Dipterocarpaceous macrofossils from Churia Group of Arjun Khola area, western Nepal and their phytogeographical and palaeoclimatical implications

Investigation of plant megafossils collected from Churia Group of western Nepal revealed the presence of fossil wood, leaves and a fruit of a phytogeographically important genus, Dipterocarpus Gaertn. f. of the family Dipterocarpaceae. These have been described under the form species, Dipterocarpoxylon siwalicus Prakash, Dipterocarpus suraikholaensis Prasad and Pandey, and three new species namely Dipterocarpus nepalensis n. sp., D. miocenicus and D. churiensis. The analysis of present day distribution of extant species of Dipterocarpus comparable to the fossil remains indicates that they do not grow in the sub–himalayan zone of India and Nepal; instead they are presently distributed in the evergreen forests of South–east Asian region (Myanmar, Malaya, Java, Borneo, etc.). This suggests that after rise of Himalaya, drier conditions prevailed so that such species could no longer survive there. Based on the present and past distribution of the comparable extant species of Dipterocarpus, the phytogeography as well as migration of this genus has been discussed. The finding of dipterocarps in the Siwalik sediments of Himalayan foot hills of Nepal suggests that they have migrated from the South–east Asian region during early Miocene and later on became locally extinct due to prevailing of unfavourable condition. An attempt has also been made to categorize the already known species of Dipterocarpoxylon having almost similar anatomical features.