NEW PLIOCENE FLORA FROM CENTRAL VIETNAM — ANCIENT ANALOG OF MAINLAND SOUTHEAST ASIA’S TROPICAL SEASONAL FORESTS

Mainland Southeast Asia (MSEA) is one of the world’s biodiversity hotspots, hosting high endemism of plants and animals under a tropical seasonal climate, yet it suffers high extinction risks due to increasing anthropogenic pressure. Effective conservation requires a better understanding of the evolutionary history of the regional vegetation to clarify its origin and development through geologic time, information that most reliably comes from the fossil record. However, paleobotanical studies in MSEA are currently limited and restricted to the northern part of the region, leaving much unknown in southern MSEA, which has broader tropical forests. Here, we report a new Pliocene flora discovered by our international team in late December 2019 from the Kon Tum Formation in Kon Tum Province, Central Highlands region of Vietnam. The flora was found in a lenticular claystone deposit, preliminarily interpreted as an overbank floodplain fill. The Kon Tum flora is represented by 572 leaf specimens entirely of angiosperm, mostly with well-preserved venation or cuticular structures. The assemblage is characterized by predominantly entire-margined leaves (only 1 morphotype toothed) with relatively small leaf size (mesophyll-nanophyll, mostly notophyll). So far, more than 40 morphotypes are recognized, with a few given tentative botanical affinities, e.g., Ficus subg. Urostigma, Dipterocarpaceae, Myrtaceae, Fabaceae, Lauraceae, and Anacardiaceae. The associated palynoflora appears to be highly diverse with more than 200 palynomorphs giving a clear signal for a tropical forest. The recognition of Loranthus, Casuarina, Alangium, Dacrydium, and multiple Poaceae pollen types indicates an age of Pliocene for the Kon Tum flora, agreeing with previous correlations. Both macro- and microfossil assemblages imply the presence of a tropical seasonal broadleaved forest in central Vietnam since at least the Pliocene, with apparently similar floristic composition to present-day forests in the area. This new flora is the first macrofossil evidence of Neogene paleovegetation in southern MSEA and indicates the antiquity of the biodiverse yet endangered Vietnamese tropical forests.