Angiospermous fossil leaves from the Siwalik foreland basins and their palaeoclimatic implications

Abstract

The angiospermous fossil leaves so far recorded from the Siwalik foreland basins of India, Nepal and Bhutan have been analysed and an attempt has been made to deduce palaeoclimate and phytogeography of the region during Siwalik (Mio-Pliocene) Period. The physiognomic characters of the fossil leaves have been critically examined in order to infer the climate of the region. The leaf features that have used mainly an aid in determining past climate, are leaf margin, drip tips, leaf size and venation density. The dominance of entire margined species (about 92%) in the Siwalik leaf assemblage indicates the prevalence of tropical climate. The presence of conspicuous drip tips and other features like leaf size, leaf texture, nature of petiole and venation density collectively suggest tropical climate with heavy rainfall (MAT 28°-29° and MAP 308 mm) during Mio-Pliocene. Almost all the fossil leaves recovered from different fossil localities in the Himalayan foreland basins have been identified with the modern angiospermous taxa up to specific level. From the present day distribution of these comparable species it is evident that most of the comparable species of the fossil assemblage are not found at present in the foot hills and became extinct due to unfavourable climatic condition after Mio-Pliocene. Moreover, on the basis of habit and habitat of comparable taxa of angiospermous fossil leaves it has been concluded that the Siwalik flora consists of three types of constituents in the forest during the sedimentation of Siwalik foreland basins: (1) Evergreen (2) Evergreen and Moist deciduous (3) Mixed deciduous. The Siwalik leaf assemblage is dominated by evergreen constituents like dipterocarps, legumes and other associated taxa in contrast to mixed deciduous constituents of the present day floral assemblage of the regions. This is most probably due to post-Pliocene orogeny of the Himalaya which brought changes in the topography and climate and thus adversely affecting the vegetation scenario of the Himalayan foot hills.