A fossil oak species,Quercus tenuipilosa Q.Hu et Z.K.Zhou,is reported from the upper Pliocene Ciying Formation in Kunming,Yunnan Province,southwestern China.The establishment of this species is based on detailed morphologic and cuticular investigations.The fossil leaves are elliptic,with serrate margins on the apical half.The primary venation is pinnate,and the major secondary venation is craspedodromous.The tertiary veins are opposite or alternate-opposite percurrent with two branches.The stomata are anomocytic,occurring only on the abaxial epidermis.The trichome bases are unicellular or multicellular.The new fossil species shows the closest affinity with theextant Q.delavayi and the late Miocene Q.praedelavayi Y.W.Xing et Z.K.Zhou from the Xiaolongtan Formation of the Yunnan Province.All three species share similar leaf morphology,but differ with respect to trichome base and stomatal densities.Q.tenuipilosa.Q.praedelavayi,and Q.delavayi can be considered to constitute the Q.delavayi complex.Since the late Miocene,a gradual reduction in trichome base density has occurred in this complex.This trend is the opposite of that of precipitation,indicating that increased trichome density is not an adaptation to dry environments.The stomatal density(SD)of the Q.delavayi complex was the highest during the late Miocene,declined in the late Pliocene,and then increased during the present epoch.These values show an inverse relationship with atmospheric CO_2 concentrations,suggesting that the SD of the Q.delavayi complex may be a useful proxy for reconstruction of paleo-CO_2 concentrations. A fossil oak species, Quercus tenuipilosa Q. Hu et ZK Zhou, is reported from the upper Pliocene Ciying Formation in Kunming, Yunnan Province, southwestern China. The establishment of this species is based on detailed morphologic and cuticular investigations. Fossil leaves are elliptic , with serrate margins on the apical half. The primary venation is pinnate, and the major secondary venation is craspedodromous. The tertiary veins are opposite or alternate-opposite percurrent with two branches. The stomata are anomocytic, occurring only on the abaxial epidermis. trichomes are unicellular or multicellular.The new fossil species shows the closest affinity with theextant Q.delavayi and the late Miocene Q.praedelavayi YWXing et ZKZhou from the Xiaolongtan Formation of the Yunnan Province. All three species share similar leaf morphology, but differ with respect to trichome base and stomatal densities.Q.tenuipilosa.Q.praedelavayi, and Q.delavayi can be considered to constitute the Q.delavayi complex.Since the late Miocene, a gradual reduction in trichome base density has occurred in this complex. This trend is the opposite of that of precipitation, indicating that increased trichome density is not an adaptation to dry environments. The stomatal density (SD) of the Q. delavayi complex was the highest during the late Miocene, declined in the late Pliocene, and then increased during the present epoch. These values ​​show an inverse relationship with atmospheric CO_2 concentrations, suggesting that the SD of the Q. delavayi complex may be a useful proxy for reconstruction of paleo-CO_2 concentrations.