Palaeobotany, Fossils and their types, Process of fossilization
Paleobotany:-
> Paleobotany is derived from Greek words paleon that means “old” and botany the study of plants. It is the branch of paleontology that deals with recovery and identification of plant remains of geological past.
> Kaspar Maria Von Sternberg:-
- He is the “Father of Paleobotany” (1761– 1838) was born in Europe.
- He established the Bohemian National Museum in Prague and is deemed to be the founder of Modern Paleobotany.
> Birbal Sahani:-
- He is the “Father of Indian Paleobotany” (1891–1949).
- He presented his research on two different areas of Paleobotany
i. The anatomy and morphology of Paleozoic Ferns.
ii. The fossil plants of the Indian Gondwana Formations.
Fossils and their types:-
> A plant fossil is any preserved part of a plant that has died long back. Fossils may be a prehistoric impression that may be hundred to millions of years old. Majority of the plant fossils are disarticulated parts of plants, it is rare to find plants to be preserved as whole.
> Importance of fossils:-
i. They throw light on phylogeny and evolution of plants.
ii. Fossil plants give a historical approach to plant kingdom.
iii. Fossils are useful in classification of plants.
iv. Fossil plants can be used in the field of descriptive and comparative anatomy.
> 8 Types:-
i. Petrified Fossils:-
- The word petrifaction means turning into stones. The fossils form when minerals replace all or the parts of the organisms. Water is full of dissolved minerals. It seeps through the layer of sediments to reach the dead organism. When water evaporates only the hardened, materials are left behind.
- There is molecule by molecule replacement of plant parts by minerals such as iron, pyrites, silicates, carbonates, sulphates etc. These minerals get deposited and impregnated inside the cells and the tissues of the plant. This type of fossil can be studied by preparing the sections and are most suitable for the study of structural details. Petrified plant organs roughly spherical in shape are known as coal balls.
ii. Molds and Casts:-
- A mold forms when hard parts of an organism are buried in the sediment such as sand, silt or clay. The hard part completely dissolves overtime, leaving behind a hollow area of organism shape.
- A cast forms as a result of the mold. Water with dissolved minerals and sediments fills the mold’s empty space or cavity. The cavity is known as incrustation and the mineral sediments that are left in the mold make a cast. A cast is opposite to its mold. These fossils are suitable for the study of the morphology of fossil plants.
iii. Carbon Films:- All living things contain an element carbon. When an organism dies and is buried in sediment, the materials that make the organism break down and eventually only the carbon remains. The thin layer of carbon left behind can show an organism’s delicate parts like leaves or plant e.g. fern fossil 300 million years old.
iv. Trace Fossils:- These fossils show the activities of the organisms. An animal makes a foot print when it steps in sand. Overtime the foot print is buried in layers of sediment. Then the sediment becomes solid rock.
v. Preserved Remains:- Some organisms are preserved in or close to their original states. These fossils are called preserved remains e.g., an organism such as an insect is trapped in a tree’s sticky resin and dies. More resin covers it sealing the insect inside. It hardens into amber. Some organisms such as a wooly mammoth dies in a very cold region. Its body is frozen in ice which preserves organism even its hair.
vi. Compression:-
- This type of fossil is common in the sedimentary deposits of rocks. It is a sort of impression where most of the organic remains of the plant remain in the fossil state. The plant or plant part gets buried and the sediments go on accumulating over the plant.
- The growing pressure of the sedimentary rocks removes the air and the watery contents of the fragment out and causes the plant tissue to compress. The compression shows the original outline of the plant or plant parts but the original thickness of the plant material cannot be determined. The buried part becomes flat due to compression or overlying pressure of the sediments.
vii. Impression:- These fossils are just impression of plants or plant parts on sediments. These fossils are useful in studying the external features of various plant parts and venation pattern of leaves.
viii. Pseudofossils:- Sometimes watery solutions of various minerals speed through the sediments and it takes the shape of some plant part or animal. Their study shows that they are neither plants nor animals. Such fossils are called pseudofossils.
Process of fossilization:- The process of formation of fossil in the rocks is called fossilization.
Common methods of fossilization includes petrifaction, molds and cast, carbonization, preservation, compression and infiltration.
i. Petrifaction:- Minerals like silica slowly penetrate in and replace the original organic tissue and forms a rock like fossil. This method of fossilization can preserve hard and soft parts. Most bones and wood fossils are petrified.
ii. Mold and Cast:- A replica of a plant or animal is preserved in sedimentary rocks. When the organism gets buried in sediment it is dissolved by underground water leaving a hollow depression called a mold. It shows the original shape but does not reveal the internal structure. Minerals or sediment fill the hollow depression and forms a cast.
iii. Preservation:- Original remains can be preserved in ice or amber (tree sap). They protect the organisms from decay. The entire plant or animal is preserved.
iv. Compression:- When an organism dies, the hard parts of their bodies settle at the bottom of the sea bed and are covered by sediment. The process of sedimentation goes on continuously and fossils are formed.
v. Infiltration or Replacement:- The precipitation of minerals takes place which later on infiltrate the cell wall. The process is brought about by several mineral elements such as silica, calcium carbonate and magnesium carbonate. Hard parts are dissolved and replaced by these minerals.
Determination of age of Fossils:-
> The age of fossils is determined by radioactive elements present in it. They may be carbon, uranium, lead or potassium. It is used in paleobotany and anthropology for determining the age of human fossils and manuscripts.
> Radioactive carbon(C14) dating method:- This method was discovered by W.F. Libby (1956). Carbon consumption of animals and plants stops after death and since then, only the decaying process of C14 occurs continuously. The time passed since death of a plant or animal can be calculated by measuring the amount of C14. present in their body.