Radiation of Middle Triassic bivalve: Bivalve assemblages characterized by infaunal and semi-infaunal burrowers in a storm- and wave-dominated shelf, An Chau Basin, North Vietnam

In North Vietnam, Lower to Middle Triassic storm- and wave-dominated shelf deposits are widely exposed and yield abundant Tethyan bivalves indicating recovery and diversification following the end-Permian mass extinction. We reconstructed the habitats of characteristic bivalve species on the basis of shell preservation and mode of occurrence. Bivalves that diversified in the Middle Triassic Anisian to Ladinian Stages in the Na Khuat Formation, An Chau Basin, can be divided into four fossil assemblages: (1) an autochthonous to parautochthonous Costatoria goldfussi mansuyi and Trigonodus tonkinensis assemblage (sandy bottom of inner shelf facies); (2 and 3) a parautochthonous to allochthonous Costatoria goldfussi mansuyi and Pteria sturi assemblage, and a Costatoria paucicostata and Hoernesia angusta assemblage (muddy bottom of inner to outer shelf facies); and (4) a typical allochthonous Costatoria ngeanensis and Leptochondria albertii assemblage. These assemblages are characterized by a diversity of species of Costatoria and Trigonodus, and by several genera of twisted Bakevellidae, such as Hoernesia, Langsonella, and Lilangina. These bivalve assemblages are composed of shallow-infaunal and semi-infaunal burrowers and epifaunal species, though in Vietnam the Lower Triassic Induan to early Olenekian bivalves are dominated by epifauna. In the eastern part of the Tethys, infaunal bivalves show a remarkable delay in recovering from the end-Permian mass extinction compared to epifaunal bivalves.