New Plio-Pleistocene flora from the Central Highlands of Vietnam—ancient analog of southern Indochina’s tropical monsoon forests

The biodiverse forests of southern Indochina face severe anthropogenic pressure, but understanding of their deep-time history remains limited due to the general lack of fossil evidence. Here, we present a new Plio-Pleistocene macrofossil flora from the Kon Tum Formation in the Central Highlands of Vietnam, comprising 565 specimens of 44 morphotypes. The assemblage is dominated by dicotyledonous leaves with well-preserved venation or cuticles and is characterized by entire-margined leaves (with only one toothed morphotype) and relatively small leaf sizes (87% microphylls and notophylls). The flora has abundant Syzygium (Myrtaceae), Ficus (Moraceae), Dipterocarpus (Dipterocarpaceae), and Sindora (Fabaceae) leaves, along with other tentative representatives from Fabaceae, Lauraceae, Phyllanthaceae, and Lecythidaceae. Both leaf physiognomy and floristic composition indicate that the Kon Tum flora represents a lowland seasonal tropical broadleaved forest linked to extant regional forests. The diverse associated palynoflora supports this paleovegetation interpretation with mixed source environments and indicates a Pliocene to early Pleistocene age of deposition in a coastal, fluvial setting with tidal influence. The study site is currently at an elevation of over 500 m, indicating significant uplift since deposition. This study marks the first Cenozoic paleobotanical collection of southern Indochina and presents an ancient analog of the extant regional lowland monsoon forests, one of the world’s most threatened forest ecosystems.