Na Duong (northern Vietnam) – an exceptional window into Eocene ecosystems from Southeast Asia

Today, the continental ecosystems of Southeast Asia represent a global biodiversity hotspot. From a deep-time perspective, however, very little is known about the formation of this hotspot. In particular, the evolutionary and biogeographic roots of numerous terrestrial biota are unknown, and ecosystemic information, especially for the Paleogene, is unavailable. The Na Duong Basin in northern Vietnam has yielded a remarkable diversity of Paleogene vertebrate, invertebrate, and plant fossils, and is thus one of the few localities in Southeast Asia allowing for multi-focused investigation of this period. We present stratigraphic, paleontologic, and paleoecologic results from the 220 m thick Na Duong section derived from magnetostratigraphy, biochronology, vertebrate, invertebrate, and plant taxonomy, and biomarker analysis. Only the upper 40 m of the section show the original magnetization, prohibiting any further magnetostratigraphic interpretation. The affinities of two newly described mammal species, Bakalovia orientalis nov. sp. (Anthracotheriidae) and Epiaceratherium naduongense nov. sp. (Rhinocerotidae), suggest an Eocene, late Bartonian to Priabonian age (39–35 Ma). High biodiversity is recorded for unionid mussels (five species), freshwater fishes (nine taxa, including Planktophaga minuta nov. gen. et sp.), turtles (five to six taxa), and crocodiles (three taxa), and long-term stability of Southeast Asian unionid and fish faunas is demonstrated. Fossil leaves, wood and resin document azonal and zonal vegetation; dipterocarp trees were identified from resin exudate spectroscopy. In-situ tree-stump horizons allow for calculation of tree density (600 specimens/ha) and maximum canopy height (35m); both values resemble those of recent Southeast Asian peat swamp forests. Environment changed abruptly from a swamp forest to a tropical to warm sub-tropical lake of fluctuating water depth. The strong biogeographic link between the Eocene mammal faunas from Na Duong and Europe highlights the importanceof Southeast Asia as a source region for trans-continental mammal dispersal along the northern Tethys margin.