Study On Miocene Equisetum And Gastropod Fossils From The Wulan Basin,Qinghai Province

As the Cenozoic crossroads of floristic component exchange in the Northern Hemisphere,the Qinghai-Tibet Plateau has become a hottest region in the study of paleontology and paleoclimate.On the basis of previous studies on animal and plant fossils in the northeast margin of the QinghaiTibet Plateau,this paper studies the Equisetum and gastropod fossils from the Miocene Youshashan Formation in the Wulan basin in the northeast margin of the plateau,and discusses their geological significance.Through the comprehensive comparative study on the morphology of current fossil specimens,living groups and related fossils,the animal and plant fossils identified in this paper belong to 3 genera and 4 new species.There are 1 genus and 2 new species of Equisetum-wulanensis Zhang et Xie sp.nov.and Equisetum kekeensis Zhang et Xie sp.nov.Two genera and two new species were identified in Gastropoda,which are Gyralus haixiensis Zhang et Xie sp.nov.and Opeas conicus Zhang et Xie sp.nov.The tuber fossils in this paper are representative of the first occurrence of Equisetum from the Miocene of the Wulan basin so far.The fossil records of Equisetum tubers in the geological historical period are summarized and illustrated.It is found that the morphological types of Equisetum tubers tend to be reduced from the Cretaceous to the present.The statistics of tuber volume and surface area on a single node of Equisetum are as follows: from Cretaceous via Oligocene and Miocene to now,the tuber volume and surface area experienced the process of first increasing and then decreasing.This paper also investigates on the fossil records of Spirulina and Spirulina,and discusses their biogeographic significance.On the basis of fossil materials,the sketches of Miocene Equisetum and gastropods are reconstructed.Based on the coexistence analysis of the Nearest living Relatives of angiosperms in Wulan flora,the Miocene paleoelevation of Wulan basin is reconstructed,and the result is that the Miocene paleoelevation of Wulan basin does not exceed 1680 m;the Miocene paleoclimate in Wulan basin is reconstructed as follows: the annual average temperature is 13.3-19.1 ?,the average temperature in the hottest month is 22.2-23.7 ?,the average temperature in the coldest month is 3.6-10.0 ?,and the annual average precipitation is 999-1736 mm.This data is much higher than the modern corresponding parameters of the Wulan Basin,indicating a warm and humid subtropical climate in Wulan area during Miocene,which is in contrast to the cold and arid environment in Wulan today.The contrasting climates may be as a synactic result of the global cooling,uplift of ELA mountainsand and the uplift of the Qinghai-Tibet Plateau in the Cenozoic.Finally,combined with fossil evidence,the scenario of Miocene paleoecological environment in the Wulan basin is figured out.The restored Miocene paleoecological environment of Wulan basin is a subtropical climate with a wide variety of animals and plants and a warm and humid climate.