistry has been born (Peters & Moldowan, 1993).
Chemical fossils are fossils made up of a group of macromolecular organic compounds, or special inorganic substances, and thus can only be separated and known by chemical means. Chemical fossils do not look like macrofossils or microfossils that have a scale range, it can become a huge ore body, such as coal, oil, etc., it can be so small that only a very sensitive chemical instrument can detect it too. Therefore, it does not need to maintain the same shape and size as macrofossils and microfossils, and only a few samples can be identified. Macrofossils and microfossils are usually named with biological names, such as ginkgo biloba, psilophyta, etc., while chemical fossils are mainly named with biochemical names, such as lignin, pigment, keratin, thrombolysis, protein, lipid, and so on (Blumer, 1973).
There are other fossils, such as subfossils that were not fully fossilized or formed too recently. And there are transitional fossils, pseudofossils and so on.
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).
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). Specifically, in addition to detailed information about the age of the fossil from some expensive dating techniques, we can also make rough predictions about the age of the fossil in our hands by identifying some of the main features of these different geological ages.
The Archean is the most distant, dating from 2.5 to 4 billion years ago. This was a time when both the sun and the earth were growing, when the sun was only about 70 percent as bright as it is today, and liquid water appeared on the earth, but the environment on the earth was very harsh, and life on the earth at this time was limited to the early simple single-celled organisms, also known as prokaryotes (Stanley, 1999). As far as fossil discovery results are concerned, Archaean fossils are mainly cyanobacteria or microfossils of archaea.
The Proterozoic eon is even more complex. It started during the famous Cambrian period, until today, so the phanerozoic spans 540 million years, the 540 million years of an age of species explosion. The phanerozoic is roughly divided into three alternating eras, the Paleozoic, Mesozoic and Cenozoic, and can be further divided into 12 periods, such as the famous Cambrian, Jurassic, and Cretaceous (Cohen et al., 2013). The earth has had so many big events in the phanerozoic that the fossils of this era are complex. Ferns, gymnosperms and angiosperms emerged, sea fish emerged and landed, insects and vertebrates flourished, dinosaurs emerged and died out, and mammals emerged and thrived today. According to the detailed geologic times, fish and amphibians prevailed in the Paleozoic era, reptiles, represented by dinosaurs, dominated the Mesozoic era, and the Cenozoic belongs to the mammal dynasty (Valkenburgh & Jenkins, 2002). So obviously, if you find a fossil that's visible to the naked eye, there's a very high probability that it's in the phanerozoic eon. As to which of the 12 periods it belongs to, you need to do some extra research.
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.
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).
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- It's extremely hard to become a fossil, by Olivia Judson, The New York Times
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