The biogeography of the late Paleozoic floras of India

Abstract

This paper reviews the biogeography of Paleozoic floras in India and addresses some of the important biogeographic questions that have been raised regarding the distribution of floras and faunas on the subcontinent. Little is known about the Silurian and Devonian floras. Plants of this age have been reported sparingly from the extra-peninsular Panjab-Kashmir Himalaya region. On the basis of the available evidence, land plants comprised a cosmopolitan world flora until the end of the Early Carboniferous. The beds above the Lower Carboniferous strata of Gondwanaland record a sudden large-scale extinction of plants due to widespread glaciation. The Gondwana glaciation is believed to have ranged from the Upper Carboniferous to the Upper Permian in different parts of the supercontinent. The earliest post-glacial fossiliferous strata of Upper Carboniferous/Lower Permian age in India are found in the Talchir Series which has been divided into two stages, the Talchir and the Karharbari. Talchir floras are depauperate and appear to have lived under unfavorably cold conditions. Approximately 20 Talchir taxa are reported. The floras are dominated by Gangamopteris. Conditions during the Karharbari in peninsular India favoured the development of diverse floras and extensive, thick peat accumulations. On the basis of floral evidence, climatic conditions are interpreted to have been milder and warmer than during the Talchir Stage. Gangamopteris still dominated but other glossopterids were becoming abundant and the pre-conifers first emerged. The Talchir series is overlain by the Damuda Series. The time during which the Damuda strata were being deposited is divided into three stages, the Barakar, Barren Measures (Kulti or Ironstone Shales), and the Raniganj. These stages are characterized by the increased diversification of Glossopteris and the waning of Gangamopteris. The tectonic movement of Gondwanaland northward away from its southern polar position resulted in gradually ameliorating temperatures during the Permian period. By the beginning of the Triassic, northward plate movement had positioned the subcontinent within an arid realm. Except for some rare occurrences of Glossopteris and Sphenophyllum, the glossopterids became extinct by the Triassic and were replaced by the Dicroidium floras. Some of the important biogeographic problems relating to the distribution of the late Paleozoic floras of India are concerned with the location of the northern, northeastern, and northwestern boundaries of Gondwanaland. The occurrence of non-Gondwanan forms at these localities could be interpreted as an indication that paleoecotones developed in the border areas between biogeographic provinces. However, an alternative hypothesis suggests that the areas of mixed floras constitute evidence that a microcontinent lay between the Eurasian plate and Gondwanaland. The occurrence of marine strata in the middle peri-Tethys region in the Salt Range and the Himalayan localities in Kashmir has long been established. However, the discovery of shelled faunas and smooth-walled acritarchs interpreted to be of marine origin throughout peninsular India indicates that the possibility of widespread marine transgressions into Lower Gondwana strata situated far from the Tethys will have to be considered.