In this essay we will discuss about the basic and applied aspects of studying fossil plants.
Essay # 1. Basic Aspect of Studying Fossil Plants:
Such studies help us to understand the past vegetation, climate and palaeoecology including the history of plant evolution, biostratigraphy, etc.
Some of the important basic aspects of such application are:
(a) To Decipher Palaeovegetation and Palaeoclimate:
Like all living organisms, plants ultimately face death and decay. Sometimes, however, they escape from final destruction and survive as fossils. There are remains which allow palaeobotanists to place together the scattered jigsaw of plant life forms. Thus, plant fossils serve as tool for past vegetational studies.
From remote geological past a correlation exists between fossil plants and their surrounding environments which serve as an index for palaeoclimate. The fossil plants are used for interpretation of the past climate, because the present-day living plants are well understood for their integral association with the idea “present is the key to the past”.
Palaeoclimate factors, therefore, are assessed on the basis of modern-day plants, showing their morphological and anatomical characteristics commensurate with climatic condition including adaptive factors. Such data reveal the character of fossil plants.
(b) Provide Evidence for Origin and Evolution of Plants:
The study of fossil plants has an importance in understanding the origin and evolution of different plant groups through ages.
There are several examples in support of the origin of land plants. A most dramatic event took place in the history of plant life i.e. the evolution of land plants during Silurian period.
To acquire terrestrial habit leaving their aquatic habitat the plants needed to be self-supportive and they had to be able to withstand the drying effect of the atmosphere leading to a series of adaptations, such as development of cuticle in their outer surface to check desiccation, formation of roots as anchoring and absorptive organ, stomata for gaseous exchange, vascular tissue for conduction.
Thus, during Silurian-Devonian periods more and increasingly complex fossil plants represented by Rhyniopsida appeared with such adaptive features. Gymnosperms became more successful than the pteridophytes by having selective advantage of seed formation.
This group of plants first appeared in the Devonian period and reached a climax in the Mesozoic Era which constituted the world’s dominant vegetation. Still later, at the end of Mesozoic Era (early Cretaceous) the angiosperms got evolved and became dominant replacing most of the pteridophytes and gymnosperms.
(c) To Ascertain Palaeophytogeography:
The important aspect of studying palaeobotany is to ascertain the palaeophytogeographic condition. The most astonishing example of such aspect is the concept of Gondwanaland, which consisted of a single land mass formed by the union of the faraway continents of the Southern Hemisphere, such as South America, South Africa, Madagascar, Australia, New Zealand, Antarctica and India. The fossil evidence shows a clear floral continuity ranging in age from the Upper Carboniferous to the beginning of the Cretaceous.
(d) To Make Biostratigraphic Correlation:
The study of fossil plants has an applied significance in understanding the biostratigraphical sequence which provides evidence to trace the plant evolution through ages.
In this context there are two aspects to be considered:
(i) Correlation of the data showing quantitative and qualitative value of retrieved fossil data, and
(ii) Use of fossils for comparison to get the empirical value in terms of appearance, duration of dominance and then gradual disappearance through migration or extermination due to climatic and other factors establishing a biostratigraphic scale.
For example, in the coal bearing strata of the Middle Carboniferous of West Europe, seven successive vegetational sequences were established using plant megafossils. Thus, an analysis of plant fossils of a given stratigraphic zone, provides the needed data to interpret its floristic composition and evolutionary sequence.
(e) Calculation of Age of the Rocks:
Radioactive elements are generally used for dating the rocks. But sometimes, index fossils are used to date a rock of unknown origin. Any index fossil the rock contains must be carefully studied with precise information about its systematics and age. Using of such index fossils the age of an unknown rock can be determined.
Essay # 2. Applied Aspect of Studying Fossil Plants:
To provide useful information in the exploration of fossil fuel like coal and oil.
The plant inhabitants of Palaeozoic- Mesozoic swamps served as a source of coal and formed coal seams. Accumulation of plant materials with a variety of minerals coupled with mud, silt and other organic materials constitute a coal bed. Many plant parts get beautifully preserved as fossils in the stratified sedimentary layers which are closely adjacent to coal layers.
A stratified scale based on fossils can be made to establish the age of coal deposits and their position in the succession of rocks. Palaeobotanical studies have served as a tool to ascertain age of coal layers, their lateral extent and quality of coal deposits. Such information are required to specify the suitability of a particular coal for energy production. In India, the palaeobotanical study has helped to demarcate the nature and quality of Raniganj coal (Permian Age) from that of Rajmahal coal (Jurassic Age).
The assemblage of fossil pollen grains and spores has contributed significantly in the field of oil exploration. The problem of oil exploration begins with search of oil reservoirs.
Exploration of oil is done by:
(i) Determining the Thermal Alteration Index:
The sporopollenin present in the walls of pollen and spores undergo post depositional thermal changes in course of the geological ages. These thermal changes brought about carbonisation resulting in changes in exine colour of fossil pollen and spores in transmitted light.
The basic idea involved is the usage of the variation in the exine colour as an indicator of the degree of carbonisation in the rocks to predict their changes of bearing reservoir. Pearson’s colour chart directly relates exine colour to a numerical index called Thermal Alteration Index (TAI) which is a measure of the degree of carbonisation. TAI having a range of 2+-3+ and exinite flourescence colour white-yellow, dark yellow-brown indicates the possibility of exinite containing rock to possess liquid petroleum and even natural gas.
To avoid unnecessary and costly drilling the determination of oil zone is made by comparing the biostratigraphic data of one to those of the others.
(iii) Defining of Ancient Shorelines:
The sediments parallel to sea shore are rich in oil. The density of pollen and spores decreases in the seaward direction. Sedimentary environment with pollen assemblages are limited to near shore marine or lacustrine waters. Thus by the study of microfossils along with marine microfossils if presents, one can determine the distance and direction of ancient shore lines, possibly bearing oil deposits.