A Study Of Two Fossil Genera Of Pteridophyta From Oligocene In The Northwestern Qaidam Basin

The Tibetan Plateau,with the highest mean altitude in the world,suffered intense tectonic movements and uplifts in Cenozoic,which results in great changes in topography in Asia and impacts on regional to worldwide climate profoundly.The Qaidam Basin,located in the northeastern plateau,is the largest inland basin on the plateau.The thick and continuous sediments have contained abundant information of geological events and environmental variation in Cenozoic.After plentiful great progress in the Qaidam Basin by previous studies,two genera assigned to Pteridophyta are reported from Oligocene Shangganchaigou Formation of Huatugou area in northwestern Qaidam Basin.Based on the comparison with extant species and fossil records in morphological characters,2 genera are recognized,which are Equisetum cf.fluviatoides,Equisetum sp.and Adiantum qaidamensis sp.nov.The existence of current Equisetum fossil supplements the fossil record of aerial stem in Oligocene Qaidam Basin.According to the fossil records and phylogenetic studies,it could be deduced that Equisetum with a Gondwanic origin,after a migration towards north hemisphere,reached the Qaidam Basin in Oligocene in the dissemination in the north.Current fossil Equisetum cf.fluviatoides may come from North America,due to the specific morphological similarity to the fossil species from North America.The migration history of current fossil together with other Oligocene fossil plants from Huatugou area,indicates the biogeographic relationship between the Eastern Asia and America and the significance of the Qaidam Basin in communication between different flora.Besides,current fossil pinnule enriches the fossil records of Cenozoic Adiantum megafossil and credible fertile pinnule in China,expanding the distribution of Adiantum in geology history.According to the growth habit of current fossil plants,the humid condition with rich water source may be a suitable habitat,such as shores of streams,rivers or lakes,and the bottom of forests.The paleoenvironment can be deduced.The result indicates that compared with modern times the environment in northwestern Qaidam Basin was humid and warm,with a low elevation.