Redescription of Plesioschizothorax macrocephalus (Cyprinidae, Schizothoracini) from the Miocene of Lunpola Basin, Central Qinghai-Tibet Plateau, and its paleobiogeographic and paleogeographical implications

Abstract

The evolution of paleo-drainages on the Qinghai-Tibet Plateau is significant for understanding the paleogeographic, topographic, and the orogenic exhumation history. Fossils of freshwater fishes are helpful for understanding the biological and geological interplays in the ecosystem because they are usually restricted to the drainages, which are in turn constrained by tectonic events. This study re-examines Plesioschizothorax macrocephalus, a fossil cyprinid fish from the Lower Miocene of the Lunpola Basin in the 1970's. Our updated phylogenetic analysis based on total-evidence dating suggests that Plesioschizothorax is most closely related to extant Percocypris, a genus currently found in the Mekong, Salween, and Upper Yangtze Rivers. Their evolutionary split, estimated to have occurred in the Late Oligocene (ca. 26.60 Ma), coincides with the intensification of Asian monsoon in the central Qinghai-Tibet Plateau. Our ancestral distribution reconstructions indicate that the common ancestor of these two fishes likely inhabited the central Qinghai-Tibet Plateau. During the Late Oligocene (26.60 Ma) to Early Miocene (21.63 Ma), their descendants dispersed to the paleo-Mekong and Salween Rivers. We assume that the enhanced precipitation due to the strengthening of the Asian monsoon had led to an expansion of the upper paleo-Mekong and Salween Rivers during the Late Oligocene to Early Miocene, creating a hydrological connection with the paleo-lakes in the Lunpola Basin and thereby providing pathways for dispersal. This scenario implies that the current internal drainage systems in the central plateau likely formed after the Early Miocene.