Fossils: Definition and Its Study | Biology

In this article we will discuss about:- 1. Definition of Fossils 2. The Process of Fossilisation 3. History 4. Types 5. Age 6. Techniques of Study 7. Significance of the Study.

Definition of Fossils:

It is difficult to precisely define the word fossil. The word fossil is derived from the Latin verb ‘fodere’ meaning to dig. Hence basically a fossil means anything that is excavated from the earth. In the present usage however fossil is restricted to organic remains obtained from earth and excludes inorganic objects or objects fashioned by humans.

In a popular sense a fossil may be defined as imprints of nature in the womb of earth. A fossil includes the remains of a whole organism or some part of the organism or the direct evidence of the prior existence of some organism in the sediments of earth.

In addition to this, anything resulting from or indicating the prior existence of organisms such as the imprints, tracks, trails, borings, coprolites and in some cases even chemical precipitates may be regarded as fossils. Chemical fossils include things such as sulphur and iron deposits indicating the former bacterial activity and limestone’s representing the deposition of calcium carbonate by algae.

The Process of Fossilisation:

Plant fossils are usually preserved in rocks composed of sediments deposited in water. Sedimentary rocks are formed at the base of oceans by the eroding activity of the rivers. Rivers flowing down the mountains in the form of torrential streams bring down with them huge rocks and boulders which break into smaller and smaller pieces as they approach the foot of the mountains.

In the plains the rivers flow slowly and leisurely with the heavier pieces of stone settling down. The finer and lighter particles remain suspended in water in the form of sand grains. The rivulets that flow into the river carry with them large quantities of fine sand and earth. All these are ultimately carried to the lake or sea to which the river falls.

At the bottom of the ocean the sand and the earth brought by the river are deposited in the form of a layer. Similar layers are being deposited continually at the bottom of seas and lakes. These layers are laid down one above the other.

During the course of ages the lower layers under the weight of the upper ones and the upper layers under pressure of the water above get compressed and gradually harden into stony rocks. Since these rocks are made up of a number of strata or layers, these are usually referred to as sedimentary rocks.

As the streams and rivers flow into the seas, depositing layer upon layer of sediment of mud and silt, they also carry with them the bodies of animals and plants that happen to fall into them for various reasons. These bodies are deposited at the bottom of the sea. To these will be added the bodies of marine plants and animals.

All these are covered by layers of sediment which prevent their disintegration and oxidative decomposition. As layers of sediment harden into layers of rocks, the remains of animal and plant bodies become embedded and preserved between them. It is very seldom that a plant body is preserved intact. Generally dismember­ment of the plant is inevitable. Leaves, fruits, seeds etc., get detached from the plant. Softer tissues are very rarely preserved.

The chief factors that govern the fossilization of a plant are mainly:

(1) The nature of the tissues and

(2) The conditions to which the tissues are subjected preceding and during fossilization.

Plants that contain hard tissues such as fibers, sclerenchyma, xylem etc., are well preserved than flowers or leaves. Soon after a material is entombed in the accumulating sediments, if there are no destructive agents (such as high oxygen content) the process of fossilization begins.

The ideal condition for fossilization of a plant material is an enclosed body of water such as a lake or a swamp in which only fine grained sediments accumu­late with sufficient speed to bring about a quick burial’. A low oxygen content with high concentration of toxic substances prevents decay. When these ideal conditions prevail a perfect fossilization occurs.

History of Fossils:

Geological time table gives in chronological order the different eras and periods in the history of the earth together with the fossils found in that period.

A geological time table is given below:

Types of Fossils:

After understanding the process of fossilization, it is but necessary to under­stand the various types of fossils.

In general, the following categories of fossils are recognised.

1. Compressions:

Of all the types of fossils these are the most common ones found in the rocks. As the very name suggests the plant gets flattened due to the pressure of the sediments. As a result a thin carbonaceous film is retained outlining the surface features of the plant parts. Generally little or no internal structure is retained. Rarely, however, the cell pattern of cutinized epidermis is retained.

Sediments of various types are involved as matrices for compressing the plant parts. These are shale, sand stone, volcanic ash, diatomaceous earth etc. Compressions help us to understand the external features of plants of the past. They provide reliable clues in establishing the affinities.

2. Impressions:

The impression is best defined as the negative of a compression. The feck strata that enclose the plant parts become hardened and when split open they show the negative Imprint of the plant part. There is no organic matter in an impression, therefore cellular details are entirely wanting. But the outlines of plant parts are very well preserved. Of all the plant parts, leaves give a very good impression.

3. Molds, Incrustations and Casts:

After the plant part is entombed in the sediment if there is no compression the surrounding sedimentary material hardens to form a three dimensional structure called the mold. Incrustations may be defined may be defined as external molds of a plant usually in some incompressible materials like sandstone, ironstone or tufa.

After the formation of mold, sometimes the plant part may decay. This space later is filled by crystalline substances such as iron pyrites, sphalarites, chalerite, opal, agate etc. Such hardened structures are called casts. Casts exhibit the same external configurations as the original plant fragment. No organic material is seen in a cast. Hence there is no question of cellular details.

4. Petrifactions:

Of all the types of fossils, petrifactions are the best, but rarest. In these fossils external form, internal structure and sometimes the substances of original plants are preserved. Cellular details are preserved due to infilteration of some minerals into the plant tissues.

During petrifaction, the entombed plant is impregnated with about twenty minerals such as carbonates, sulphates, silicates, phosphates, iron pyrites etc. Solidi-faction of plant tissues takes place ultimately. Petrifactions are of great importance because they can be cut into sections for anatomical studies.

5. Paper Coal:

A paper coal or leaf coal consists of thin dead leaves dispersed in an organic matrix. Sometimes decomposed stems and their cuticles have also been retained. The matrix consists of carboniferous limestone. The best paper coal has been found at Tovarkovo, in Toula in Russia.

6. Coal Balls:

Petrifactions of spherical specimens are generally termed coal balls. During the formation of coal balls the plant material in swamps gets infiltrated with car­bonates of calcium or magnesium, so that the debris of plants will not get converted into coal. Coal ball plants are of great value in palaeobotanical studies.

7. Compactations or Mummified Plants:

Plant fragments compressed by vertical pressure are called compactations. Aggregations of plant material found in peat, lignite etc., represent large scale mummifications.

8. Objects of Organic Origin:

Any organic material that is related with ancient plant life, also comes under the category of fossils. Fossil resin amber obtained from coniferous plants is one such example, Resinous substances exuded from plants due to injuries, insect bites etc. fall on the ground, harden, sometimes enclose small plant fragments and preserve them.

9. Chemical Fossils:

Remnants of minute amounts of organic compounds in parts of fossilized structures without undergoing any or minimal chemical change are called chemical fossils.

10. Pseudo Fossils:

These are fake structures mistaken for plant remains. Because of the similarities with plant organs these are often mistaken for fossils. These may be products of minerals in rock crevices or creation of ancient human beings.

Age of Fossils:

In the investigation of fossils, finding out their age is very important. It gives us the knowledge of the existence of a particular plant life in the remote past. Usually age of fossils is approximated by finding out the age of sedimentary rock in which the fossil is encased.

When observing strata of sedimentary layers, we can easily assume that the lowest is the oldest and the top most is the recent. The successive layers (from below) will be younger and younger. This of course is subject to certain reservations.

For example, geological changes involving moun­tain formation, earthquakes etc., have always been taking place. As a result, an ocean bottom, due to upheaval may become land so that sedimentation stops. Once again an ocean may be formed due to geological changes. In such cases the layers of sediment will not be sequential. So when one is finding out the age and sequence of plant life all these have to be borne in mind.

Generally there are two methods in finding out the age of fossils. In the first method (uranium/lead method), the age of the rock is measured which gives an approximate idea about the age of the fossil and in the second method (C¹⁴ dating method), the age of the fossil is measured directly.

1. Uranium/Lead Method:

Uranium 235 is a radioactive element. It gradually decays and gets converted into an isotope of lead (Pb 206). If in a given rock, both the U 235 and Pb 206 are present, by measuring their relative volumes (since the rate of decay is known). One can measure the lime since the solidification of the rock. This gives an approximate Idea of the age of the fossil embedded in the rock.

2. C¹⁴ Method:

All living things contain carbon and s certain percentage of this is the isotope carbon (C¹⁴) which is produced in the earth’s atmosphere by the cosmic ray bombardment. The percentage in any living body is always the same, because the body absorbs C¹⁴ from the air at the same rate at which the isotope decays.

As soon as an animal or plant dies, C¹⁴ is no more absorbed; and its amount begins to drop slowly. Since the original percentage of C¹⁴ in an old piece of wood or bone is known a nuclear chemist can figure out how long ago the object started to decay.

If there is exactly half as much C¹⁴ in the specimen as there should have been originally, the chemist can assumes that the object is 5,568 years old (the half-life of C¹⁴ is 5,568 years. The rate at which a radioactive material decays to reach one half of its original volume is called half-life). By careful uses of C¹⁴ method the age of any fossil can be measured upto 50,000 years.

Techniques of Fossil Study:

There are several methods to study the fossil specimens. While only mor­phological studies are possible with all other types of fossils, in the case of petrifactions it is possible to study the internal structure. The petrified specimens are cut into thin slices by different methods. In one of the methods each slice is attached to glass plate and ground into sufficient thinness so that it can be viewed under the microscope.

Another method of study of petrifactions is to prepare thin films by adopting special techniques. The first step is to smoothen the surface of the slice of the petrified specimen. Then depending on the mineral content of the specimen, either 5% HCl (if the material has calcium carbonate) or 10% Hydrofluoric acid (if the material has silica) is allowed to act on the surface for 5 and 10 minutes respec­tively.

Due to this treatment the surface becomes rough and any organic matter remaining on the surface can be treated with hot gelatin. After the gelatin dry’s up, the thin film with the cellular details can be removed from the specimen and studied under the microscope. This method is useful where a petrified specimen still retains some organic matter.

Significance of the Study of Fossils:

The study of fossils provides an insight into plant life of the past; their structure, distribution and reproductive features. This helps us to understand and appreciate the present day flora better. Paleobotany introduces us to many extinct plants which may be looked upon as documents of nature recording the progressive changes and modifications undergone by various groups of plants through millions of years.

This helps us in understanding the history and evolution of modem vegetation. From the academic view point perhaps the best irrefutable evidence to evolutionary theory comes from the fossils. Fossils arranged in a chronological order clearly reveal the relationship between one group and other which are very diverse in the present day.

Applied aspects of paleobotany are many. The study of fossil spores and pollen (Palaeopalynology) have a marked bearing in the exploration of oil and coal. Microfossils in coal deposits help us to measure the literal extent by matching them with another coal whose microfossil quantity is known. Micro-fossils have also been of immense help as stratigraphic markers in location of strata in relation to a known oil bearing strata and its mapping.