The bivalve faunas from the Upper Triassic Xujiahe Formation in the Sichuan Basin

The bivalve faunas abound in the middle and late Upper Triassic Xujiahe Formation in the Sichuan Basin,where the strata are extensive,well-defined and contain commerical coal beds. Two bivalve assemblage zones have been distinguished on the basis of stratigraphic and fossil data obtained from the field work in recent years. They are Burmesia lirata-Myophoria (Costatoria)seperata-Yunnanophorus boulei and Yunnanophorus baoxingensis -Permophorus emeiensis-Weiyuanella rhomboidalis. In the present paper, the characteristics, distribution and depositional environments of these fossils are discussed,and five new species, i. e. Yunnanophorus baishuiheensis, Yunnanophorus emeiensis, Yunnanophorus pengzhouensis,Unionites yangcunensis and Mokiolus rugosus are described in detail. These fossil plates are published for the first time. This study will contribute to a deeper understanding of stratigraphic division and correlation,and palaeogeographic framework in the Xujiahe Formation,Sichuan Basin.