Fossil Conifers From The Pliocene Of Tengchong, Yunnan Province, China And Their Palaeobiogeographic Significances

Conifers are the most flourishing gymnosperms.They are widely distributed in plains and mountainous areas with different latitudes and different altitudes.Severe crustal movement and sea-land change provide a moderate humidity condition for the prosperity of angiosperms,which rapidly flourish in the Late Cretaceous,replacing the dominant position of conifers.In the Cenozoic,the climate in general showed cold and dry trend.During the period of warming and cooling,the range of plant distribution was expanded or withdrawn,and the geographical distribution was finally formed.Yunnan province is located in the southwestern frontier of China.Due to the large-scale uplift and disintegration of the Qinghai Tibet Plateau,the rivers and moutains are densely distributed.The complex geomorphology structure make it unique to the vegetation landscape and the stereoscopic climate characteristics.Because of its unique geographical environment,it often becomes a plant refuge during the cold period of the Cenozoic.And the stratigraphic sequence is fully developed and the fossil remains rich in the inter mountain basin,which is an ideal place to study the evolution and distribution of the Cenozoic plants.The coniferous fossils studied in this paper are collected from the Mangbang Formation in the Tuantian Basin,Tengchong,Yunnan,which is assigned to the Pliocene.4 species of 4 genera within 3 families are identified,based on comprehensive analysis of leaf macromorphology and cuticle characteristics,including a new species-Pinus plioarmandii,two sibling species of extant plants,Podocarpus cf.forrestii and Nageia cf.nagi,one Known fossil species,Calocedrus lantenoisii.Based on the coniferous fossils studied in this paper and previous fossil record,we have deduced the palaeobiogeography.(1)The fossil record indicated that the white pines had migrated to southern China by at least the Late Eocene.However,Asian white pines do not occur in or adjacent to our fossil site in western Yunnan today.Therefore,we consider that the glacial events during the Pleistocene are the possible factors for white pines to retreat from western Yunnan,and the rapidly uplifted mountains and deep river valleys might have posed a significant geographic barrier for the genetic exchange during the warm interglacials.(2)The genus Podocarpus distributed in the Southern Hemisphere during Eocene,then moved northward.It was distributed in the mid-high latitudes of the Northern Hemisphere during the Late Eocene to Oligocene.Afterwards,it dispersed to the low and intermediate latitude areas,it distributed in the southern China during the Miocene-Pliocene.Nageia might originated in the northeastern part of Asia in the Early Cretaceous and dispersed to South China at least in the Eocene,which may be one fo the centers for early diversification of Nageia.They gradually disappeared until the climate became warmer during the Miocene-Pliocene and redistributed in the southern China,as the climate became cold.(3)The same as the Pinus,Calocedrus North America flora has close relationship with East Asian flora,but has a distant relationship with European flora,which may be the result of the communication between the North American flora and the East Asian flora through the Bering land bridge in the Cenozoic era,while the seaways that extended the full north-south length of the continents divided the continents into separate phytogeographic provinces.The habitat of Podocarpus,Nageia,Calocedrus,the subgenus Strobus is the warm and humid envoirment.And their distribution is larger during the Pliocene than it is today.We infer that the climate of Tengchong is warmer and more humid during the Pliocene.