The Late Triassic Yangcaogou Flora From Beipiao,Liaoning Of China And The Reconstruction Of Plant Ecosystem

The Late Triassic Yangcaogou flora is located in the Yangcaogou Basin near Yangcaogou Village of Changheyingzi Township in the southeast of Beipiao City of western Liaoning.The Yangcaogou Basin is a small-scale Late Triassic-Middle Jurassic continental sedimentary basin.A set of Late Triassic strata mainly dominated by the fluvial and lacustrine with intercalation of the swamp deposits were developed in the basin,which is called the Yangcaogou Formation.It is characterized by the composition of the khaki thick conglomerate,feldspar-quartz sandstone,siltstone,mudstone with intercalation of the grey and yellow green sandstone,siltstone and the thin coal,and indicates an alluvial fan sedimentary system,including sedimentary microfacies of the debris flow,channel sediment,braided-river channel,the sheet flow,small-scale shallow lakes and the swamp.According to the rock assemblage,the Yangcaogou Formation can be subdivided into the conglomerate member,the sandstone-siltstone member and the sandstone member in ascending order,and yielding fossil plant within each member.The results of this study indicate that the Yangcaogou Flora is a Late Triassic flora with rich biodiversity.It provides the important fossil evidence for studying the Late Triassic ecosystem,the climatic feature and the geographical environment in the northeastern margin of Eurasia.Furthermore,the results also have a valuable reference to explore the Triassic land plants recovery process,the Late Triassic plant geographic division,the land plant evolution,mass extinction and succession during the Late Triassic and the Early Jurassic,the terrestrial Triassic and Jurassic boundary etc.The fossil plants are collected from the fine sandstones,the siltstones and the mudstones of the Upper Triassic Yangcaogou Formation and all of them are preserved as impression.According to the systematic studies,the flora is composed of 77 species belonging to 34 genera(including 2 new genera and 5 new species),Bryophytes(1 genus,Czekanowskiales(5 genera,7 species,9.09%),coniferopsida(4 genera,15 species,19.48%),the flowers and seeds of gymnosperms and the unknown group of the form genus(2 genera,4 species,5.19%).The statistical results indicate that the Yangcaogou Flora is dominated by gymnosperms and ferns.Among Gymnosperms,Cycads and Conifers have the largest number of species,but the number of the genus is lessExcept for a small number of Dipteridaceae,Ferns is dominated by the rich biodiversity of the Filicopsida incertae sedis Cladophlebis.According to the distribution of the known species,for example,Neocalamites rogosus Sze,Clathropteris obovata Oishi,Cladophlebis grabauiana Sze,Cl.gracilis Sze,Cladophlebis kaoiana Sze,Pterophyllum ambabiraensis(Srebrodolskaja)Schorochova,Glossophyllum?shensiense Sze,Cycadocarpidium erdmanni Nathorst,C.swabii Nathorst,C.tricarpum Prynada,Podaozamites schenki Heer,P.distans(Orersl)Braun,P.giganteus Sun,Stenorachis bitchuensis Oishi,Taeniopteris linearis Mi et Sun and so on are all the index elements of the Late Triassic flora at home and abroad.The occurrence of them demonstrates that the geological age of the Yangcaogou Formation should be the Late Triassic.The Yangcaogou Flora represents a Late Triassic Flora in the northern part of the western Paleo-Pacific,northeastern margin of Eurasia.Based upon the compositional features and elements of the flora,The Yangcaogou Flora can compare with those of the Late Triassic distributed in the northern China,Japan and the far east region of Russia,such as the Yanchang Flora destributed in the North China,Northwest China and the floras from Northeast China,including the Luoquanzhan Flora from Dongning of northeastern Heilongjiang,Tianqiaoling Flora from Wangqing,Jilin,the Xiaomifengdingzi Flora from the upper part of the Xiaomifengdingzi Formation of Bamingshi coal mine,Shuangyang,Jilin,the Dajianggang Flora of Shuangyang,Jilin,the Beishan Flora from Shiren of Hhunjiang(now Baishan city),Jilin,the Xiaoyingzi Floral from Soshuzhen of Bashan,Jilin,the Laohugou Floral of Lingyuan,Liaoning,the Nariwa Floral from Japan and the Mongugai Flora from the far east region of Russia.These floras are mainly founded in the continental sedimentary basin(or bearing marine interlager)or continental volcanic-sedimentary basins of the northern part of the western Paleo-Pacific zone,northeastern margin of Eurasia.Due to the complicated geotectonic background and 1 species,accounting for 1.30% of total species),Sphenopsida(5 genera,7 species,9.09%),Ferns(6 genera,19 species,24.68%),Cycads(7 genera,17 species,22.08%),Cordaitopsida(1 genera,1 species,1.30%)),Ginkgopsida(3 genera,6 species,7.79%),different ecological environments,the floras from the deferent localities in this region show certain similarity and differentiation.The similarity is that the dominated groups and the index elements from different floras are approximately same.This manifests mostly identical of the climatic zone and geological age of these floras.The differentiation is that the composing proportion of plant groups and the elements is not identical.In addition to the factors of fossil preservation and collection,the reasons of differentiation are mainly related to the complicated geographical condition and local ecological environment of the Late Triassic in the northern part of the western Pacific.As today's terrestrial ecosystems,besides the vegetation types of the geological time were mainly controlled by the latitude of the earth,the factors of the position of ancient continent,local ecological environment,mountain system,volcanic activity and the warm current from ocean,also influence composition of plant groups and the elements from various localities greatly,showing up the differentiation among the floras from the different localities.According to the analysis of the compositional characteristics,the Yangcaogou Flora is composed of Ferns,Cycas,Ginkgo phytes,Czekanowskiales and Conifers.Except for a small number of Dipteridaceae elements,the main component groups and elements display the features of the floras from the warm temperate-temperate zones in northern China,and obviously different from those of southern China.The characters of the Yangcaogou Flora demonstrate that Yangcaogou was located in the continental climatic zone with obvious seasonal changes in the warm temperate zone in northern China during the Late Triassic,but because of the impact of the Paleo-Pacific warm current and volcanic eruption,the local warm and humid climate occurred in the Yangcaogou basin,except for making the flora shows the main characteristics with seasonal variation in warm temperate of northern continent,a small number of the plant groups mostly occurred in the tropical-subtropical floral of southern China are also found in the Yangcaogou Flora.Based on the comparison between the living habits of the fossil plant groups and the living plant groups,and the component group analysis of the Yangcaogou Flora,it can be inferred that Yangcaogou Flora represents a Late Triassic forest ecosystem with seasonal and rainfall changes in a warm temperate zone.In this ecosystem,the trees dominated by gymnosperm,such as Ginkgo phytes and Conifers,constitute the main body of the forest ecosystem.The shrub and herbage,such as cycads,Czekanowskiales,ferns and bryophytes,are also the associated groups in this forest ecosystem.According to the ecological analysis of plant groups,the forest ecosystem of the Yangcaogou can be subdivided into five plant communities: 1.The highland coniferous community.The dominant group is coniferous,which is similar to the living coniferous forest in the warm temperate-temperate zones,and usually grew in the higher place of the mountainous area around the basin.2.The slope broad-leaved and coniferous mixed community.Most are the broad-leaved and coniferous trees and shrubs,and the dominant groups were ginkgop phytes,Czekanowskiales,conifers,and some ferns may grow in the lower wet sites under the crown and the moist forest ground of the slopes in this community.3.The lowland cycads-ferns community.The dominant group are cycads and ferns.They grew in the areas with low topography,moist soil and good perviousness.4.The waterside-wetland horsetails community.The dominant elements belong to horsetails,which mainly grows in the humid land and the margin of pools and rivers in the basin,such as the edge of the small lake,bank of the river,the floodplain,swamp or the low-lying humid area.The aquatic and semiaquatic elements of Marsileales grew in a swamp or shallow waters.5.The bryophytes community.Only Thallites were found.Its ecological environment is similar to living liverworts,which mainly grows in warm and wet environment,such as the surface of soil,rocks and tree trunks under the crown and the wet areas.