Miocene Leguminosae (Albizzia,Podocarpium,Cercis) Fossils From Lincang Of Yunnan And Zeku Of Qinghai And Their Paleophytogeography

Yunnan Province is situated at the lower latitudes with abundant rainfall and smaller seasonal temperature difference.These favorite nature conditions boosted diverse extant plant species.In geohistorical background,Yunnan acts as one of important resource pools of both fossil plants and extant ones in China and even in the world.Lincang of Yunnan and Zeku of Qinghai are located in the southeastern margin and northeastern margin of the Qinghai-Tibet Plateau,respectively.The issue about the uplift and environmental effects of the Qinghai-Tibet Plateau has being argued for a long time in geological field.Plant fossils can provide important evidence for the uplift of the Qinghai-Tibet Plateau and paleoclimatic changes.On the basis of the systematic investigations on the fossils collected from Lincang of Yunnan and Zeku of Qinghai,some conclusions were drawn as follows: One revised species,one similar species and one indeterminated species of Albizzia were identified as Albizzia scalpelliformis Guo,Li and Xie Emended,Albizzia cf.kalkora(Roxb.)Prain and Albizzia sp.,and also the origin and the migration of the genus Albizzia were discussed.It is suggested that the fossils of Albizzia originated in the African region from the Eocene.Afterward,due to the leguminous plants has the salt-tolerant ability and could migrate in long distance,the genus might migrated through the Indian Ocean to the Indian plate.In the Eocene,when the Indian plate collided with the Eurasian continent,the genus was able to land from the Indian subcontinent to the Eurasian continent.To the Miocene,this genus was distributed widely in Yunnan,Shandong,South Korea and Japan,which implicated that the genus first migrated to the Yunnan region.Then they expanded northward into other regions in favor of the Miocene climatic optimum.After the Pliocene,the fossil distribution is reduced.A known pod species—Podocarpium podocarpum(A.Braun)Herendeen 1992—was recognized.The changes of distribution range through different geological periods were discussed,and the possible origin and migration routes in the geological history were also discussed.That is,this fossil has first discovered in the Eocene,then with the Oligocene close of Tulgai Strait,it immigrated from Asia to Europe.During the Miocene,the Eurasian continent in mid-latitudes was possessed of good climatic conditions,and this genus was widely distributed in the Eurasia.To the Pliocene,only a few of fossil records were left.No fossil record occurred after the Pliocene,which shows a possible post-Pliocene extinction.A known species of Cercis miochinensis Hu et Chaney based on pods from the Miocene of Zeku,Qinghai was identified.On the basis of this genus' fossil records in different geological history,the origin and migration route of the genus was speculated,that is,Cercis originated in the Eocene at North America,and then across the North Atlantic Land Bridge migrated into the European region.In the Oligocene,the Tulgai Strait that separated Europe and Asia was gradually closed.Thus the genus Cercis was able to enter the Asian region.In the Miocene,arid zones in mid-latitudes of Eurasian continent were disappeared and the climatic condition was favorite for plants.Cercis then reached its maximum distribution in geological history.