Miocene flora from the Siwalik of Arjun Khola area, Nepal and its palaeoclimatic and phytogeographic implications

Abstract

An extensive study on the plant fossils (leaf and fruit impressions/compressions) collected from the Miocene of Siwalik sediments, Arjun Khola area, western Nepal has been carried out. It revealed the existence of a variety of angiosperm plant taxa represented by 43 genera and 47 species belonging to 24 families. Of these, 34 species are reported for the first time from the studied area. Quite a number of fossils out of the entire lot, were described much prior to the present study from other represented by the Siwalik localities. Most of the leaf and fruit specimens are assignable to families of dicots but a single fossil leaf Clinogyne Salisb. has been assigned to the monocot family Marantaceae. The families which are highly represented in the assemblage are Fabaceae, Sapindaceae, Dipterocarpaceae and Annonaceae. Fabaceae which appeared in Upper Paleocene became a major component of the evergreen forest during middle Miocene. A majority of the families are generally represented by leaf forms but Sindora Miq. and Butea Roxb. ex Willd. of the Fabaceae and Terminalia L. of the Combretaceae are represented by fruits. Most of the fossil leaves in the assemblage showed large to medium size with drip tips, entire margins and apparent greater venation density. All the above characters collectively indicated the existence of tropical to sub–tropical type of vegetation during the Miocene all along the Himalayan foot hills of Nepal. Out of the identified specimens, the majority (more than 69%) resembled modern taxa, distributed in the tropical evergreen forests of different geographical regions of the world and recovery of similar looking fossil elements from the Arjun Khola area, indicated existence of similar forest types in and around the area at the time of Siwalik sedimentation. In contrast, the present day vegetation in and around the area is quite different from the past as it includes elements that generally constitute plants of dry deciduous forests. A number of plants migrated from south–east Asia to India via Myanmar and vice–versa. Consequently, many taxa, especially members of Dipterocarpaceae and Fabaceae, existing in south–east Asia in the Paleogene appeared on the Indian subcontinent and Nepal in the Neogene. Phytogeographical data also suggests that taxa flourishing in the Himalayan foot–hills during Siwalik sedimentation have migrated elsewhere in modern times possibly towards south and southeast in search of favourable conditions for luxuriant growth.

Present day distribution of all the recovered species from the Siwalik Foreland Basins of Arjun Khola area shows that they are found today in the tropical forests of North–east India, Bangladesh, Myanmar and Malaysia where favourable climatic conditions exist and only little amount of taxa are found to grow presently in the Himalayan foothills. Thus, it may infer that a fair change in floral composition has taken since the Miocene. The nearest living relative (NLR) method of palaeoclimate construction suggests that the Arjun Khola area enjoyed a tropical climate having Mean Annual Temperature (MAT) 22–28ºC and Mean Annual Precipitation (MAP) 2200–3200 mm during the deposition of the sediments.