FOSSILIZATION AND TYPES OF FOSSILS

                   FOSSILIZATION

A fossil in general, may be defined as remains of plants and animals, which lived in remote pass, in many cases,are preserved in the rocks in one form or the other.

The study of fossils is known as palaeobotany. 

The process of preservation of living beings Or their parts in the form of fossils is known as fossilization. The process of fossilization is a continuing process. 

Two theories have been proposed to explain the fossilization process in plants. 

1 Replacement theory= According to this theory, fossilization takes place by the replacement of the molecules of the original substances of the plant one by one by the molecules of minerals in soil solution. The replacement occurs due to hydrolysis or weathering of the organic substances present in the plant body. Several known fossils show such type of replacement. 

2 Infiltration theory= According to infiltration theory, fossilization takes place as a result of infiltration and precipitation of minerals through the cell wall. After burial, the plant body undergoes partial disintegration,and the free carbon released in this process forms carbonates by reacting with infiltrated calcium, magnesium, etc. Fossilization by infiltration depends upon the disintegration of organic substances present in the cells. 

Types of Fossils:-

1 Compression fossils= These are perhaps the most common type of fossils. As the name indicates, these fossils are formed as a result of burial of plants or its parts in sediments. The buried parts become flat due to compression or overlying pressure of the sediments. Sand stone, diatomaceous earth and volcanic ash are some of the materials that cause compression. However, if the compressed plant organ is thin and flat ( e. g., leaves) , it does not show any apperciable change in its shape. 

2 Petrification fossils= 



Those fossils which preserve both external form and internal structure are called petrifications. In this process, every cell of the plant tissue is preserved and perfectly distended as if it is living. There is molecule - by - molecule replacement of plant parts by some 20 minerals ( in the form of carbonates, sulphates, silicates, phosphates, etc.) . 

3 Incrustation fossils= 

These are the commonest types of fossils, suitable for the study of morphology.of fossil plants. In these fossils, external form of the plant is preserved as a cast, but the internal structure is destroyed. 

4 Impression fossils= 

These fossils are just impression of plants or plant parts on sediments. Hence, they do not contain organic materials as in compressions. Impressions formed in fine and soft materials show better details. 

5 Coal balls= 

Petrified plant organs of roughly spherical shape are known as caol balls. Most of the fossils present in the coal occur in this form. Coal balls are formed by infiltration of calcium carbonate, magnesium carbonate, iron sulphide, etc., in buried plant parts. 

6 Amber=

It is a resinous excretion of certain fossils coniferous trees which flowed due to injuries caused by boring insects or from the decaying branches. 

7 Actual remains=

Such plant fossils are of relatively young age. Their complete decomposition has been prevented by low temperature and absence of air ( i.e., vaccum). Usually materials such as cuticle, spore walls, etc., which show remarkable resistance to natural decomposition, are preserved in this form. 

8 Pseudofossils= 

Sometimes rocks take the shape of sone plant part or animal during their formation and as such give an illusion of fossils. Their detailed studies, however, clearly reveal that they are not plants or animals but are mineral depositions mistaken for plant or animal remains. Such specimens are known as Pseudofossils. 

 Determination of Age of Fossils:-

Two general methods;

1 Uranium/ Lead method= In this method uranium 235 is a radioactive element. It gradually decays and get converted into an isotope of lead 206.The rate, if in a given sedimentary rocks, both the 235u and 206 Pb are present, the age of rock and the plant present can be calculated by measuring relative quantities of radioactive isotopes and the stable isotopes. 

2 Carbon dating Method= In this method 14C isotope is used in estimating the age of the specimens. As carbon is the basic Cosituent of plant life. There are three forms of carbon occurring in living plants. Carbon 12 (99%); Carbon 13 (1%) and Carbon 14(less then 1%). Carbon 14 isotope occur in the nature is constantly produced in upper atmosphere due to bombardment of nitrogen 14 by high energy cosmic rays. Upon deaths C-14 in tissues begins to decay. By measuring the remaining amount of Carbon -14 in the fossilized rocks, age of fossils can be determined. 

Importance of fossils:-

1 Fossils throw light on phylogeny nad evaluation of plants. 

2 Fossils are helpful in classification of plants. 

3 Fossils can be used in the filed of comparative anatomy. 

4 Fossils plants give a historical approach to plant kingdom. 

5 Fossil interpret how earth has changed over time. 

6 They also provide missing link in the evolutionary chain. 

Submitted to - Mrs. Nisha Mam

Submitted by- Vartika Sharma

Semester- 2nd

Roll no. - 89

Section- A

College - Govt. Degree College , 

                      Kathua.