Contribution to the siwalik flora from Surai Khola sequence, western Nepal and its palaeoecological and phytogeographical implications

Abstract

A systematic study of plant megafossils comprising mostly leaf-impressions and few fruits from a sequence of the Siwalik sediments of Surai Khola area, Rapti Anchal, western Nepal has revealed 33 taxa belonging to 15 dicotyledonous families, viz., Anonaceae, Flacourtiaceae, Clusiaceae, Rutaceae, Meliaceae, Sapindaceae, Anacardiaceae, Fabaceae, Anisophylleaceae, Myrtaceae, Rubiaceae, Apocynaceae, Ebenaceae, Euphorbiaceae and Moraceae. Amongst them, Goniothalamus, Harpullia, Anisophyllea, Anthocephalus, Diplospora, Chonemorpha and Cleistanthus are new genera to the Indian Tertiary flora. The megafloral assemblage of the Surai Khola sequence, including 21 taxa reported earlier, now comprises 53 taxa of angiosperms. Most of the extant species resembling the fossil taxa are distributed in tropical evergreen to semi-evergreen forests of Western Ghats, northern India, Andaman Islands, Bangladesh, Myanmar and in Southeast Asia which suggest that warm humid climate with higher precipitation prevailed in the area during Middle Miocene-Pliocene. Looking at the floral assemblages of different formations of the Surai Khola sequence, from base to the top sediments belonging to the Lower, Middle and Upper Siwaliks, one may find a gradual shift in the vegetation pattern. The basal sediments of Bankas and Chor Khola formations have preponderance of a lowland mixed mesophytic broad leaved evergreen to semi-evergreen elements. An increase in deciduous elements in the floral composition consequent to the disappearance of dipterocarpus and other significant evergreen taxa is noticed towards the close of Middle Siwalik and the beginning of Upper Siwalik. This change in the vegetation pattern reflects the changes in climatic conditions which are mainly due to northward movement of the Indian Plate and further uplift of Himalaya.