A New Species of Exbucklandia (Hamamelidaceae) From the Pliocene of China and Its Paleoclimatic Significance

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Eight fossil leaves identified as Exbucklandia tengchongensis sp. nov. (Hamamelidaceae) were collected from the Pliocene Mangbang Formation in Tengchong, Yunnan Province, Southwest China. The fossil leaves are characterized by the overall rounded lamina with entire margin, actinodromous venation, and cyclocytic stomata, which suggest the affinity within the genus Exbucklandia, particularly with E. populnea. A survey on the cuticles of the sun and shade leaves of modern E. populnea indicates that the shade leaves generally possess more pronounced undulate anticlinal cell walls and a much lower stomatal density than the sun leaves. Two morphotypes, i.e. sun vs. shade types, of the fossil leaves were therefore recognized. The distribution of the modern Exbucklandia suggests that the genus lives under a warm climate with a mean annual temperature (MAT) from 13 °C to 27 °C and a mean annual precipitation (MAP) from 800 mm to 2500 mm. Hence, E. tengchongensis might also live under a similar climatic condition in the Pliocene. Leaf margin analysis on the Tengchong flora supports this result. The little change of Neogene MAT in Southwest China is therefore supported.