Pliocene Fossils Of Populus L.and Acer L.from Tengchong,Western Yunnan And Their Geological Significance

The uplift of the Qinghai-Tibet Plateau profoundly influenced the global environment and biodiversity changes during the Cenozoic.Since the Late Cenozoic,the southern part of the Hengduan Mountains has been influenced by the uplift of the Qinghai-Tibetan Plateau with many sedimentary basins formed and Cenozoic strata well developed,which preserved rich plant fossils.This thesis studied leaf and samara fossils in detail from the Pliocene Mangbang Formation of Tengchong,western Yunnan.Previous studies showed that the flora is dominated by subtropical broad-leaved evergreen forests with some temperate components under warm and humid climate.This thesis focuses on a palaeobotanical taxonomic and palaeo-biogeographical analysis of the temperate components represented by Populus L.in Salicaceae and Acer L.in Sapindaceae within the flora,which holds significant scientific value as it provides insights into the composition,nature,palaeoelevation,and palaeoecological evolution of the Pliocene floras in western Yunnan.By comparing the gross morphology and cuticular structures of both extant plants and fossil records,along with conducting cluster analysis of their morphological traits,this thesis identifies the studied leaf fossils as Populus cf.rotundifolia Griffith in the genus Populus L.,and the samara fossils as Acer cf.caesium Wallich ex Brandis,Acer cf.laurinum Hasskarlha,Acer cf.cappadocicum Gleditsch,and two new fossil species,Acer pliocenicum sp.nov.and Acer trigonusfructus sp.nov.,all belonging to the genus Acer L.By integrating fossil records and molecular phylogeny,this thesis investigates the geohistorical evolution of the genera Populus and Acer.Populus may have originated in the late Eocene,with the primitive differentiation of its major taxa occurring on the northern Tibetan Plateau from the late Eocene to the early Oligocene,followed by a notable diversification in Eurasia from the Oligocene to the Miocene.Based on the studied fossil materials,P.rotundifolia may have diverged from Populus davidiana Dode and originated in the southern part of the Hengduan Mountains at least by the Late Pliocene.Acer may have originated from the late Cretaceous to the early Eocene,with its major taxa diverging in western North America during the Eocene and evolving with significant diversity in Eurasia from the Oligocene to the Miocene.A.caesium,A.laurinum,and A.cappadocicum may have emerged before the Pliocene and experienced further migration and evolution from the Pliocene to the Pleistocene in western Yunnan.Considering the distribution and habitat of the nearest living relatives of the studied fossil species,the area surrounding the fossil site may have reached or nearly approached its current elevation at least by the Late Pliocene.The diversity of some certain temperate components within the flora indicates that Tengchong climate during the Pliocene might have been influenced by the global cooling trend,but yet remained relatively warm and humid.