| Themes > Science > Paleontology / Paleozoology > Fossils And Fossilisation > What are fossils? |
Fossils are the remains, impressions or traces of ancient animals or plants, which have been preserved in the earth's crust for thousands of years. Fossils are thus, simply put, the remains or other evidence of ancient life forms. Many creatures found as fossils are extinct, while some may even be found today. Fossils as old as 550 million years have also been found. The study of fossils is called Palaeontology. How are fossils made? Only a few of ancient organisms are preserved as fossils. This is because usually only the organisms with a solid and resistant skeleton are easily preserved. Most fossils comprise the bones, teeth and shells of ancient animals and are found in sedimentary rocks. These rocks are formed when sand or other particles are carried to a place by water or wind and accumulated over time. After hundreds of years, these get hardened and form rocks. Shale, sandstone, and limestone are the most common types of sedimentary rocks. Besides sedimentary rocks, fossils may also be found in other materials, such as ancient tree resin (amber), tar pits, ice, volcanic lava, or even in a dried, mummified state in loose soil. After an animal or plant dies, its body is decomposed by bacteria or other organisms. But if the creature is buried quickly after death, as in case of a flood, landslide, volcano, or snowstorm, or gets embedded in tar or tree resin, the body is not decayed. Hard parts of a body, such as teeth, bones and shells, are more likely to be preserved than soft parts. These are then subjected to various climatic changes. In the course of thousands of years, minerals may get deposited in the pores of the shell or bone. The remains may also be subjected to intense pressure and other factors, which may fossilise the remains. In some cases, the original skeleton may be completely replaced by other mineral matter. The soft parts of animals or plants are very rarely preserved. However, there are some striking examples of soft tissues getting fossilised. The embedding of insects in amber and the preservation of Mammoths in ice are some of the examples.Majority of the fossils are preserved in a water environment, chiefly because those on land are more easily destroyed. Anaerobic (oxygen-less) conditions at the bottom of the seas or other water bodies are especially favourable for preserving the forms, since very few microorganisms are present there to destroy the remains. In general, for an organism to be preserved two conditions must be met: (1) rapid burial in order to slow decomposition and prevent the action of scavengers on the dead body (2) possession of hard parts, which can be easily fossilised. This entire process by which dead organisms or their parts are transformed into fossils is called fossilisation. Some places, such as the Grand Canyon at northern Arizona in the USA, have interesting sights of fossils. One can observe thick and nearly horizontal layers of sediment on the seafloor, which have been deposited over millions of years. Each layer here contains fossils that are distinct from those of the layers above and below it. By comparing overlapping sequences, the scientists can build up a continuous record of animals and plants that existed during various time periods. What are the uses of fossils? Fossils provide information about the history of life on earth. By studying the fossil record, or the data recorded in fossils found over the world, scientists trace important information. Some of the uses of fossils are: (1) They help us find out the progressive changes within an animal or plant group. Thus, the evolution of that group can be studied. For example, a fossil called Archaeopteryx, which showed certain characteristics of both reptiles and birds, was a transitional form between birds and reptiles. With the help of this fossil, scientists determined that birds evolved from reptiles. (2) Through fossils, one can quickly and easily find out the age of the geological strata or the rock layers in which they occur. The accuracy with which this may be done depends on the nature and abundance of the fossils in that strata. Thus fossils can be used to identify geologic relationships. (3) Fossils may provide information about the climate and environment of the site where they were deposited and at the time of their preservation. (4) Fossils are useful in the exploration for minerals and mineral fuels. Geologists can find out the presence of oil and natural gas deposits at a place by analysing the microfossil samples obtained from these locations. |
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