Plant diversity and climate during Siwalik (Miocene-Pliocene) in the Himalayan foot hills of western Nepal

摘要

A detailed study on plant megafossils comprising leaf impressions from the Siwalik sediments of the Suraikhola area, Rapti Anchal, western Nepal has been done. The qualitative and quantitative method of comparison of morphological features between the fossils and extant taxa reveals that they are comparable to 31 species belonging to 18 angiospermous and one pteridophytic families. 22 of these species are new to the Tertiary flora of the Indian subcontinent. With the addition of 31 taxa described in this text, the megafossil assemblage of Siwalik of Suraikhola now comprises 85 species belonging to both monocotyledonous and dicotyledonous families of angiosperms and one pteridophyte. On the basis of the present assemblage as well as on already known data from there, the palaeoclimate and phytogeography of the Suraikhola region in the Himalayan foot hills of western Nepal during Mio-Pliocene have been deduced. The significance of the physiognomic characters of the fossil leaves in relation to climate has also been discussed. An analysis of the floral plants of different formations of the Suraikhola sequence belonging to the Lower, Middle and Upper Siwaliks indicates that there is a gradual change in floral composition, the Lower Siwalik Formation like Bankas and Chor Khola have evergreen to semi-evergreen elements. An increase of deciduous elements in the floral composition is noticed towards the close of the Middle Siwalik and the beginning of the Upper Siwalik formations. This change in the vegetation pattern of different formation reflects the climatic change during Mio-Pliocene times.