A new Quercus(Fagaceae) morphospecies is described based on well-preserved fossil leaves. The fossils were collected from Yangyi Formation of the Upper Pliocene at the Yangyi coalmine, Baoshan, western Yunnan, China. Details of the microstructure of mesophyll tissue are preserved because lithification of the fossils was not complete. The fossil laminas possess typical characteristics of Quercus sect. Heterobalanus: quite thick and coriaceous cuticle; secondary veins bifurcating near the margin in the middle and top of the leaf; and upper epidermis with adaxial hypodermis. Based on a detailed morphological and anatomical comparison with all living and fossil species of Quercus sect. Heterobalanus, the fossil leaves prove to be different in their sparse and garland-shaped multicellular trichomes on the lower epidermis, and so it is described as a new species Quercus yangyiensis He, Li et Sun sp. nov. The much sparser trichomes of our fossils compare well with those of living Quercus sect. Heterobalanus and indicate a more humid climate during the deposition of the Yangyi Formation in the Late Pliocene.
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