Fossils are remains of animals and plants that existed in past geological periods. These remains have been preserved in the earth crust for millions of years. The word fossil comes from the Latin word fossilis, which means 'dug up' signifying that many fossils are found by digging the earth. They can vary in size from microscopic to the size of dinosaur. The fossil remains give us plenty of information on history of life on earth, movement and behavior of animals, geological history and other subjects.
Fossils exist in various forms. The skeletal structures we see in fossils are usually hard rocks that have preserved the shape of the skeletal parts. The impressions of the animals and plants, such as footprints are also preserved in fossils. Frequently these impressions are deformed and vary in size and shape from the original impressions. In rare cases parts of bones, teeth, shells, or softer body tissues are present in the fossils.
Formation of Fossils
There are certain conditions for the formation of fossils. For this process to start the body of the organism is prevented from extensive decay which occurs naturally in the atmosphere. Before substantial decay can take place the body sinks in mud or water, or is covered by sand, thereby obstructing direct contact with atmosphere. Scavengers feed on the softer parts of the body leaving behind harder parts such as bones, teeth and shells. Gradually more layers of soil and silt are deposited on top and the physical remains of the organism are buried deeper and deeper.
The hard part of the organism buried in sediments may be subjected to various changes. The pores in bones and shells are filled with mineral infused water. The minerals are deposited and fossilize the remains. This process is called permineralization.
In other cases the original skeletal material is totally replaced by mineral matter, a process known as mineralization. The fossil has same shape as the original object, but is actually rock. This process also results in loss of original color.
Majority of fossils are preserved in water environment. The dead organism falls to the floor of the water body and gets covered by silt and sand. The decay process is much slower in water. On the other hand the remains on the land are more easily destroyed due to ravaging by scavengers, chemical actions and erosion.
We have seen that essentially two conditions must be met for organism to be preserved in fossilized form - rapid burial to retard decomposition and destruction, and possession of hard parts that can be fossilized.
Types of Fossils
There are different types of fossils depending on the way they were formed and preserved. We already know about fossils created by permineralization and mineralization. There are cases where an organism is preserved in unaltered condition. For example insects trapped in tree sap can get preserved in unaltered condition when sap turns into amber. The amber resists chemical and physical attack on the trapped insects for a very long time.
Carbonization or coalification is another process in which all elements except carbon are removed from the body of the organism. Fossil moulds and casts are formed when organisms have completely rotted away leaving behind only the impressions.
These were examples of fossilized body parts. There are other types of fossils called ichnofossils, which are fossilized traces of the movement of animals, such as footprints, tooth mark, dung and nest.
Recovery of Fossils
All fossils are formed and preserved deep inside the earth crust. Over a period of time the lower layers of the earth crust are pushed up making it easier to find fossils in those layers. Many fossils are exposed when top layers of soil and rocks are removed by erosion. At many places people have found fossilized skeletal remains of large animals.
A lot more fossils are found and recovered by digging. After a fossil is found it has to be removed from its rocky matrix in which it had been lying for thousands or millions of years. There are specialized tools for digging and recovering large and small fossils. The fossils have to be handled delicately to avoid any damage. Very often the fossil is removed along with its rocky case from the site and taken to the lab where final recovery is done.
Why Study Fossils?
There are several reasons why study of fossils is important. They give information about the evolutionary process, the changes in life forms that have occurred progressively, and the movement and behavior of extinct life forms. The so-called index fossils help geologists in quickly assigning the age of the strata where fossils were found. The fossils also provide information on the climate and atmosphere of the site where they were deposited. In addition, they help in exploration of minerals and mineral fuels.
Our geological and evolutionary history is preserved in the fossils.
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