What are Fossils and How They Form.

Fossils are remains or impressions of ancient animal or plant which have been preserved within the sedimentary rock. Fossils may comprise the remains of the complete animal but that is very rare. Such fossils chiefly include insects preserved in amber, animals in asphalt, and mammoths and other mammals frozen in ice. But such fossils are of rare occurrence and are recent origin.

Conditions Favorable For Preservation.

We know that millions of animals and plants had lived, died and were destroyed without leaving trace. But it has been observed that two factors are favorable for the preservation of the organism as fossils:

  • The possession of hard parts such as shells and bones, and
  • Quick burials of the remains by different processes to prevent destruction by scavengers and decay.

Any animal or plant satisfying the above two conditions can be preserved as fossils under normal conditions. The condition in which fossils occur depends on their original composition and on the material in which they are assembled.

Types Of Fossils

Fossilization may occur in several ways. Sometimes the soft parts remain unaltered in fossilization; sometimes only hard parts remain unaltered and sometimes the hard parts also altered.

Unaltered-soft-parts (Actual remain).

In such cases the whole of the organism, including its soft parts is preserved as such. It may be possible due to entombment of the animals under a thick cover of ice. Sometimes insects become entangled in soft and sticky secretions (resin) of trees. On exposure, this hardens and changes to amber and with it the entangled are perfectly preserved.

Unaltered hard parts.

Shells and internal skeletons are frequently preserved for long periods of the time. Many of the best of these are fossils of marine animals that fell into the soft sediments on the sea-floor when they died. Of the land dwellers, those that live near swamps, lakes or sea are most likely to be preserved. Corals, mollusca and Protozoan are example of this category.

Fossils of bivalves
Fossils Of Bivalves in Limestone.

Altered hard parts.

In this case, actual remains of an organism are likely to undergo changes through the time. These changes are fostered by the slowly circulating ground waters that carry elements in solution. It includes the following processes:

 1. Petrifaction:-  It is a slow process which involves removal in solution, of each individual molecule of the material constituting the hard parts and simultaneous precipitation of an equivalent quantity of the replacing mineral. This molecule by molecule replacement of one substance by another helps in preserving even the most delicate organic structures, as such. In this manner the bones, shells or plant tissues transformed into calcite, silica or pyrite and the processes are known as calcification, silicification and pyritisation respectively.

petrified wood
Petrified Wood

 2. Carbonation:- In this process, the organic is decomposed and it loses nitrogen, oxygen and other volatile constituents, as a result, it is enriched in carbon and is said to have been carbonized. Coal seams are the carbonized remains of plants.

 3. Moulds and casts:-  Sometimes, the hard parts preserved within the accumulating sediments, may be totally removed in solution. As a result, hollows are left within the rock beds which are called moulds. Porous and permeable rocks bed are able to have seepage of sub-surface water can remove in solution the shells of organism; facilitate the formation of the moulds. When the moulds are filled up subsequently with mineral matter, it is known as cast. Thus casts only
retain the external form of the hard parts.

 4.Impression:- Plants and animals devoid of hard part do sometimes leave a record for their existence, in the form of imprints within the rock beds. Impressions of leaves, feathers of extinct birds are the examples.

 5.Tracks and trails:- While moving on soft and damp ground, the footprints of trail of the animals are entombed in mud and when this ground hardens into a rock the footprints present are preserved. Even though they do not from any part of the animal, yet they are regarded as fossils.

Uses of fossils:

  • The studies of fossils provide evidence in favor of organic evolution and migration of plants and animals through ages.
  • They help in establishing the geological ages of rock beds and their correct order of succession in any area.
  • In the reconstruction of palaeogeography of earth, fossils are paramount significance.
  • Fossils also help in ascertaining the palaeoclimate.
  • In correlating rock beds of one area with those of another.
  • The relics of ancient life can be used to establish the time sequence of sedimentary rocks.
  • Fossils provide evidences for “Ontogeny recapitulates Phylogeny”. Ontogeny means the
    development of the individual, recapitulates means brings and phylogeny means the race history.This is obvious as the fossils include the ancestors of modern forms.
  • The study of fossils constitutes an integral part of any investigation leading to the discovery of new deposits of coal and petroleum.
  • The study of fossils animals and plants is of the highest importance to the biologist not only because they include the ancestors of modern species but because among fossil forms we find many groups which often throws light on the relationship of existing animals and plants.


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