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 importance of Southeast Asia as a source region for trans-continental mammal dispersal along the northern Tethys margin.Heutige kontinentale Ökosysteme Südost Asiens sind ein Hotspot der globalen Biodiversität, über dessen geohistorische Entstehung jedoch sehr wenig bekannt ist. So sind die evolutionären und biogeographischen Wurzeln der meisten terrestrischen Lebensformen unbekannt und Daten zu Ökosystemen, speziell aus dem Paläogen, fehlen. Das Becken von Na Duong im nördlichen Vietnam beherbergt bemerkenswert diverse Biota fossiler Pflanzen, Mollusken und Wirbeltiere und stellt für das Paläogen eine der wenigen Regionen Südost Asiens dar, welche einen multi-disziplinären Untersuchungsansatz erlauben. Auf der Grundlage von Paläomagnetik, Biochronologie, Wirbeltier-, Mollusken- und Pflanzen-Taxonomie, sowie der Analyse von Biomarkern präsentieren wir hier unsere stratigraphischen, paläontologischen und paläo-ökologischen Ergebnisse eines 220 m mächtigen Profils der Braunkohlengrube Na Duong. Nur die hangenden 40 m der Sektion zeigen die ursprüngliche Magnetisierung, wodurch eine magnetostratigraphische Interpretation verhindert wird. Die Verwandschaftsverhältnisse von zwei neu beschriebenen Säugetierarten, Bakalovia orientalis nov. sp. (Anthracotheriidae) und Epiaceratherium naduongense nov. sp. (Rhinocerotidae), verweisen auf ein eozänes Alter (oberes Bartonium bis Priabonium, 39-35 Ma). Starke biogeographische Bezüge beider Säugetiere aus Na Duong zu europäischen Arten unterstreichen die Bedeutung Südost Asiens als eine Quellregion paläogener transkontinentaler Säugetier Ausbreitungen entlang des nördlichen Randes der Tethys. Hohe Diversitäten werden für unionide Muscheln (fünf Arten), Süßwasserfische (9 Taxa, darunter Planktophaga minuta nov. gen. et sp.), Schildkröten (fünf bis sechs Taxa) und Krokodile (drei Taxa) belegt, wobei eine taxonomische Langzeit-Stabilität der Unioniden- und Fisch-Faunen Südost Asiens belegt wird. Fossile Blätter, Hölzer und Harze dokumentieren die azonale und zonale Vegetation, letztere repräsentiert durch spektroskopisch an Harzen belegte Zweiflügelfrucht-Gewächse (Dipterocarpaceae). In-situ Baumstubben-Horizonte erlauben die Berechnung der Baum-Dichte (600 per ha) und der maximalen Kronendach-Höhe (35 m), welches in guter Übereinstimmung zu heutigen südostasiatischen Sumpfwäldern steht. Die Umwelt veränderte sich abrupt von einem Sumpfwald zu einem tropisch bis warm-subtropischen See mit fluktuierender Wassertiefe

Biochronological and palaeobiogeographical significance of the earliest Miocene mammal fauna from Northern Vietnam

Current scientific knowledge of Tertiary fossils from south of the Ailao Shan-Red River shear zone is extremely poor, in sharp contrast with the situation nowadays, as the area of Laos and Vietnam is regarded as a global hotspot of biodiversity. In this context, the few localities that yielded fossil assemblages are of first importance for the understanding of Cenozoic palaeobiogeography and the tectonic and palaeogeographical evolution of the region. Hang Mon 1 (Son La Province, Northern Vietnam) was the first site that provided evidence of Tertiary mammals, but its age remained very controversial, interpretations ranging from Oligocene to Late Miocene. Herein, we re-investigate the mammal fauna of the locality based on newly collected material and previously published fossil mammals. A new outcrop, Hang Mon 2, provides evidence of the rhinoceroses Pleuroceros blanfordi and Bugtirhinus sp. Together with the earlier finds of uncommonly small-sized Protaceratherium, these fossils allow a correlation to the earliest Miocene (most probably ranging from ~23 to ~21 Ma; Aquitanian) based on faunal comparison with the Sulaiman Province of Pakistan. The revision of the mammals from Hang Mon 1 is in agreement with this stratigraphic proposal. In addition, the discoveries from Vietnam (the rhinocerotid assemblage and Hyotherium) further support the hypothesis of strong biogeographical and environmental affinities between Europe, the Indian Subcontinent and Southeast Asia (Vietnam) during the Aquitanian.The excavations and fossil preparation were financially supported by the Deutsche Forschungsgemeinschaft (DFG; Grant Numbers BO 1550/11-1, 2). Several of the authors are members of the GDRI (International research Network) entitled “Paleobiodiversity of South East Asia” sponsored by the National Center of Scientific Research (CNRS).Peer Reviewe