Explain the three major categories in which fossilization occur ?

Explanation

The three major categories of fossil formation are :
(i) (a) The original hard parts such as bones, teeth or shells of an organism are preserved as such .
(b) The hard parts of marine animal such as bones, shells etc., preserved under sediments of the ocean.
(c) Salinity of ocean prevents the deterioration of those parts .
(d) The sediments of the ocean become hardened to form definite layers or strata.
Eg: Woolly mammoth that lived 22 thousand years ago were preserved in the frozen coast of Siberia .
(ii) Petrifaction : Petrifaction is a process of fossilization, in which the original body of the animals, when they die, may be replaced by minerals and original substance of the body being lost thorugh disintegration . Iron pyrites, silica, calcium carbonate and bicarbonates of calcium and magnesium are the main minerals involved in petrifaction .
(iii) Natural moulds and casts : The moulds are the impressions caused by the animal in soft mud, which become hardened into stones. The hallow portion of the hardened impressions are filled by hard minerals and get fossilized. This is known as cast . Coprolites are the hardened faccal matter of animals as tiny pellets. This enables to understand the nature of food on which the prehistoric animals thrived .