| Other Abstract | The important period for the global icehouseto greenhousetransformation is the Permian period, during which terrestrial plants flourished and the climate zonation characteristics are quite obvious. According to the characteristics of the environment and plant groups, it can be divided into four major floras. During this period, the paleoclimate and vegetation changed gradually. The extinction event at the end of the Permian changed the ecological environment completely. Therefore, it is of great significance to understand and discuss the biological evolution and environmental characteristics of this period. Plant fossils are the most direct and effective evidence of plant evolution. Sporopollen is the reproductive organ of higher terrestrial plants, which has the advantages of large quantity and easy preservation. It is the most direct and sensitive response to the environment and climate. These two are the important basis for studying the characteristics of flora and environmental changes in geohistory. There are abundant plant fossils in the Permian continental strata in Yongchang of Gansu Province. The systematic study of the plant fossils and sporopollen assemblages in the Shanxi Formation of Yongchang provides sufficient evidence for the study of plant diversity evolution and paleoenvironmental characteristics of Cathaysia flora in Northwest China.
A large number of plant fossils were collected from the Shanxi Formation of Yongchang, Gansu Province. The macro- and microstructures analysis of the well preserved compression fossils were carried out, including vegetative leaves and reproductive organs. 22 genera and 40 species of plant fossils were identified and described by cuticle analysis, in situ pollen analysis, cluster analysis and principal component analysis, and 4 new genera and 8 new species (including 2 new species published online). 43 sporopollen samples from Shanxi Formation were treated by traditional sporopollen analysis method of hydrochloric acid and hydrofluoric acid soaking. 78 species of 53 genera of sporopollen fossils were identified and described. Three assemblages were established according to the change of sporopollen abundance. Based on a large number of fossil records and ancient block data, the origin and paleogeographic distribution characteristics of some genera and species are discussed in depth. Meanwhile, the nature, geological age and paleoenvironment of flora are comprehensively analyzed in combination with the characteristics of large fossil assemblage, cuticle analysis results and sporopollen assemblage.
The morphological characteristics of many compression fossils, such as Lepidodendraceae, Calamitaceae, Filices, Pteridospermopsida, Cordaitopsida and seeds, were studied. According to the characteristics of microstructure, the accurate classification and identification are carried out, including Lepidostrobus, Calamites, Macrostachya, Asterotheca, Pecopteris, Cladophlebis, Sphenopteris, Mariopteris, Taeniopteris, Neuropteris, Neuropteridium, Alethopteris, Cordaianthus, Samaropsis, Acanthocarpus and Carpoliths. A new species Lepidostrobus yongchangensis sp. nov. has been established, which is similar to the species of Lepidostrobus found in the Pennsylvanian strata of Illinois, USA.
The stem and strobilus of Calamitaceae were analyzed in detail. The strobilus was identified as Macrostachya sp., and combined with a large number of fossil records of Calamitaceae, it is inferred that this kind of plants originated in South China of Late Devonian, and spread throughout Europe, America and even China during the Carboniferous-Permian period, and Calamostachys is the ancestor group of other strobili types. A new genus Hexistrobus yongchangensis gen. et sp. nov. was established based on the morphology of strobili and in situ spore characteristics. It is speculated that this kind of Noeggerathiales originated from the Devonian in Europe and widely distributed in Cathaysia flora during the Cisuralian of China, and its distribution area is consistent with its ecological environment characteristics.
The characteristics of in situ spores of Asterotheca yongchangensis sp. nov. were studied. The results show that the synangium of Asterotheca is composed of four sporangia and its evolution can be divided into five stages. According to the Paleozoic fossil records of Pecopteris and Asterotheca, it is inferred that the decline period of Pecopteris in North China was no later than the Guadalupian, and the prosperity of Asterotheca was closely related to the migration and distribution of Pecopteris. The paleogeographic distribution of Sphenopteris shows that the genus originated from the Late Devonian in South China, and its paleogeographic distribution and paleostructural characteristics show a strong coupling relationship. The microstructure characteristics of three species of Taenopteris found in the study area indicate that the genus may be the leaves of early cycad, and the fossil records show that Taenopteris distributed in North China in the Cisuralian, and widely appeared in South China in the Lopingian. The Taenopteris is a hydrothermal plant.
The branches and reproductive organs of early conifers in Yongchang flora were studied in detail. Three new genera of Shencladia longshoushanensis gen. et sp. nov., Hexicladia yongchangensis gen. et sp. nov. and Palaeovoltzia minuta gen. et sp. nov. have been established. At the same time, the reproductive organs with severe carbonization were divided into three types of pollen cones and four types of ovulate cones. The structure of pollen cones and ovulate cones showed that the ovules of early conifers did not have short stalks, which provided a new evidence for Schweitzer&rsquos view that there was no evolutionary relationship between early conifers and Cordaitopsida. According to the characteristics of dwarf-shoots of reproductive organs, the pollen cones and ovulate cones of early conifers are structurally equivalent, which is consistent with the viewpoints of Hernandez-Castillo and Rothwell.
According to the comprehensive analysis of plant fossils and sporopollen assemblages, the results show that Filices and Pteridospermopsida are dominant in flora, followed by conifers. Based on the comparison with the flora of the same period and the existence of the unique elements of the Cathaysia flora in the plant assemblage, it is confirmed that the flora has the features of middle-term Cathaysia flora, accompanied by the infiltration of the elements of Euramerican flora and Angara flora. It is speculated that the geological age of the study area should belong to the Artinskian kunguran period in the middle and late Cisuralian.
The analysis of paleoenvironment shows that the cuticle characteristics of a large number of Filices and Pteridospermopsida have the same characteristics, that is, the vegetative leaves are all in the whole, the upper epidermis is thicker than the lower epidermis, and the stomatal apparatus is not obviously sunken, indicating that these plants are all growing in the humid climate environment. The characteristics of trichomes on the cuticle of a new species of Taeniopteris and reproductive organs of some conifers indicate that the plants in this period have the structural functions of preventing water evaporation and protecting reproductive organs, and this structure indicates that the plant fossils are parautochthonous burial. In addition, the true Filices and Pteridospermopsida, which are dominant in the flora, live in the humid environment of the hot temperate zone. The other components, such as Lycophyta, Sphenopsida, Noeggerathiales, belong to the plants that like to wet and distribute on the edge of the water body. Only conifers belong to the drought resistant plants and distribute on the outer edge. Therefore, according to the comprehensive characteristics of flora combination and the results of cuticle analysis, it is speculated that Yongchang flora in the middle and late Cisuralian lived in the plain lowland swamp environment, with the characteristics of warm and humid climate with seasonal drought. |
