Taxonomy Of Fossil Conifers And Quantitative Reconstruction Of Palaeoclimate During The Permian-Triassic Transition In South China

The Permian-Triassic transition is an important period in the history of the Earth's evolution.The structure of terrestrial ecosystems has undergone a tremendous transformation during this period,from the domination of pteridophyta to the domination of gymnosperms.Conifers are the most abundant and widely distributed taxa of living gymnosperms,which were once widespread worldwide during the Permian.However,previous research during the Permian-Triassic transition in South China mainly focused on the terrestrial coal-bearing strata and the typical components of Cathaysian flora.Conifers in this area were poorly researched and the available information is mainly about definition and classification.Furthermore,the published genera are much less than in other coeval floristic regions.Therefore,the study of conifers in this period in South China not only provides more reference materials for basic research,but also helps to comprehensively understand the changes in the composition of plant assemblages during the Permian-Triassic transition.On the base of the abundant fossil conifer remains founded in the Late Permian strata of several marine sections in Guizhou province and in the Early Triassic strata in Yunnan province,this study carries out the taxonomic study of fossil conifers during the Permian-Triassic transition in South China and discusses the origin and evolution of conifers in this area after comparison with studies in other regions of China.This study also collects the information about the distribution and preservation of fossil conifer remains during the Permian-Triassic transition in South China and rebuilds the possible living environment and burial process for fossil conifers in South China.The following summarizes the main work and conclusion about fossil conifers:1.There are only 4 species in 3 genera of fossil conifers currently founded in the Permian strata in South China,including Szecladia multinervia,Pseudoullmannia frumentarioide,Pseudoullmannia bronnioides and Pagiophyllum heshanense.In this study,a large number of specimens of a new genus Anshuncladus gen.nov.have been collected from the Dalong Formation of four marine sections:respectively the Xinmin section,the Kejiao section,the Duanshan section-A and the Duanshan section-B.In this new genus,three new species were defined,namely Anshuncladus xinmingensis gen.et sp.nov.,Anshuncladus contiguus gen.et sp.nov.and Anshuncladus aduncatus gen.et sp.nov.Anshuncladus gen.nov.is characterized by spiral-arranged scale-like leaves with a distinct median keel.The leaves are amphistomatous.Epidermal cells are polygonal with smooth periclinal walls.Stomata are evenly distributed in discontinuous rows.The number of subsidiary cells is commonly four or five,rarely three and six.Cuticles on the surfaces of subsidiary cells are thicker than on other parts,forming a distinct ring.According to these characteristics,it can be distinguished from other genera in the same period.2.The marine sediments were widely distributed during the Early Triassic in South China.The fossil confers were mainly found in the late Early Triassic strata of Qionghai County,Hainan Province,represented by Voltzia and Albertia.In this study,only Voltzia was founded on the top of the Feixianguan Formation and the preservation status was much worse than those in Hainan.3.During the Permian,conifers in North China,Northwest China and Junggar region were consistent in composition with those in Euramerican floral province.But in South China,there is no species to be founded outside this area.While during the early Triassic,the species of fossil conifers in southern and northern China are similar and both showed the main features of those of the Euramerican floral province.The distinctive species of conifers in South China only appeared in the Permian,and didn't show greater differences in the Early Triassic,suggesting that these“special”conifer types in South China are more likely to be related to climatic conditions rather than being an independent branch of evolution.It is the adaptability of plants in terms of morphology and function under special climatic conditions,thus forming endemic genera.This particularity then disappeared when the global arid climate was popular in the early Triassic.4.Some specimens of the newly discovered species Anshuncladus contiguus gen.et sp.nov.in the Dalong Formation in South China showed approximately decussate-arranged leaves.But this character in the geological record can only be traced back to the late Triassic.The conifers in South China showed this trait in the Late Permian,suggesting that the unique climatic conditions in the region may make the local conifer plants relatively"advanced"in evolution.5.The Permian fossils conifers in South China are mainly preserved in the marine or coastal clastic facies?terrestrial-marine transitional facies?strata,often associated with marine invertebrate fossils.There are only pollens founded in the terrestrial strata.And coniferous fossil woods are relatively rare,and found mainly in transitional facies.In addition,the large number of specimens of conifer remains found in the Dalong Formation with multi-level branching structures and well-preserved cuticles can eliminate the possibility of long-distance transportation by rivers.Based on these characteristics,it is speculated that these conifers in South China during the Permian are likely to grow in the coastal zone with the large-leaved gigantopterids and the lycopods,and are affected by the advance and retreat of seawater,thus forming this special distribution and preservation.For the early Triassic conifers,which are of the same type as those of the Euramerican floral province and adapted to relatively dry environment,are not well preserved.Most of them are small and broken,so they may grow in the highlands and deposited after a long distance transportation.At the same time,the fossil conifers found in the Dalong Formation have well-preserved cuticles,providing us good materials to quantitatively reconstruct the palaeoclimate.So on the base of fully understanding of these fossil conifers,this study uses the stomatal parameters of conifers to reconstruct the atmospheric CO₂concentrations during the Changhsingian,by the well-developed traditional stomatal ratio method and the newly emerging leaf gas exchange theory model method.In addition,in order to compare the reconstruction results between different sedimentary facies,and to extend the reconstruction of paleo-atmospheric CO₂ concentration to a larger time range,especially to the Permian-Triassic transition,the study also adds three terrestrial and terrestrial-marine transitional Permian-Triassic boundary sections with abundant plant fossils.Using plant carbon isotope proxy,this study quantitatively reconstructs the atmospheric atmospheric CO₂ concentration from the Late Permian to the plant extinction boundary,and discusses the changes of the paleo-atmospheric CO₂concentration both in time and in space.The main research processes and conclusions are as follows:1.Anshuncladus xinmingensis gen.et sp.nov.and Anshuncladus contiguus gen.et sp.nov.collected from the Dalong Formation of the Duanshan section-A and section-B have been used to reconstruct paleo-atmospheric CO₂ concentration by the stomatal ratio method.Araucaria luxurians?Brongniart et Grisebach?de Laubenfels,a native conifer of New Caledonia,which has leaf macro-morphological and cuticle micro-morphological similarities with Anshuncladus,including leaf size,stomatal distribution,stomatal complex type,and numbers of subsidiary cells,was finally selected as the nearest living equivalent?NLE?.In total,more than 58 leaves from 16specimens derived from the Duanshan section-A and over 140 leaves from 34specimens derived from the Duanshan section-B formed the fossil leaf database.Observation was carried out using a Leica DM2500 light microscope with an epi-fluorescence module and digital images were captured with an attached Leica DFC300FX camera.Consequently,333 and 520 photos from section-A and section-B respectively were obtained and used to count stomatal density.The lower and higher boundaries of reconstructed pCO₂ of the section-A are ca.347 and 513 ppm,while the corresponding values for the section-B are ca.300 and 438 ppm.2.Franks'model was only used for the section-A.The key input parameters needed for this model include:A₀?the CO₂ assimilation rate of the extant plant based on the data from NLE?,the carbon isotope composition of the fossil plants?tasting the fossil cuticles directly?,the carbon isotope composition of paleo-atmospheric CO₂?calculated by the equilibrium relationship with the marine carbonate isotope in the same period?,SD?measuring in the former method?,the stomatal size?data from NLE?.We import all the parameters in to the software R to calculate the palaeo-atmospheric CO₂ concentration and the error range.The result of the palaeo-atmospheric CO₂concentrations consists with that from the stomatal ratio method.3.We collected the fossil plant cuticle samples to evaluate the palaeo-atmospheric CO₂ concentration by testing the carbon isotope composition.The samples are from the lower part of Xuanwei Formation and the part around the plant extinction horizon?terrestrial and transitional deposits?and from the Dalong Formation?marine deposits?.The results from terrestrial and transitional deposits indicate that the palaeo-atmospheric CO₂ concentration had no prominent variation in the lower part of Xuanwei Formation and near the plant extinction horizon,but increased fast above the horizon.Similarly,the reconstructed pCO₂ based on fossils from marine deposits showed no variation from early Changhsingian to late Changhsingian.In the three sedimentary facies,the estimated palaeo-atmospheric CO₂ concentrations during the Late Permian?Changhsingian?are consistent,with the range of 300-400 ppm.The results also consist with those from the former two methods.4.Using materials from the Duanshan-A and Duanshan-B sections,the absolute values of palaeo-atmospheric CO₂ concentrations calculated by the three methods are more or less different.It is related to the uncertainty of each method.Overall,they have no huge differences in the order of magnitude and are consistent in the range of ca.300-500 ppm.And the three curves of paleo-atmospheric CO₂ concentrations show the same trend,decreasing firstly and then increasing.5.The palaeo-atmospheric CO₂ concentrations in the Changhsingian range from300 to 500 ppm.The low atmospheric CO₂ concentrations in this study suggest a cool climate during this period.It's supported by the coeval oxygen isotope record of conodont apatite from South China,which also suggests a cool climate with seawater temperatures?SSTs?decreasing from ca.26 to 22?.