The Origin, rise and decline of Glossopteris Flora: with notes on its palaeogeographical northern boundary and age

Abstract

In Africa impressions of two leaves of Gangamopteris were found below the glacial conglomerate and in Australia Gangamopteris and Schizoneura were found interbedded with the glacial beds at Bacchus Marsh. Microfossils, including pollen grains like those of the Glossopteris Flora, have been reported from the tillites, particularly from the Bacchus Marsh tillite. Similar microfossils have also been reported from the beds of the Talcher stage lying immediately above the glacial beds. All these fossils indicate that they were the pioneers of the Glossopteris flora which may have come into existence in unglaciated and protected locations of Gondwanaland during the Ice Age itself. The early elements of the Glossopteris Flora thus seem to have lived in a cold temperate climate alongside glaciers. The flora of this stage was relatively poor but as the climate warmed up the forests became richer and their plants more diversified. However, this flora was living in a climate where a cold winter alternated with a warm summer and it abounded in deciduous plants. The climax of the Gondwana vegetation was reached during the Raniganj stage when a much warmer and more humid climate supported a rich forest vegetation again abounding in deciduous trees. Thereafter, there was a sudden change of vegetation during the Triassic when the plants of the Glossopteris Flora yielded place to new elements of the Dicroidium Flora.

The disappearance of the Rhacopteris-Lepidodendropsis Flora with the onset of the glaciation during the Carboniferous-Permian times is easily understood but the sudden appearance of an entirely new Glossopteris Flora after, or even during the glaciation itself, and its almost simultaneous spread in all parts of Gondwanaland raise difficult questions about its origin and phylogeny. Besides dealing with the answers to these questions, the author discusses the factors and implications of the gradual decline of the Glossopteris Flora and the rise of the Dicroidium Flora during the Triassic. The estimates of the geological age and palaeogeographical limits of the Glossopteris Flora are also scrutinized in the light of recent work.