Fossil freshwater mollusks from the Early Pleistocene (Calabrian) of northern Taiwan

Despite the extensive fossil mollusk records in Taiwan, over 99% are marine species. Among the few freshwater deposits, the Tananwan Formation (1.46-1.24 Ma) in northern Taiwan has been recognized as one of the most prolific fossiliferous strata. In a 1-meter thick fossiliferous mudstone layer within the formation, we identified five molluscan taxa. The assemblage is primarily dominated by Sinotaia quadrata (Benson, 1842) and the less common Melanoides aff. tuberculata, which corresponds to the current freshwater ecosystem in northern Taiwan. This study presents detailed descriptions and images of fossil freshwater molluscan assemblage from Taiwan, providing insights into the origins of contemporary freshwater biodiversity in the subtropical West Pacific. Additionally, in several fossil S. quadrata specimens, juvenile shells were found within female shells, marking the second known case of child-carrying in fossil viviparids.