Desmanthus And Acer From The Oligocene Of Northwestern Qaidam Basin,China And Its Geological Significance

The Qinghai-Tibet Plateau is famous as ‘the roof of the world' because its average elevation is above 4000 m.Since the late Cenozoic,the uplift of the plateau has significantly affected climate and environmental changes in interior of Northwest China and has created a unique highland biological habitat.The Qaidam Basin,largest inland basin on the northern Tibetan Plateau,has experienced continuous aridification caused by uplift in the Cenozoic.Recently,well-persevered fossil plants,including fruits,leaves and;winged fruits,were found in the Oligocene Shangganchaigou Formation from the northwestern Qaidam Basin.These fossils provide valuable paleoclimatic and paleogeographic information.Based on the morphological study and comparison,current fossil fruits were identified pods as Desmanthus cf.virgatus Willd.(Leguminosae Juss.),leaves as Acer sterculiaceum Wall.(Aceraceae Juss.)and winged fruits as Acer sp.(Aceraceae)respectively.Current fossil species,Desmanthus cf.virgatus,represents the first fossil record of the genus and adds a new fossil record into Leguminosae.According to the ocean-crossing ability of Leguminosae pods,it is suggested that Desmanthus was originated in Africa then transported to Asia by India Plate.Current fossil leaves and winged fruits also enrich the fossil record of Aceraceae,and provide helpful information about its origin and past distribution.It corroborates the previous assumption that Aceraceae originated in south China then dispersed and transported to northeastern Tibetan Plateau in Oligocene at least.Fossil plants indicate a wider distribution of Leguminosae and Aceraceae in Oligocene than today.And the subsequent paleoenviromental and paleoclimatic changes account for their extinction.Based on the habitat and distribution of the living analogues of current fossil plants and the fossil assemblages from the study region,a warm and humid environment during the Oligocene is interpreted in northwestern Qaidam Basin.We investigated the elevation of the nearest living relatives species of current fossil plants,and speculated the palaeoelevation of study region in Oligocene is less than 2200-2500 m.As above,the Indian and East Asian monsoons might affect the northern China area and was suitable for the vegetation of the Basin in Oligocene.It might indicate that the East Asia Monsoon was established no later than Oligocene.