Fossils: Meaning, Formation and Types

In this article we will discuss about Fossils. After reading this article you will learn about: 1. Meaning of Fossils 2. Formation of Fossils 3. Types 4. Various Names Assigned 5. How are Fossils Studied? 6. How are Fossils Reconstructed and Named? 7. Determination of Age.

Contents:

1. Meaning of Fossils
2. Formation of Fossils
3. Types Various Names Assigned to Fossils
4. How are Fossils Studies?
5. How are Fossils Reconstructed and Named?
6. Determination of Age of Fossils

1. Meaning of Fossils:

Remains or vestiges or traces of plants and animals of the past are called fossils. These remains of organisms from past geological ages remain preserved in sedimentary rocks either as actual structures or as impressions, casts or molds.

The word ‘fossil’ is derived from the Latin word “fossilis” which means “to dig up”. In the earlier studies, therefore, a large number of things dug out of earth’s crust were called fossils. These things also included minerals and rocks besides remains of plants and animals. Later on, however, study of fossils were made restricted to only animals and plants.

Study of fossils is of great importance because:

(i) They furnish evidence of the prehistoric life, and

(ii) They also provide missing links in the evolutionary chain.

Plant fossils are rarely as well preserved as animal fossils because their tissues normally do not contain calcified structures. They are usually, therefore, completely decomposed before the process of fossilization.

The fossils or remains of large or macroscopic structures, such as leaves, branches, fruits and seeds, are called mega fossils while those of very small or microscopic structures (e.g. spores, pollen grains, etc.) are termed as microfossils.

The species of plants or animals which no longer exists is called extinct. On the other hand the species which exists at present is called extant. If a fossil cannot be assigned to any genera containing extant species then its genus is termed as an organ genus. Similarly, if it cannot be assigned to a family, it is placed in a form genus.

Study of fossils is now an established science. It has helped in the construction of phylogenetic classification schemes. It has also thrown light on how some of the complex structures of extant plants and animals have evolved. A full-fledged research institute, devoted fully to the study of fossil plants, now exists in our country. It’s name is Birbal Sahni Institute of Palaeobotany, Lucknow.

2. Formation of Fossils:

In the basic process of fossilization, the physical part of any plant or animal must be buried within a well-protective matrix in the crust of the earth. This matrix in the earth’s crust is usually sedimentary. The sedimentary environment of this kind can be of several types such as lake, stream, inland sea or estuarine, etc.

In several cases it has also been observed that diatom frustules also get incorporated in deposits of deep sea basins. In rare cases, the sedimentary environment is in the form of volcanic deposits or other subaqueous conditions. The portions of the organisms (plant or animal) preserved in sediment become stony or lithified during course of time.

How Are Sedimentary Rocks Formed? Accumulation of rock particles results in the formation of sedimentary rocks. Weathering and mechanical abrasion of existing rocks take place and give rise to the rock particles.

Chemical weathering and flooding also help in the formation of these particles. These rock particles or sediments accumulate and water is squeezed out of them. During course of time, this makes them much more compact or rocky structure. Such a rocky structure is called sedimentary rock.

Some other conditions which favour fossilization include:

(i) Anaerobic conditions,

(ii) Low pH,

(iii) Forest fires in the form of fossil charcoal, and

(iv) Presence of sedimentary materials such as carbonates, silicates, salts of iron, etc.

3. Types of Fossils:

(i) Sedimentary Rocks (e.g. Coal):

Majority of plant materials are preserved as fossils in sedimentary rocks. Coal is the best known example of sedimentary rock. Sediments of plant origin are crushed by overlying pressure and form coal. Present coal belt in the world, therefore, represents dense forests of the world of earlier times. Least metamorphosed coal shows maximum details of fossilized or preserved plant material.

Therefore, lignite’s (early stage of coal formation) carry less crushed plant parts and their details can be studied easily. Plant parts get excessively crushed in bituminous coal and anthracite coal because they show more degree of metamorphosis than lignite coal. Bituminous coal is of great importance in the study of palynology because pollen grains are best preserved in this type of coal.

(ii) Amber:

The fossilized plant resin secreted by coniferous trees that grew in very early times is called amber. This “very early time” in the geological past ranged from Carboniferous (i.e. about 345 million years ago) to Pleistocene (i.e. about 2.5 million years ago).

Fungal spores, pollen grains, etc. were trapped in this resin before fossilization. The resin fossilized into amber and inside this were left spores, pollen grains, etc. Amber is, therefore, an example of fossils within fossil.

(iii) Diatomite:

Diatoms are unicellular algae belonging to class Bacillariophyceae. Their walls have silicon deposits. The sedimentary rock formed by the remains of diatoms is called diatomite. In due course of time, diatoms keep on depositing at the base of sea, oceans or lakes and form sedimentary rock.

(iv) Pseudo-Fossils or Dendrites:

Pseudo-fossils or dendrites are completely inorganic structures of various types. They often resemble plant organs. Their formation takes place by the deposition of minerals due to seepage or percolation of water in rock crevices. They superficially resemble leaves of ferns.

(v) Mummification:

The process of the formation of fossils in ice-frozen environments in the polar regions is termed as mummification. The moisture of the tissue of the organism gets converted to very small or microcrystals of ice. It is almost a process similar to deep freezing.

(vi) Biochemical Fossils:

These are the fossils which consists of chemical substances like chlorophyll, amino acids, aromatic acids, flavonoids, branched hydrocarbons and steroids. These have been reported to be present either in the fossilized remains of organisms or in the rocks. Niklas (1981) reported biochemical fossils of the substances related to sporopollenin, lignin, cutin, cellulose, etc.

4. Various Names Assigned to the Fossils:

On the basis of the type of fossilization, various names have been assigned to the fossils. Some of them are discussed below:

(i) Impressions:

This is a type of fossil in which impression or negative replica of the plant part is clearly visible on the rock upon splitting. The surface of the plant part involved is visible in this type of fossil. The entire shape of the organ is clearly visible in an impression but cellular details are not visible. During the course of the formation of impression type of fossil there is no involvement of the organic matter.

(ii) Molds:

As the name indicates, molds are formed by three-dimensional structures, dissolved by seepage of ground water, leaving a hollow cavity in the rock. Such hollow cavities resemble with the original organ in size and shape. External features of the plant in three-dimensional view are best seen and studied in molds, e.g. ornamentation of seeds and fruits. The process of mold formation is similar to modern day sculpture making.

A sculptor makes the “original” with wax or wood and prepares the “mold”. At the time of preparation of mold, the original statue of wax or wood is removed from inside. The molten material is now poured into the mold to make statue of this material.

Exactly in the similar fashion, molds are formed in the conditions when, prior to the crushing of plant part, the sediment surrounding it hardens. During course of time, the internal plant material dissolves, and the hollow of the sediment is the “mold”.

(iii) Casts:

Decay of tissue in an organism results in the formation of a hollow. When this hollow gets filled with mineral matter i.e. sediments, it results in the formation of a cast. An exact replica of the original plant material is thus resulted.

Like molds, there is also no involvement of the actual part of the plant in casts. Fruits, hard seeds and tree trunks are commonly fossilized as casts. In casts also, a three-dimensional view of the organ is seen similar to molds.

(iv) Compressions:

When bulk of the plant material gets compressed in layers of sediment, the fossils are called compressions. By the laying of the additional sediments from above, water squeezes out of the parts of organisms, and this makes them more compact and flattened. Ultimately, a thin carbonaceous film remains in the compression fossil, and this corresponds to the original outline of the parts of the organisms.

Differing from impression type of fossils (in which no cellular details can be seen) some cellular details can be seen in compression type of fossils. These cellular details include epidermal hairs, cuticularized epidermal cells, spores, etc.

If compression type of fossils are formed in low pressure and low heat, some more cellular details (e.g. plasmodesmata, chloroplasts with grana, nuclei with chromatin, micro-fibrillar organization of cell wall, etc., may be observed, although rarely. Much of the organic matter of the plant is preserved in compression type of fossils.

(v) Petrifactions:

Petrifactions are formed when parts of the plant are completely submerged in water reservoirs containing dissolved minerals. In the process of their formation, several types of soluble minerals infiltrate the cells and intercellular spaces replacing the water and organic molecules. The soluble minerals include carbonates, silicates and iron compounds.

The minerals surrounding the cellular remains precipitate and form the rock matrix. This precipitation is resulted due to gradual evaporation of moisture. A state of super-saturation is resulted. In this way a petrified fossil is a mass of plant tissue infiltrated with the hardened mineral substances so that a large part of the internal structure is preserved.

It is like that of a process of embedding the plant material in paraffin wax in the laboratory. The air and water in tissues and cells are replaced by the impregnating material in a petrifaction. The cellular details are much better preserved in petrifaction than in a compression.

5. How are Fossils Studied? 

(i) Impressions:

These are studied generally by preparing artificial casts which are also called positives.

(ii) Molds:

These are studied exactly like that of impressions.

(iii) Compressions:

These can be studied either by transfer technique or by maceration in Schulze’s solution, or by first softening and then cutting thin sections by microtomy. Microtomy is done by embedding the material in plastic.

(iv) Petrifactions:

Petrified materials are studied by peel section method. In the earlier days, palaeobotanists used to study fossils by very hard and time-consuming techniques. They used to cut thin sections of rock containing fossils by a circular toothless saw. Periphery of this saw had diamond particles.

Etching technique or peel method was then discovered In this method polished surface of the rock is etched by sulphuric acid or nitric acid. This surface is then flooded with acetone and a film of cellulose acetate is rolled down.

After some time, when the film is dry, it is removed from the surface of specimen. A clear impression of specimen develops on the film. By definite methods, it is made permanent and studied under microscope.

(v) Some Modern Methods of Studying Fossils:

Fossils are now studied in modern laboratories by using transmission and scanning electron microscope, interference microscope, phase contrast microscope and methods of X-ray analysis.

6. How are Fossils Reconstructed and Named?

Fossils of the entire plants have only rarely been reported. Generally an organ or a part of the plant is seen preserved as fossils. Reconstruction of the fossils of various vegetative and reproductive parts in the form of a single plant is, therefore, a very big problem of the palaeobotanists.

A detailed scientific knowledge is required to reconstruct a plant. Besides this, some direct or indirect evidences are also applied to conclude that the organ belongs to a particular plant.

Some Factors Which Help in Reconstruction of Plant Fossils:

Some of the factors, on the basis of which the fossils are reconstructed, are under mentioned:

1. Discovery of an organic connection between the fossils of two parts. It acts as a direct evidence to conclude that these two parts belong to the same plant.

2 Structural similarity between the fossils.

3. Regular occurrence of the same type of fossils in the same area.

4. Ovules of different fossils containing similar type of pollen grains.

5. Relationship between different fossils is also assigned on the basis of pollination.

Concept of Form Genus or Organ Genus:

Separate generic names are given by the palaeobotanists to the fossils of the detached organs or fragments. Each of these organs or fragments is called a form genus or organ genus. Similar to the present day living plants, binomial system of nomenclature is also applied to name these form genera.

Considering the organ genera of Pentoxylales as an example, various organs of the Type genus Pentoxylon have been named as under:

(i) Stem has been named as Pentoxylon sahnii.

(ii) Leaf has been named under organ genus Nipaniophyllum raoi.

(iii) Male fructification has been named under organ genus Sahnia nipaniensis.

(iv) Female fructification has been named under organ genus Carnoconites compactum.

Using all these organ genera, Sahni (1948) named the complete plant as the Type genus Pentoxylon.

7. Determination of Age of Fossils:

By finding the age of rock, the age of fossils is calculated. The age of rock is calculated by using radiometric dating techniques. In these techniques, various radioactive isotopes of Uranium (²³⁶U, ²³⁸U), Thorium (²³²U) and Potassium (⁴⁰K) are used. All these radioactive isotopes are also called “geological clocks”.

The radioactive isotopes decay and lead to stable isotopes. In this process, the energy is released. The rate of decaying of any radioactive isotope and its giving rise to stable isotope, is always constant. The age of rock and plant can thus be calculated by measuring relative quantities of radioactive isotope and the stable isotope.

The use of radioactive carbon is called carbon dating. Carbon dating technique is used in ascertaining the age of specimens of plants and animals, back to about 60,000 years. It is because of the fact that half-life of carbon is 5568 + 30 years.