The Richest Diversity and Highest Abundance of Freshwater Bivalve Fossils from the New Fossil Locality of the Early Cretaceous Sao Khua Formation at Roi Et Province, Northeastern Thailand

A new fossil freshwater bivalves locality in the Sao Khua Formation was discovered at the top of a small hill named Phu Kum Khao in the Pho Chai District, Roi Et Province, northeastern Thailand. The fossils bed is a mud-nodule conglomeratic sandstone of the Sao Khua Formation and has proved to be both of high abundance and species diversity (6,637 specimens of nine species). The most abundant species is Pseudohyria (Matsumotoina) somanai Tumpeesuwan, Sato, and Nakhapadungrat, 2010, which is not only the dominant species of the Sao Khua Formation but also the index fossil representative of the Late Barremian age for the formation. The bivalves from this locality are preserved as both articulated and disarticulated shells. Associated vertebrate fossils are preserved as bone fragments and micro remains. According to the fossil assemblage and its taphonomy and orientation, lithology, and geometry, we interpret this fossil bed as having been deposited by a crevasse splay of a meandering river system.