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What are fossils? How to determine the age of a fossil?

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Hint: When an organism dies, its body decomposes and hence is lost in the environment. But when any body apart or complete body gets preserved in the rocks or any other form, it becomes a fossil.

Complete Answer:
The fossils are the remains or impressions of the ancient organisms preserved by natural means in some medium. The medium in which the fossils occur include sedimentary rocks, amber, asphalt, volcanic ash, ice, peat bogs, sand and mud. Palaeontology furnishes the most direct and most reliable evidence for evolution, as it deals with the actual organisms that lived in the past.

The age of rocks and fossils can be determined by the following methods :
A) Relative Dating Method: relative dating of the rocks was carried out by estimating the position and their rate of erosion in a particular environment.
B) Absolute dating method: This method is based on the property of spontaneous decay of unstable radioactive nuclei into stable radioactive nuclei at a constant and known rate.
C) Uranium-Lead technique: This method was introduced by Boltwood in 1907. Radioactive uranium occurs in mineral zircon. Uranium($U^{238}$) decays spontaneously to lead ($Pb^{206}$). It has a half life of 4-5 billion years.
D) Carbon ($C^{14}$) Dating Technique: Radioactive carbon ($C^{14}$) occurs naturally. Its half-life is about 5600 years.

Note: During the formation of sedimentary rocks, the dead animals of the sea or large lakes get buried in the rocks.