Paleoclimate significance of the Middle Jurassic flora in the Western Hills of Beijing

Abstract：Plant fossils of 38 species in 22 genera in Middle Jurassic were collected from Lower Yaopo Formation, Upper Yaopo Formation and Longmen Formation on the southern slope of the Jiulong Hill in Mentougou district, Beijing. Statistical data show that the Middle Jurassic flora of the Western Hills in Beijing is dominated by Filicopsida（27.6%）, Cycadopsida（25%）and Ginkgopsida（25%）, among which Coniopteris with 5 species and Cladophlebis with 17 species were found. The Nearest Living Relatives Method and morphologic features analysis were used to analyze palaeoclimate. The result indicates that the nearest living relative plants of dominant fossils are Osmundaceae, Dicksoniaceae and Cycadopsida, which mainly live in a humid-warm tropical or subtropical climate, and warm-humid climate is also identified from the diversity of Cladophlebis, large feathery compound leaves of Cycadopsida and coarse stems of Equisetales, etc. Consequently, the Middle Jurassic flora in the Western Hills of Beijing reflects a humid-warm subtropical climate. In addition, the climate had changed into drier and warmer according to the species and the quantity reduction of plant fossils from the Lower Yaopo Formation through Upper Yaopo Formation to Longmen Formation.