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Steppe vegetation represents a key marker of past Asian aridification and is associated with monsoonal intensification.Little is however,known about the origin of this pre-Oligocene vegetation,its specific composition,and how it changed over time and responded to climatic variations.Here we describe themorphological characters of Ephedraceae pollen in Eocene strata of the Xining Basin,and compare the pollen composition to the palynological composition of Late Cretaceous and Paleocene deposits of the Xining Basin and the Quaternary deposits of the Qaidam Basin.We find that:a) the Late Cretaceous steppe was dominated by Gnetaceaepollenites,b) in the transition from the Cretaceous to the Paleocene Gnetaceaepollenites became extinct and Ephedripites subgenus Ephedripites dominated the flora with rare occurrences of Ephedripites subgen.Distachyapites,c) the middle to late Eocene presents a strong increase of E.subgen.Distachyapites,and d) the Quaternary/Recent is marked by a significantly lower diversity of Ephedraceae(and Nitrariaceae) compared to the Eocene.In the modern landscape of China only a fraction of the Paleogene species diversity of Ephedraceae remains and we propose that these alterations in Ephedreaceae composition occurred in response to the climatic changes at least since the Eocene.In particular,the strong Eocene monsoons that enhanced the continental aridification may have played an important role in the evolution of E.subgen.Distachyapites,triggering an evolutionary shift to wind pollination in this group.Conceivably the Ephedraceae-Nitrariaceae dominated steppe ended during the Eocene/Oligocene climatic cooling and aridification,which favored other plant taxa.