First records of freshwater rissooidean gastropods from the Palaeogene of Southeast Asia

This paper provides the first records of freshwater rissooidean gastropods from the Palaeogene (uppermost Eocene to Lower Oligocene) of northern Vietnam (Cao Bang Basin). The most abundant element of the assemblage is the new genus Bacbotricula (Pomatiopsidae: Triculinae?), comprising numerous well-preserved specimens belonging to two new species, B. nhamaygachensis and B. dongbangensis. The assemblage also contains poorly preserved specimens of two other species, one of which is tentatively assigned to the Bithyniidae. The Cao Bang assemblage is among the earliest Cenozoic freshwater gastropod faunas described from Southeast Asia and Bacbotricula is the only pomatiopsid that has been recorded for the Palaeogene. The occurrence of Bacbotricula in northern Vietnam during this period of time suggests that the palaeo-Red River may have served as an early dispersal corridor for the Triculinae. The gastropods likely lived on deltaic plains in Lake Cao Bang and were preyed upon by cyprinid fishes.