Intensified East Asian summer monsoon facilitated the formation of the world's largest Late Miocene Hipparion fossil assemblage

The Linxia Basin, located in the northeastern of the Tibetan Plateau, spans the Late Eocene to the Quaternary and provides an important window into the evolutionary history of the East Asian Monsoon and Eurasian vertebrate faunas. The sedimentary facies and stratigraphic framework of the Linxia Basin is best known in the northern part of the basin, however, in the basin's southern part, where rich fossils are preserved, the geological context is under-researched. Furthermore, the Late Miocene Yangjiashan fauna, which contains the world's most extensive Hipparion fossil assemblage, mostly occurs in the southern part of the Linxia Basin. This unique palaeontological assemblage raises intriguing questions regarding the exceptional preservation conditions and ecological factors that facilitated such an extraordinary accumulation of Hipparion fauna remains in a specific depositional environment. As an indicative fossil assemblage, the Yangjiashan fauna reflects the environmental characteristics of an open and dry-hot savanna ecosystem, providing an important historical framework for investigating the origin and evolution of the modern African savanna fauna. We applied a cyclostratigraphic approach to derive orbital ages for the Milankovitch cycles in the Qianshanliang section from the southern part of the basin, which preserves abundant fossils of Yangjiashan fauna. We analyzed the evolution of the East Asian Summer monsoon (EASM) by comparing climate indicators from the Bohai Bay, South China Sea, Linxia Basin, and Tianshui Basin.Additionally, we investigated the sedimentary characteristics of the classical sections containing abundant Yangjiashan fauna fossils in the Linxia Basin. We report an astronomically tuned age for the Qianshanliang section as 7.68–6.15 Ma, with the age range for the fossil bed precisely constrained to be 7.49–7.45 Ma. We also propose that the East Asian Summer Monsoon (EASM) experienced significant intensification around 7.7–7.2 Ma and 6.6–6.2 Ma. Combined with other magnetostratigraphic results from the Linxia Basin, these two periods of intensification of the EASM can be well-matched with the occurrence of the fossil layer of the Yangjiashan fauna and are frequently accompanied by coarse sedimentation. Based on these findings, we suggest that enhancements of the EASM in the Late Miocene played a crucial role in facilitating the formation of the Yangjiashan fauna fossils.