Plant megaflora from the Siwalik (Upper Miocene) of Darjeeling District, West Bengal, India and its palaeoclimatic and phytogeographic significance

Abstract

Qualitative and quantitative analysis of fossil assemblage so far collected from the Middle Siwalik sediments of Darjeeling District, West Bengal revealed the occurrence of 35 new taxa belonging to 18 angiospermous families. On the basis of present data as well as already known data from there, the reconstruction of vegetation scenarios of Himalayan foothills during Siwalik time has been done. We also discussed problems related to plant diversity, endemism, and migratory pathways of mainly phytogeographically important taxa. The analysis of present day distribution of all the recovered taxa from the region shows that they are mostly known to occur in Northeast India, Bangladesh, Myanmar and the Malayan region where the climatic conditions are favourable. About 1/3 taxa of total assemblage are found to grow presently in the Himalayan foothills of the eastern region and the remaining 2/3 taxa are locally extinct. This indicates that the climatic changes must have taken place after Mio–Pliocene. The dominance of evergreen elements in present fossil assemblage indicates the prevalence of tropical warm humid climate with plenty of rainfall during the deposition of sediments. Foliar physiognomic approach for reconstruction of palaeoclimate further suggests that the Oodlabari area in the Himalayan foothills of West Bengal enjoyed a tropical climate (with MAT 28.9º C and MAP 448 mm) during the Miocene Period. This is, however, contrary to the present day climate of the area with reduced precipitation. On the basis of the present fossil assemblage, the coexistence intervals of different climatic parameters, i.e. Mean Annual Temperature (MAT), Warmest Month Temperature (WMT), Coldest Month Temperature (CMT), and Mean Annual Precipitation (MAP) have been estimated as 22º C–26.5º C, 17.8º C–20º C, 25º C–30º C, and 2650–3200 mm, respectively. However, Leaf Margin Analysis (LMA) suggests the MAT value as 28.9º C for the area during Upper Miocene.