Explain the three major categories in which fossilization occur ?

Explanation

Fossilization is the process by which plant and animal remains are preserved in sedimentary rocks. The fall under three main categories.
(1) Actual remains
(i) The original hard parts such as bones, teeth or sheels are preserved as such in the earth.s atmosphere.
(ii) This is the most common method of fossilization. When marine animals die, their hard parts such as bones, sheels, etc., are covered with sediments and are protected from further deterioration.
(iii) The salinity in ocean prevents decay.
(iv) The sediments become hardened to form definite layers or strata. For example, Woolly Mammoth that lived 22 thousand years ago were preserved in the frozen coast of Siberia. (2) Petrifaction
(i) When animals die the original protion of their body may be replaced molecule for molecule by minerals and the original substance being lost through disintegration.
(ii) This method of fossilization is called petrifaction.
(iii) The principle minerals involved in this type fossilization are iron pyrites, silica, calcium carbonate and bicarbonates of calcium and magnesium.
(3) Natural moulds and casts
(i) Even after disintegration, the body of an animal might leave indelible impression on the soft mud which later becomes hardened into stones. Such impressions are called moulds.
(ii) The cavities of the moulds may get filled up by hard minerals and get fossilized, which are called casts.
(iii) Hardened faecal matter termed as coprolites occur as tiny pellets.
(iv) Analysis of the coprolites enables us to understand the nature of diet the pre-historic animals thrived on.