Cretaceous trigonioidid (non-marine Bivalvia) assemblages and biostratigraphy in Asia with special remarks on the classification of Trigonioidacea

Non-marine Cretaceous bivalve Trigonioidacea, having an anterior pedal retractor scar distinctly separated from the anterior adductor scar, are subdivided herein into seven families and subfamilies, and 26 genera and subgenera. The general ornament pattern at the family and subfamily level, and features of the radial ribs, including the VA (angle of V-shaped ribs) and hinge teeth at the generic and subgeneric level serve as the basis for classification. They have a wide distribution in Asia, particularly in Middle and East Asia. Seven trigonioidid assemblages are recognized and dated on the basis of morphological features and stratigraphical occurrence of the fossils as well as associated intercalated marine deposits. The seven assemblages are as follows: (1) Hauterivian-Barremian and ?Valanginian Nippononaia (Eonippononaia) tetoriensis-Koreanaia (Eokoreanaia) cheongi-Danlengiconcha elongata assemblage; (2) Late Barremian-Early Albian, but mainly Aptian, Nippononaia (Nippononaia) ryosekiana-Koreanaia (Koreanaia) bongkyuni-Trigonioides (Wakinoa) wakinoensis-Trigonioides (Trigonioides) kodairai-Trigonioides (Trigonioides) quadratus-Plicatounio (Plicatounio) naktongensis-Pseudohyria (Matsumotoina) matsumotoi assemblage; (3) Aptian-Early Albian, but mainly Aptian, Trigonioides (Trigonioides)-Plicatounio (Guangxiconcha) suzukii assemblage; (4) Late Aptian-Early Cenomanian, but mainly Albian, Trigonioides (Diversitrigonioides) diversicostatus-Pseudohyria (Pseudohyria) subovalis assemblage; (5) Albian, probably extending down into Aptian, Trigonioides (Trigonioides) heilongjiangensis bed; (6) Cenomanian, but possibly going up into Turonian, Trigonioides (Trigonioides) paucisulcatus-Trigonioides (Trigonioides) matsumotoi-Trigonioides (Trigonioides) mifunensis-Plicatounio (Guangxiconcha) tamurai assemblage; and (7) Turonian-Early Maastrichtian, but mainly Santonian-Early Maastrichtian, Plicatounio (Plicatounio) hunanensis-Pseudohyria (Pseudohyria) turberculata-Pseudohyria (Pseudohyria) cardiiformis-Pseudohyria (Pseudohyria) aralica assemblage. This biostratigraphy permits correlation and age determination of trigonioidid-bearing formations throughout Asia.