An Overview of the Middle Miocene to Early Pleistocene Flora of the Siwalik Sediments in Nepal

Floral taxa from the middle Miocene to the early Pleistocene have been thoroughly documented in the Siwalik sediments in the Nepal Himalayas and are well recorded from 1983–2022. In the present article, we attempted to compile the information about the paleofloristic and paleoclimate changes during the middle Miocene to early Pleistocene period and compile the information related to plant remains recorded so far throughout the Neogene period. The Siwalik Group contains a total of 434 species, comprising 180 genera of 53 families of ferns and angiosperm megafossils and 459 species belonging to 84 families of gymnosperms, angiosperms, pteridophytes, and algal and fungal palynofloral fossils. Due to the fact that palynomorphs have deformed and distorted surface ornamentation and aperture arrangements, most species can only be recognized at the family level. Most of the fossilized families and taxa still exist in the area where the fossils were found. However, some of them, like Calophyllum polyanthum, Dipterocarpus, Cinnamomum tavoyanum, and Filicium decipiens, have disappeared because of changes in rainfall and temperature seasonality after the rise of the Himalayas. Based on the reconstructed climate data, the distribution pattern of fossil assemblages looks more or less similar in most of the Siwalik Group. However, there are still some gaps in our knowledge about how the floral assemblages in the eastern and far-western parts of Nepal changed and developed over time. The overall fossil assemblages of the Siwalik Group suggest that there were tropical and subtropical evergreen rain forests with warm and humid climates that received high rainfall. The plant fossils also suggest that the vegetation shifts occurred due to changes in regional aridification and temperature seasonality.