Middle Triassic (Anisian) diversified bivalves: depositional environments and bivalve assemblages in the Leidapo Member of the Qingyan Formation, southern China

The Middle Triassic (Anisian) Leidapo Member of the Qingyan Formation, which crops out in southwestern China, consists of basin-floor and slope deposits with abundant benthic fossils and records two transgressions and one regression. The well-preserved bivalves are grouped into four assemblages. The allochthonous Cassianella–Elegantinia and Protostrea–Cassianella assemblages as well as the autochthonous Palaeonucula assemblage are found in slope deposits. The allochthonous assemblages exhibit several modes of occurrence in sediment gravity-flow deposits. The parautochthonous Posidonia assemblage is preserved in slope and basin deposits. The genus-level compositions of these bivalve assemblages are similar to those from the Early Carnian of Tethyan Europe. It is probable that a significant diversification of bivalves took place in the Anisian.