Study On Paleoecology And Paleogeography Of Middle Jurassic Flora In Baojishan BASIN, Gansu Province

In the biological composition of terrestrial ecosystem,plants,as producers,largely determine the function of terrestrial ecosystem,and become an ideal carrier to depict paleoclimate and reveal the succession of ecological structure under environmental change.Therefore,studying on fossil flora composition and characteristics to explore plant diversity and the evolution of ecosystems in the geological history has important scientific 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 Baojishan basin,Gansu Province,China.After collection,classification and identification,the composition of Baojishan 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,the plant geographic division of northern China and plant geographic attribution of the Baojishan flora were discussed.The main conclusions as follows:(1)The plant fossils identified and described in the laboratory are 50 species in 19 genera.Combining with previous research data,the composition of the Middle Jurassic flora in Baojishan basin is sorted out,and the results show that there are 73 species of plant fossils in 27 genera.Among which,Ginkgopsida(41.10%)and Filicopsida(35.62%)are flourishing,Coniferopsida(8.22%)account for a certain proportion,and both Sphenoposida(6.85%)and Cycadopsida(5.48%)were found,Fructus et Semina fossils are poor(2.74%)and Pteridosperms do not exist.Based on the age indication of the fossil compositions and compared with the similar flora at home and abroad,the geological age of the present fossil horizons of the Yaojie Formation in Baojishan basin should belong to the Aalenian to Bajocian(Middle Jurassic).(2)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.According to the living environment of all kinds of living plants,the ecological environment of Middle Jurassic flora in Baojishan basin of Gansu Province is restored,and four plant communities are divided: Shoreline-Wetland Sphenoposida plant community,Lowland Filicopsida-Cycadophytesplant community,Slope land broad-leaved plant community,Highland coniferous plant community.Comprehensively analyze the ecological environment reflected by various groups of flora,the paleogeographical distribution of plants,the characteristics of leaf physiognomy and the sedimentary environment of yaojie formation,it is shown that Baojishan basin had a warm and humid climate in the Middle Jurassic.(3)The regional division and comparison of 12 main plant fossil sites in Middle Jurassic in China are made to explore the geographical attribution of plants in Baojishan flora of Gansu province by cluster analysis.We found the Middle Jurassic floras in northern China can be divided into two geographical units: the area I includes Beijing Xishan and Henan Yima;the area II includes north-west,north-east and west of North China of northern China.Moreover,the flora in Baojishan is closely related to the flora in Huating of Gansu.The geographical relationship of Baojishan flora,Huating flora and Yaojie flora in Lanzhou was analyzed by branch biogeography.The results showed that the geographical relationship between Baojishan flora and Huating flora in Gansu was closer than that of Yaojie flora,which was consistent with the conclusion of cluster analysis.