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Vol. 5 No. 4 - October 1999

Vegetational Scenario and Climate of

The Coal-Bearing Gondwana

By: Usha Bajpai

One of the most important periods in the history of the earth, or rather the Southern Hemisphere, that is, the Gondwana Supercontinent, was the Permian Period (280-230 million years before present) when the major coal deposits of the region were laid down. In India, the most important coal deposits are approximately 280 million years old and were laid in basins aligned along important river valleys of peninsular India. Major coal-fields are in Son, Damodar, Mahanadi, Satpura and Wardha and Godavari basins. During this period India lay between southern latitudes 70 and 40.

Coal is a compression fossil having been formed through an accumulation of plant material that became crushed and compacted due to sediment load. Lignite represents an early stage in coal formation. Coal contains bits of wood, pieces of bark, leaf cuticles, resin bodies, spores and pollen. In rare instances coal is formed entirely of cuticular fragments and amorphous organic material. This type of coal is known as paper coal. One is always interested to know the ecological and climatic conditions, besides the depositional environment under which coal formation takes place.

Climate may be defined as the prevailing weather condition that results from the exchange of moisture, and heat between the earth and the atmosphere. Plants live under various ecological environments on the land, from aquatic continental to upland condition in extreme dryness, etc., which are reflected in the anatomy and morphology of the plants. Thus, periodic changes in the earth's climatic conditions can be deciphered from records of vegetation in time and space. This has helped the palaeobotanist to decipher and interpret the past climate.

The study of the fossil plants found in the Gondwana rocks, the Glossopteris Flora, provides convincing evidence for the ecological and climatic conditions. Before the rise of the Glossopteris Flora, there was more or less cosmopolitan vegetation, the Lepidodendropsid Flora, in the Carboniferous, this flora vanished from the areas that came under the influence of the glaciers. The retreat of the glaciers in the earliest part of the Permian saw almost sudden and enigmatic arrival of the Glossopteris Flora. Its ancestry is presently not known and is probably rooted in rapid mutations that resulted due to the glacial episode. The temporal and spatial spread of the Glossopteris Flora was facilitated by amelioration of the climate due to rapid deglaciation. The flora comprised different members of the Bryophyta, Lycophyta, Arthrophyta, Filicophyta, and Gymnospermophyta. The gymnosperms formed the most dominant group of plants and included the divisions Cordaitopsida, Cycadopsida, Coniferopsida, Ginkgopsida and Glossopteridopsida. The most important group was the glossopterid group of plants with tongue-shaped leaves, several genera of which are on record, for example, Gangamopteris, Glossopteris, Maheshwariphyllum, Palaeovittaria, Rhabdotaenia and Rubidgea. The wood that has been recovered almost invariably shows an araucaroid organisation with clearly defined growth rings. It indicates the prevalence of a temperate climate. In the earliest Permian though, the presence of meagre vegetation comprising small plants indicates a cold xeric environment. Later part of the Early Permian saw warming of the climate and transgression of the sea. The climate became more humid, vegetation proliferated, and the first coals were laid. The study of the leaf epidermis of different plants has indicated that the cold temperate climate of the Early Permian gradually changed over to a warm temperate climate of the Late Permian. In between, at the end of the Early Permian, an inexplicable paucity of vegetation is noticed which is sometimes ascribed to hot and dry spells. However, the recovery of rich palynological assemblages, with lycopod megaspores in large numbers, from the sediments of this period shows the presence of favourable ecological niches. The prevalence of hypostomatic leaves shows increase in precipitation, while the appearance of petiolate leaves possibly indicates a rise in wind velocities in the Late Permian. It was during this period that the last workable Gondwana coal was deposited. The association of fungi and bacteria with the coal indicates that probably it was not deposited in very deep waters.

Dr. Usha Bajpai is a scientist at Birbal Sahni Institute of Palaeobotany, Lucknow, India.

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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