Unionidae (Bivalvia: Palaeoheterodonta) from the Palaeogene of Northern Vietnam – exploring the origins of the modern East Asian freshwater bivalve fauna

Eocene/Oligocene sediments deposited in two Cenozoic pull-apart basins in northern Vietnam, i.e. the Na Duong and Cao Bang basins, yield several rich and well-preserved assemblages of unionid bivalves. This study outlines the taxonomy of the Unionidae from these communities, and all taxa are described and illustrated in detail. As far as we are aware, this is the first comprehensive treatment of Cenozoic Unionidae from South-East Asia. Eight of the nine taxa are described as new to science. Four species – Cuneopsis quangi sp. nov., Cristaria mothanica sp. nov., Cristaria falcatocostata sp. nov. and Lamprotula hungi sp. nov. – can be confidently assigned to modern genera. The generic position of the remaining species is equivocal (?Nodularia cunhatia sp. nov., ?Cuneopsis sp., ?Lanceolaria sp., ?Lamprotula sp.) or remains tentative (Anodontini sp.). The phyletic relationships of the fossil taxa to modern species are discussed and interpreted in light of the Cenozoic tectonic history of South-East Asia and the intimately associated evolution of drainage systems. The results confirm a close relationship of the fossil unionids to the modern mussel fauna of the Yangtze, Pearl and Red River catchment areas. Moreover, the fossils from Cao Bang and Na Duong are among the earliest confirmed records of all genera identified, and thus document an important, early stage in the development of the modern East and South-East Asian bivalve faunas.