Floral diversity and environment during the middle Siwalik sedimentation (Pliocene) in the Arunachal sub-Himalaya

Abstract

A comprehensive morphotaxonomical evaluation of diverse angiospermic dicotyledonous leaf impressions recovered from the middle part of the Siwalik succession (Subansiri Formation: Pliocene) of Arunachal Pradesh, eastern Himalaya, India, shows that the leaf remains are comparable to modern Glochidion J. R. Forst. and G. Forst. (Phyllanthaceae), Bauhinia L., Callerya Endl. (Fabaceae), Mitragyna Korth. (Rubiaceae), Beilschmiedia Nees (Lauraceae), Uvaria L. (Annonaceae), Neolamarckia Bosser (Rubiaceae), Sorindeia Thouars (Anacardiaceae), Lagerstroemia L. (Lythraceae), and Premna L. (Lamiaceae). Among these taxa, seven species are new to the Neogene floras of the Indian subcontinent. Analyses of the floral assemblage, with respect to the present-day distribution pattern of modern equivalent taxa and the physiognomic characters of the recovered fossil leaves, suggest that a tropical evergreen forest was growing in a warm humid climate in the region at the time of deposition. This qualitative climatic data is also corroborated by our previously published quantitative data obtained from a CLAMP (climate leaf analysis multivariate program) analysis on the middle Siwalik floral assemblage. The presence of some Southeast Asian elements in the fossil assemblage provides clear evidence of free exchange of taxa across southern Asia in the Pliocene.