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.