Middle Miocene flora from Siwalik foreland basin of Uttarakhand, India and its phytogeographic and palaeoclimatic implications

摘要

Morphotaxonomical study on the plant fossils (leaf and fruit impressions) collected from Lower Siwalik sediments of Tanakpur area, Uttarakhand revealed the occurrence of 57 species of 25 angiospermous families. Of these, 39 species have been recorded new to the fossil flora of Himalayan foot hills and remaining 18 species are reported already from different Siwalik fossil localities of India, Nepal and Bhutan. The family Fabaceae (Legume family) represented by 15 species is the most dominant family in present assemblage followed by Annonaceae (8 species), Sapindaceae (5 species), Lauraceae and Euphorbiaceae (4 species) and Dipterocarpaceae and Rutaceae (3 species). The family Fabaceae which appeared in Upper Palaeocene became a major component of the evergreen forest during Middle Miocene times all along the Himalayan foot hills. In the present assemblage the evergreen floral taxa (54%) are dominant in contrast to mixed deciduous taxa at present. The predominance of evergreen elements in the Siwalik fossil assemblage indicates the prevalence of warm and humid climate with plenty of rain fall during the deposition of Siwalik sediments. The analysis of present day distribution of all the recovered species from the Siwalik foreland basins of Tanakpur area shows that they are mostly known to occur in North-east India, Bangladesh, Myanmar and Malaysia wherever favourable climatic conditions exist. Only about 14% taxa of the total assemblage are found to grow presently in the Himalayan foot hills and the remaining 86% taxa are locally extinct, suggesting changes in the climatic condition. Nearest Living Relative (NLR) suggests that the Tanakpur area enjoyed a tropical climate having Mean Annual Temperature (MAT) 24-28°C and Mean Annual Precipitation (MAP) 2100-2800 mm during the deposition of the sediments.