Study On The Paleoecology And Paleogeography Of Middle Jurassic Flora In Yaojie,Gansu Province

Plants mainly rely on their own mediation to adapting to the climate change;fossil plant can reflect the environment and climate in the geological history accurately.Studying on fossil flora composition and characteristics to explore the plant paleoenvironment and palaeogeography in the geological history has important significances for understanding the future trend of the global environment and ecosystems.The fossils studied in this paper were collected from the Yaojie Formation in Lanzhou,Gansu Province.After collection,classification and identification,the composition of Yaojie flora was clarified.On this basis,the geological age and paleoenvironment of the flora were studied.By using the cluster analysis and branch biogeographic analysis method,combining with the spatial and temporal distribution characteristics of the genus Gonatosorus in the Mesozoic of China,the plant geographic division of northern China and plant geographic attribution of the Yaojie flora were discussed.The main conclusions as follows:(1)The collected plant fossils were identified as 18 genera and 31 species,combining with previous research data,a total of 31 genera including 50 species were identified,including newly discovered Gonatosorus shansiensis.Ginkgopsida(30%)and Filicopsida(26%)were the most abundant compositions in this flora,Coniferopsida(18%)and Cycadopsida(16%)occupied for some proportion,Sphenoposida(8%)were also found.(2)On the basis of flora compositions and geological age of the flora indicated by the fossils,compared with some similar flora at home and abroad,it is considered that the geological age of the Yaojie flora in Lanzhou should belong to the Aalenian to Bajocian.(3)Through the analysis of burial condition of plant fossils,it is shown that most of them are in situ or sub-situ burial and can be used as a basis to reflect the ecological environment.By studying factors such as the ecological environment reflect by various groups of flora,plant geographical distribution,depositional environment of Yaojie Formation,and other factors that influence the formation of climate,indicating a warm and humid climate of Yaojie basin in the Middle Jurassic.(4)The Middle Jurassic floras in northern China can be divided into two geographical units: the area I include Beijing Xishan and Henan Yima;the area II includes northwest,northeast and western North China of northern China.The results show that Yaojie flora belongs to the Siberian flora region and has a close relationship with Gaotouyao flora in Inner Mongolia,this two floras can be classified as a small plant subordinate in Siberian flora region,and the boundary between the Siberian and Europe-China flora region is supposed in the south of Yaojie flora.