Microstructures Of The Late Permian Plants From Yumen Of Gansu And Paleoatmospheric CO₂ Reconstruction

Collision and amalgamation of the Tarim-North China Plate and the Siberia Plate happened in mid and late Permian,led to the exchange of the Cathaysian and Angara floras resulting in a mixed Cathaysian-Angaran flora in the northern Qilian region.The formation,development and evolution of the mixed Late Permian Cathaysian-Angaran flora,is important for the reconstruction of the continental position,the restoration of the paleogeography and paleoenvironment,and the indication of paleoclimatic conditions.However,up to date,investigation on the fossil plants of the Late Permian in this area has been confined to the macrostructures,their microstructures being largely unknown,which greatly limits the identification of some fossil species in this flora and the extraction of paleoenvironmental and paleoclimatic data from the fossil cuticles.Plant fossils were collected from the Sunan Formation of Yumen in Gansu Province.The microstructures of most of the fossils were investigated.Knowledge of the cuticular structures of Comia,Tychtopteris,Compsopteris,Zamiopteris and Iniopteris studied fill the void and inadequate knowledge of the epidermal features of these form-genera.The microstructures of these genera suggest that Comia,Tychtopteris,Compsopteris,Zamiopteris should be assigned to the Peltaspermales,while the form-genus Iniopteris perhaps belongs to the Pteridospermopsida.All the genera of Coniferopsida in the flora are also studied in detail,including their morphological characteristics and anatomical structures.Based on the features of the vegetative and reproductive organs,a new conifer,Gansuvoltzia minuta gen.et sp.nov.,is described and proposed as the type species of a new genus of the Voltziales.In addition,three new conifer species Pseudovoltzia yumenensis sp.nov.,Ullmannia qilianshanensis and Quadrocladus pendulinus sp.nov.are also described.The female cone of Gansuvoltzia minuta is composed of at least 9 helically arranged cone scales,which have an erect ovule produced on the middle part of the adaxial surface,but does not have bract-dwarf shoot complexes.The fact that the ovules of Pseudovoltzia yumenensis are attached to the cone scale by its free seed stalks provides new evidence for supporting Florin's evolutionary series of early conifers.The ovule and a few pollen grains were observed on the same cone scale of Ullmannia qilianshanensis,suggesting that the female cone may be in the process of being pollinated.Species of Sphenopsida,Filicineae,Cycadopsida,Ginkgopsida,and few Pteridospermopsida,which are preserved as impressions or have especially thin cuticles,are described only from their macrostructures.On the basis of the morphological characteristics and microstructures of the fossil plants,58 species belonging to 29 genera of 6 classes,including one new genus and eight new species,are identified and described.Combined with the fossil species reported in previous studies,the plant composition of the Sunan Formation in Yumen is analyzed.The result shows that the most abundant element of the flora is the Pteridospermopsida,followed by the Coniferopsida,and then the Sphenopsida.Filicineae,Cycadopsida,and Ginkgopsida are all less common.Based on the composition of all species in the flora,the author recognizes that the present flora represents a mixed Cathaysian-Angaran flora.The Angaran elements are slightly more common,and some Euro-American elements also occurred.On the basis of the age range of all species,except for the new species,the geological age of the flora is suggested to be the Late Permian.Stomatal parameters of leaves are sensitive indicators of environmental change,which have been widely used by paleobotanists and geologists to trace the change of paleoatmospheric CO2 concentration during geological history.The reconstruction of paleoatmospheric CO2 by stomatal parameters of fossil plants is becoming more and more prevalent and has lead to important achievements in the reconstruction of paleoatmospheric CO2 concentrations in the Cenozoic and Mesozoic.However,due to material constraints and other factors,less research on late Paleozoic atmospheric CO2 concentrations using this method has been carried out,with the exception of a few foreign paleobotanists who have explored the potentials using fossil plants of Pteridospermopsida and Coniferopsida.Until now,almost no research in this field has been carried out in China.Fortunately,well-preserved cuticles of Pteridospermopsida and Coniferopsida provide good material to reconstruct the paleoatmospheric CO2 concentrations of the Late Permian using stomatal parameters.As stomatal frequency of fossil conifers and seed ferns are generally considered to be potential indicators of pCO2 values of the late Paleozoic,the species Comia dashankouensis(Pteridospermopsida)and Quadrocladus solmsii(Coniferopsida)from the Sunan Formation in Yumen are used to reconstruct the Late Permian paleoatmospheric CO2.Based on the ratios between stomatal indices of fossil cuticles and those of their modern analogs(nearest living equivalent),and after using the Carboniferous and Recent Standardizations to explore ranges,the pCO2 value estimated by Comia dashankouensis is 1164-1941ppmv,while the value estimated by Quadrocladus solmsii is 700-1166ppmv.The results suggest that the Carboniferous Standardization is more suitable for conifers and the Recent Standardization is more suitable for seed ferns when reconstructing the pCO2 of the late Paleozoic.