Paleobotany

 

Fossils are the remains of the plants and animals that lived in the past millions years ago.
The science of the study of fossils is called paleontology. The word paleontology is derived from the Greek Palaeso- ancient, Orta- existing things and Logos, to discourse or study.
The study of fossil plants is called Paleobotany.  It reveals the evolutionary history of plants in the past. It deals with the morphology and anatomy of the plants which were there in past geological time, their bioactivities, geography, ecology etc.
Prof. Birbal Sahani is known as Father of Indian Paleobotany. And he established the Paleontological Society of India in Lucknow.
Fossils
Any body or traces of body, animal or vegetable buried and preserved by natural causes is called Fossils. The word fossil is derived from the latin word fossilium which literally means anything dug out of the earth.
Fossil may be entire organism which got buried in the remote past, a mould or cast of the entire organism or its parts or their replica, foot prints.
Process of Fossilisation:
The process of preservation of living beings or their parts in the form of fossils is known as fossilization. This is a continuous and slow process.
The terrestrial animals or plant either die in the sand near the stream or in a stream bed and soon become buried in the sediment and water. Their soft parts gradually degenerate and the bones are gradually replaced by mineral matter molecule by molecule.
Generally acidic or cold water and low oxygen concentration create adverse conditions for microbial decay. Eruptions of volcano, the animals and plants found in its vicinity are buried in its ashes. Petroleum springs, amber and resins from the trees, sand and ice also preserve the organisms unchanged. Due to pressure of heavy sedimentation, fossils become highly compressed. .

Types of Fossils:
1. Petrifaction:
Petrifaction is the commonest method of fossilization. They are considered to be the best type of fossils. The river water seeps sand and mud from the land and deposits in the sea, where these sink to the bottom. When an aquatic organism dies or a terrestrial animal accidentally drowns , may be by chance they get covered quickly by sediments.
After the burial, the soft parts of the body like muscles, nerves and internal organs gradually decay and are carried away by the seepage of water. The organic matter in bones, teeth or shells of animals and leaves, trunks and roots of plants are preserved.
Within the sediments layers, the hard parts may retain much of their original composition, but their organic constituents disintegrate leaving the structure porous. Water seeps the interior of these porous structures and mineral matter dissolved in water is slowly deposited in the pores. Thus organic substance of hard parts is replaced particle by particle with silica or lime dissolved in water.

The process proceeds so gradually that even the outlines of cellular structure and almost all the histological details are preserved.
The fossils preserved by petrifaction are called petrified fossils. Both external and internal structural details are preserved in these fossils. By the process of the petrifaction hard parts like bones and teeth of vertebrates, shells of molluscs, exoskeleton of arthropods are preserved as fossils.

Coal balls are one type of petrified fossils. They occur in chunks of coal. Each ball is a mass of carbonate of calcium and magnesium and iron sulphate. These balls show delicate plant parts that remained intact.

Compression:
They are formed when the whole plant or part of a plant gets buried under sediments of different types. As a result of the weight of these sediments the plant part becomes flat resulting in the formation of a carbonaceous film revealing the outlines and external features of the plant or plant part.
A very little or no cellular detail is preserved in such fossils. They usually reveal the external characters of the plant or its part.
The pressure results in driving out all water and air present in the tissues. In curse of time the organic substances of the plant like cellulose and lignin become coal where as the carbohydrate change into hydrocarbons. Cuticles and spore coats remain unchanged.
Coal is also a kind of compression formed by an extensive accumulation of plants and plant parts, in body of water, that is gradually compressed.. Spores, pollen grains, cuticle, bark fragments, are the common plant parts preserved in coal.
Fossil plants from carboniferous period are compressed fossils.

Impression or Imprints: Plant or plant parts leave prints on coming in contact with soft clay. It is just as we put our foot on a wet clay. In course of time impression becomes permanent, when the clay turns into stone.
They are aptly regarded as negatives of the compression. Such fossils only reveal the outline of a plant or its part. The cellular details are not preserved.

Incrustation Fossils
Moulds
Naturla moulds are formed by the hardening of material surrounding the buried organisms. Their body disintegrate and are removed by seepage of the ground, leaving hollow cavities which are filled with other sediments. These form the moulds and depict the exact external features.
The soft animals like jelly fishes, wings of insects, leaves, etc., are usually preserved by this methods.

Casts:
During compression sometimes the plant material within decay and disintegrates leaving a space which is filled with other sediments. These filling sediments harden to form a cast. The cast has the same external appearance as that of plant or its parts and reveal only the external appearance in outline.
Fossil of Pompelli city which was buried by volcanic ash from Mount Vesuvius in 79 AD, present moulds and casts of men and their domestic animals.

The casts and molds help us reveal or reconstruct the external features of the plants. They do not preserved the organic material and cellular details.

Amber
Usually insects get entangled in sticky secretions of trees, like resin secreted from the coniferous tress. On exposure, resin hardens and changes to amber, and with it the entangled insects are preserved forever. Such fossils are so perfectly preserved that their colour and even the minutest histological detals are as clear as they are seen in freshly fixed specimens.

Importance of Fossils:
1. Fossils provide us the information about the organisms that lived on Earth from the time of the oldest fossils.
2. Fossils help us to understand the past climates, including ice age and periods.
3. Fossils are preserved in the different strata of the rocks and helps in the estimation of the age of the rocks. The fossils are the marker elements to understand the age and arrangement of soils layers are called ‘Index fossils’. This knowledge is useful to prepare the geological time scale.
4. Study of microscopic fossils in rocks help in the discovery of oil and gas reserves.