Fossils are the preserved biological remains and activities of ancient animals and plants. Paleontologists study fossils to determine anatomical and behavioural characteristics of past organisms. Fossils are classified into three categories of body, trace and chemical. The fossilisation process is rare and occurs through a combination of biological, chemical and physical processes. Fossilisation processes are moulds, casts, drying, freezing, petrification, polymerisation and carbonisation.
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Body fossils are the preserved remains of an animal or plant. The soft parts such as skin, fur, internal organs and plant fleshy parts do not survive long after death. Therefore, body fossils are more commonly found as skeletal remains and other hard parts such as shells, horns, claws and teeth. However, freezing, drying and polymerisation are fossilisation processes by which the full soft and hard parts can be fully preserved. Perimineralization and polymerisation are also fossilisation processes by which body fossils are formed.
Trace Fossils (Ichnofossils)
Also known as ichnofossils, trace fossils are the fossilised biological activity left behind by historical animals. Some examples are tracks, trails, dwellings, feeding marks, nests and faeces. Different species produce similar appearing trace fossils, which are found separate from the source animal. Therefore, scientists commonly categorised this type of fossil is commonly according to the biological act rather than by the producer. Preserving trace fossils involves casts, moulds, drying, freezing and the petrification fossilisation process.
Chemical fossils are the preserved organic compounds, which can be traced back to biological origin. The decay of plants and animals includes a series of biochemical processes leaving biochemical traces commonly found in rock. The presence of chemical fossils in geological strata indicates the existence of life at the time a particular rock was formed. The carbonisation fossilisation process preserves chemical fossils.
Molds and Casts
An organism lying, moving or decomposing in sediment creates fossilised moulds and casts. As the surrounding sediment hardens, a mould forms as the organic remains decay and trace markings remains intact. The external mould is a recreation of the organism's surface, which provides detail of external characteristics and anatomy. When the internal portion of the mould fills with sand or clay, it creates an internal cast of trace fossils, such as tracks or burrows.
Freezing and Drying (Dessication)
Freezing fossilisation occurs when an animal falls into a pit or land void, of freezing temperature, and remains frozen for a long time. Animal and plant remains, which the freezing process fossilises, are discovered with fully preserved hard and soft parts. Drying fossilisation process occurs to an organism's remains left in arid and dry environments, such as caves or deep pits. This results in mummification, which preserves the hard and soft parts of an organism.
Petrification and Polymerization
Petrification occurs when water dissolves the remaining skeletal and hard parts, replacing them with by mineral deposits. The result is a detailed recreation of the original remains in stone or similar materials. Petrification fossilisation occurs by replacement and permineralization processes. Both processes create fossils by replacing the remains with minerals. However, in permineralization much of the original skeleton remains as part of the fossil. Polymerisation occurs when tree sap converts to an amber resin. The fossil resin material preserves small insects, plants and bacteria by trapping and encasing them in a plastic-like amber shell.
The carbonisation fossilisation is a common preservation process of plants and animal soft parts. The carbon is a by-product of decayed organic remains and creates an outline of the organism in the rock.
Rarity of Fossilization
The occurrence of fossils are rare. Trace biological evidence and dead plants can decay or get eaten by scavengers soon after death. Time, location, and environment all contribute to the fossilisation process. Immediate burial, sediment accumulation, low oxygen and low heat slows the decaying process allowing fossilisation to occur. Organic remains on land or in water can become fossilised.
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