Fossils : Fossilization, Evolutionary significance of fossils, List of Important fossils

The fossils are the preserved remains of animals, plants or their parts found in various strata of earth. Fossils may be of entire organisms or a part which got buried, a mould or cast, foot prints or imprints of a leaf on a stone.

Fossilization :- Fossils can result by several methods. However these meth-ods are purely accidental. There are many methods of fossilization.

1. Petrifaction :- It is the commonest method. In this method dead and buried organisms turn into stones. This is due to formation of sedimentary rocks un-der water. While the soft parts disappear due to decaying, hard parts get pre-served due to mineralisation. The preservation happens in the original strata.

2.  Petrifaction of soft parts :- Under certain conditions muscles and other soft organs may get mineralised and form rocky fossils. Several such plant fossils had been obtained.

3. Preservation of foot prints :- Moving animals on soft mud can leave foot prints. These prints, if left undisturbed, can get hardened and form rocky fos-sils. A study of such imprints can provide clues regarding the body form and characteristics of the extinct animal.

4.  Moulds and casts :- Fossilized moulds are found in volcanic ashes. Sev-eral invertebrate fossils had been obtained as moulds. They provide details about the exact physical features of the animal.

5.  Fossilization in resins and amber :- Normally, insects get entangled in soft sticky secretions of trees called resin. The dried material can get fossilized. These fossils can even reveal the colour of the organism.

6.  Preservation in ice :- Entire animals can get frozen and may be pre-served in ice. In such fossils the body parts remain intact without change. Fossils of woolly mammoth from Siberia are classical fossils of this nature.

7. Dating of fossils :- The age of the fossils to a large extent can be assessed accurately, using radioisotope method. It is known that all elements exist as isotopes. Isotopes are atoms having slightly different atomic weights. e.g c12 and c14. Some isotopes remain unstable. They undergo decay and release sub atomic � and � particles till they become more stable. Examples are

Uranium238    �          lead206

Potassium40    �          Calcium40+argon39

Carbon14        �          Carbon12

The time required for the breakdown of half the given quantity of unstable isotope is called the half-life. The half-life of many isotopes is useful to assess the age of the rock accurately by measuring the ratio of unstable : stable nuclides. Eg. c14 : c12 of the rock in which the fossil was collected.

Evolutionary significance of fossils

1. Fossils tell us the full story of evolution. Fossil studies reveal the course of evolution.

2. Through fossils the origin and evolution of specific groups of organisms can be understood e.g. Horse evolution.

3. Fossils provide us clues regarding climatic conditions of various prehistoric periods.

4. Study of fossils simplifies phylogenetic discussions.

5. Some fossils like woolly mammoth can provide vital clues regarding geneti-cal make up.

Important fossils

Ichthyostega - interconnecting link between fishes and amphibians.

Seymouria - Interconnecting link between Amphibians and Reptiles.

Archaeopteryx - Ancestral form of birds

Dinosaurs - Extinct group of reptiles.

Hyracotherium - Early ancestor of horses.