Pliocene Fossils Of Hamamelidae From Tengchong,Yunnan ——Phylogeny,Phytogeography And Paleoclimate

Tengchong County in western Yunnan is located in the western part of the Hengduan Mountains and the southeastern edge of the Qinghai-Tibet Plateau.The influence of strong tectonic movements since the Neogene has formed the ecological environment with complex topography,diverse and three-dimensional climate,and unique and abundant vegetation.The Neogene stratigraphy in western Yunnan is well developed and extremely abundant in the preservation of plant fossils,which is the best place to study the present and Cenozoic plant fossils in China.By analyzing the macro and micro structures of leaves,the fossil leaves and their associated fungi collected from the Pliocene Mangbang Formation in Tengchong Tuantian were identified in detail.The fossils of two genera and four species of angiosperms in the family Hamamelidaceae were identified,including two species of Exbucklandia(one new fossil species E.pilosus sp.nov.)and two species of Rhodoleia(one new fossil species Rh.multivenia sp.nov.).Also,a new fossil species of fungi(M.tengchongensis sp.nov.)of the genus Meliolinites in the family Meliolaceae,associated with the fossil Rhodoleia,has been identified.It is concluded that the genus Exbucklandia most probably originated in the Late Cretaceous of East Asia,spread to other European regions through three divergences in the Cenozoic,and was restricted to the subthermal-tropical regions of East and Southeast Asia after the Pliocene,probably due to the ice age.The genus Rhodoleia most likely originated in the Late Cretaceous of central Europe and spread to southwestern China at least before the Late Pliocene and probably spread to other parts of East Asia after the Late Pliocene.The epidermal hairs in fossil species E.pilosus are abundant,and in E.tengchongensis are absent,while leaves of extant Exbucklandia have rather few epidermal hairs.The abundance of epidermal hairs in the genus Exbucklandia shows a tendency of decrease.The epidermal hairs in Rh.multivenia are rather few,and in Rh.tengchongensis are abundant,while leaves of extant Rhodoleia usually have a large number of epidermal hairs.Thus,the abundance of epidermal hairs in the genus Rhodoleia shows a tendency of increase,which might be a result of insect herbivory defense.All the four fossil species showed different degrees of insect herbivory on the leaf surface,and the appearance of fungi of the family Meliolaceae on the leaf surface of the fossil Rhodoleia may be related to insect herbivory.In this paper,we also reconstructed the palaeo-atmospheric CO₂ concentrations in the Pliocene of Tengchong using three different modeling approaches based on the stomatal parameters and carbon isotope values of four fossil species and their recent extant counterparts.The mean value of palaeo-atmospheric CO₂ concentrations obtained by the stomatal ratio method of four fossil plants of Hamamelidae was 423.1ppmv,the mean value of 342.0 ppmv by the photosynthetic gas exchange modeling method the reliability of the results obtained by the three methods is discussed and compared.In addition,the water use efficiency(WUE)of fossil and extant plants was reconstructed from the carbon isotope values of fossils and their near-extant counterparts,and it was found that the WUE of fossil plants was lower than that of extant plants,and such reconstructed results may support the warm and humid climate of the Pliocene of Tengchong.