Fossil Schizothoracines From The Oligocene Of Northwestern Qaidam Basin And Their Implications

The rapid uplift of the Tibetan Plateau,caused by the India-Asia plate collusion,during the late Cenozoic has profoundly influenced the terrestrial ecosystem in the region.The Qaidam Basin is the largest inland basin in the north-eastern Tibetan Plateau,China,and it is key to understanding the Cenozoic climatic and biological changes that have occurred on the plateau because of the continuous Cenozoic sediments with a maximum thickness of ?12,000 m in the basin.Compared with the abundant results from the Cenozoic geological research based on sedimentology,tectonics,magnetostratigraphy,palynology,and isotope geochemistry,the records of Paleogene megafossil are comparatively rare.In recent years,we collected many fossil fishes from the Shangganchaigou Formation(=Upper Ganchaigou Formation)in the Huatugou area of the north-western Qaidam Basin.We compared these fossils with related extant and fossil taxa,and erect a new fossil genus of the subfamily Schizothoracinae(Cyprinidae): Paleoschizothorax;including two fossil species: Paleoschizothorax qaidamensis and Paleoschizothorax diluculum(published online).The geological researches indicate that the sedimentary time of the Shangganchaigou Formation in Huatugou area is Oligocene;therefore,Paleoschizothorax is the earliest credible fossil schizothoracine by far.It sheds new light on the origins and early evolution of schizothoracines,which are a type of endemic freshwater fish in and around the Tibetan Plateau.Paleoschizothorax has three rows of teeth on pharyngeal bone with a pattern of 5/3/2;it shares many osteological characters with barbines and primitive schizothoracines,and it closely resembles the latter in the size and shape of the scales: body scales are small and oval;the number of lateral line scales exceeds 100;and the long-oval scales from the pre-anal region are very similar to the ‘anal scales',which are unique to schizothoracines.A phylogenetic analysis,which included 13 extant genera and two fossil forms of barbines and schizothoracines and 70 morphological characters,also supports the close relationship between Paleoschizothorax and primitive schizothoracines.Fossil fishes and concomitant fossil plants indicate that the elevation of fossil site in the Oligocene might be no higher than 2500 m;and the climate conditions at that time were much warmer and more humid than that of the current,it is similar to some regions in modern central-eastern Sichuan Province and central Yunnan Province.These fossils also suggest that there used to be a deciduous broad-leaved forest,which provide suitable ecological environment and rich nutrients for the fishes.By learning about the spatial and temporal distribution of fossil barbines and schizothoracines,and the significant impact of the global cooling at the EoceneOligocene transition(EOT)on the northeastern Tibetan Plateau,we deduce that the origin of primitive schizothoracines is closely related to this global climate shift.Moreover,we speculate that the primitive schizothoracines likely originated in the northern Tibetan Plateau based on the fossil records and molecular phylogeny of the extant taxa.