Fossil

Composition
Fossils of animals, plants, or protists occur in sedimentary rock. In a typical fossil, the body form is retained, but the original molecules that made up the body have been replaced by some inorganic material, such as calcium carbonate (CaCO3) or silica (SiO2). 

The fossil feels like, and is, made of rock. It has been mineralised or petrified (literally, turned into rock). A fossil may also be an imprint or impression of a living thing remaining in the fossilised mud of a long-gone age. It takes about 10,000 years for fossils to form. Fossils are most commonly found in sedimentary rocks.

Examples

The parts of an organism  that fossilize well are the hard parts, not the soft parts. The most common fossils are those of organisms that have hard parts. The hard, calcitic shells of molluscs (such as clams and snails) and of now-rare brachiopods – also known as lampshells – are examples.

These sea-dwelling shellfish have produced many fossiliferous (fossil-bearing) chalky layers of limestone in the earth. Soft-bodied organisms can fossilise in special circumstances: the Ediacaran biota is a good example.Levin, Harold. 2006. The Earth though time. 8th ed, Wiley N.Y. Chapter 6, p117.

The best-known fossils for the general public are those of the giant, prehistoric dinosaurs. The fossilized bones and fossilized tracks of these huge, ancient reptiles can be seen in many museums of natural history and earth science.

Academic
The study of fossils by geologists and biologists is known as paleontology. If the study puts living things in their ecological context it is called paleobiology. Examples of fossils include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, hair, petrified wood, oil, coal, and DNA remnants. A very famous palaeontologist is Mary Anning she is from the United Kingdom and found fossil remains on the Jurassic Coast.  

Applications
People have found many uses for fossils.  Some fossils are found and sold.  Some rocks, such as limestone, are mostly just masses of small fossils and are used as construction materials and for other purposes.  Some fossils are used as a marker to determine formation.  Some fossils are used as a source of fuel and called fossil fuel.

Preservation
There are some sites where fossils have been found with remarkable details, or in large numbers. Palaeontologists call these sites by the German term Lagerstätten. The La Brea tar pits in Los Angeles, California, is such a place. So are the Solnhofen limestone quarries in Bavaria. In the Earth’s structure a very important place for fossils is the Burgess Shale. It is known for its many important fossil discoveries.

Types
Microscopic, or very tiny’', fossils are called “microfossils”. Larger, macroscopic fossils — such as those of seashells and mammals — are called "macrofossils". Sometimes natural stones look like fossilized organisms, but they are not fossils at all. Rather, they are called "pseudofossils".

Although most fossils are formed from the hard parts of organisms, there are also indirect signs of prehistoric life. Examples such as a worm's trail or an animal's footprint are quite common. These are known as  trace fossils. Fossilized excrement, faeces or dung is known as a coprolite. A chemical trace of prehistoric organisms is called a chemofossil. Objects made by prehistoric people are called artifacts but these are not fossils.

Even when the remains of soft-bodied animals are gone, there may be impressions, molds or carbon traces which remain permanently. So, in special cases, we can see fossils of even small, soft invertebrate animals. Sometimes a fossil is produced because of dryness (desiccation), freezing, or pine resin.

Mummified animals, ice-covered wooly mammoths, and insect-filled amber are examples of such fossils. Living fossils, however, are not'' fossils at all. Instead, they are modern-day organisms which look like their prehistoric ancestors of many millions of years ago. The ginkgo tree, the coelacanth and the horseshoe crab are good examples.

Early mentions
Many pre-scientific peoples noticed fossils, but not all thought they were the remains of living things. Perhaps the first to leave a record of his thought was the Ancient Greek philosopher Xenophanes (about 570BC–470BC).Guthrie W.K.C. 1962. A history of Greek philosophy. vol 1: The earlier presocratics and the pythagoreans. Cambridge.p. 387 His ideas were reported by later writers:
"Shells [are] found in the midst of the land, and on mountains. In the quarries of Syracuse the impressions of a fish and seaweed have been found; on Paros the impression of a bay-leaf in the depth of a stone, and on Malta the flattened shape of sea-creatures [have been found]. These, he says, were formed when everything, long ago, was covered in mud, and the impression dried out in the mud". Guthrie p. 387

These ideas were rediscovered in the 17th century in Europe. Nicolas Steno in the Netherlands and Robert Hooke at the Royal Society in London both wrote and gave lectures about fossils. In the 18th century fossil-collecting began, and serious thinking on geology began to make progress. In the 19th century geology became a modern science, and fossils played a part in the theory of evolution.

Related pages
 Evolution
 Extinction
 Lagerstätten
 Earth history
 Palaeontology
 List of extinction events

 References 


More reading


 Other websites 

 Fossil – Citizendium

Category:Paleontology
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