Fossil samaras of Acer L. (Sapindaceae) from the Upper Pliocene of western Yunnan, southwestern China

The genus Acer L. (Sapindaceae Juss.) nowadays thrives in the temperate-subtropical areas of the Northern Hemisphere, especially in Yunnan and its adjacent areas in southwestern China. However, the fossil records of Acer from China, especially fruit fossils, are rather limited compared to its modern prosperity and high species diversity there. In this study, we described three taxa of Acer, viz. A. cf. caesium Wall. ex Brandi, A. cf. laurinum Hassk. ex Miq. and Acer sp., based on samara fossils from the Upper Pliocene of Tengchong, Yunnan, southwestern China. The samaras of Acer cf. caesium are characterised by large size, suborbicular to elliptical nutlet shape, and acute divergent angle; those of Acer cf. laurinum are characterised by large size, acute divergent angle, long elliptical nutlet shape and 5-7 veined nutlet surface; and those of Acer sp. are possibly underdeveloped and distinguished from all extant and fossil species of Acer by small size, distinct sulcus, and specific sub-triangular to semicircular nutlet shape. The occurrence of these fossils from Tengchong indicates that Acer was prosperous with high species diversity in western Yunnan during the Late Pliocene. The migration and differentiation of the genus Acer in southwestern China might have been accelerated by the rapid uplift of the Hengduan Mountains (HDM) and the post-Pliocene global cooling trend.