Early Cretaceous Plant Fossils From The Laiyang Basin Of Shandong And Their Paleoecological Characteristics

The Cretaceous is an important"greenhouse climate"period.During this period,a series of geological tectonic activities happened,which led to significant changes in the global environment,climate and vegetation.Accordingly,the study on the Cretaceous plant fossils has a constructive contribution for understanding the terrestrial ecosystem,palaeoenvironment and palaeoclimate during this stage.The Laiyang area is located in the central region of the Shandong Peninsula.A set of lacustrine-dominated strata were developed in this area.Recently,abundant fossils collected from this region show that the Early Cretaceous terrestrial ecosystem with a high level of biodiversity was presented in the Laiyang area.In this study,detailed taxonomic studies of plant megafossils and sporopollen fossils are presented from the Laiyang area,which provided an important fossil evidence for analyzing paleoclimate,paleoecology,palaeogeography of the Laiyang region during the Early Cretaceous.In addition,the paleoatmospheric CO₂ concentrations of the Early Cretaceous are reconstructed by using the stomatal parameters of Coniferopsida plant fossils from Laiyang Formation for the first time,which has important guiding significance for predicting the trend of global climate change in future.All the fossils were collected from calcareous shale in the middle part of the Early Cretaceous Laiyang Formation.Based on the comprehensive study on the morphological features and microstructures of vegetative and reproductive organs,21 species of 10 genus from 5 family?including 3 new species?are reported in total.Meanwhile,combined with related fossil reports,global plate tectonic structure and environmental changes,the palaeogeographic and paleoenvironmental significances of Equisetites,Pararaucaria,Elatides,Pagiophyllum and Ephedra are discussed.A new species Equisetites shandongensis sp.nov.is established on the bases of well-preserved Equisetites aboveground stems,sporangiate spike and rhizomes fossils from the Laiyang Basin of Shandong Province.Combined with the morphological evolution and geological records of the fossils of Equisetaceae,it can be inferred that Equisetites could be an transitional group on the evolutionary direction of the extant Equisetum.Pararaucaria laiyangensis sp.nov.is the first female reproductive organ fossil found in the Laiyang Basin.There is a small branch with scale-leaf foliage at the base of this female cone,which provides an important evidence toward the whole-plant reconstructions of the Pararaucaria plant.In addition,in view of the palaeogeographic distribution of Pararaucaria plant fossils,it can be inferred that Pararaucaria is adapted to growing in semi-arid areas.In this paper,systematical studies on the microstructure and the macro characteristic of the Elatides foliage and female reproductive organ fossils?Elatides laiyangensis,Elatides cf.curvifolia and Elatides sp.?have been conducted.It can be reckoned that this genus spread from Europe through Ural Mountains to Xinjiang,and then its distribution scope expanded further.It finally arrived in the northernmost province of Heilongjiang.Meanwhile,due to the obstruction of the relatively high ancient Kunlun–Qinling Mountains in the Early Jurassic,this genus spread from Northern China,passing through Shandong,and finally arrived in South China in the Early Cretaceous.Ephedra shandongensis sp.nov.is first-time recognized and described in Shandong area.Based on the reliable fossil record of Ephedra and molecular biology,it can be indicated that the origin time of Ephedra is no later than Barremian in the Early Cretaceous.With the appilication of mechanistic–theoretical model and the stomatal ratio,the paleoatmospheric CO₂ concentration of the Early Cretaceous?Hauterivian–Barremian?is reconstructed based on three species of Brachyphyllum,Pagiophyllum and Cupressinocladus,with good indication of CO₂ concentration.The results of palaeo–CO₂ reconstruction show that the range of the CO₂ concentration during the Early Cretaceous Laiyang Basin was from 529 to 1252ppmv.Compared with the results previously derived from other methods or proxy sources,it can be found that the current palaeo–CO₂ concentration value is within the error range of each reconstruction proxy,which further indicates that this mechanistic–theoretical model is available for the reconstruction of the palaeo–atmospheric CO₂ concentration during the Mesozoic.Moreover,the lower paleo–atmospheric CO₂ concentration of this area suggests a cooling trend of the palaeoclimate during the Early Cretaceous?Hauterivian–Barremian?of the Laiyang Basin,and provides the hypothesis that terrestrial plants could response to the oceanic Weissert anoxic event?Weissert OAE?.Combined with the cluster analysis with the same period flora,the paleoflora of the Early Cretaceous Laiyang Formation is described and reconstructed based on plant macrofossil and spore–pollen assemblages of the Laiyang Formation.It can be inferred that the dominate elements of the Laiyang flora are the Coniferopsida,Cycadopsida,Bennettiopsida,Filicinae and Pteridospermae.Otherwise,Ginkgopsida and Ephedraceae are rare.The characteristic of Laiyang floral is similar to that in the Early Cretaceous mixed–type floristic provinces of China,which contain the characteristics of both Northern Floristic Province and Southern Floristic Province,and is closer to the Southern Floristic Province.Based on the plant assemblage,the study area is located in a holistic dry subtropical and locally humid climate during the Early Cretaceous.