The Miocene Plant Fossils,Paleoatmospheric CO₂ And Plant-Insect Interations From Fujian Province

The origin and evolution of land plants are regulated and constrained by their living environment.Plant species richness and their assemblage characteristics can reflect the environmental features during plant growth period.Meanwhile,they are treated as important indicators reflecting climatic changes.Stomatal parameters of plant fossils provide unique insight to study the changes of pCO₂ concentration,and it becomes one of the most reliable proxy indices for reconstituting pCO₂ concentrations.Plants and insects are the two important components of the terrestrial ecosystems.The paleontological data of plant-insect interaction documented evolutionary history of the terrestrial ecosystems.The traces of insect feeding behavior are the direct evidence of plant-insect interactions and provide valuable information for studying nutritional symbiosis relationship between plants and insects.The abundant plant fossils from the Miocene Fotan Group,Fujian Province are regarded as good materials to trace the climatic change during this period.Insect-herbivore diversity in fossil leaves provides substantial evidence to study insect herbivories during the Neogene era.We made a systematic study for plant fossils collected from the Miocene Fotan Group,Zhangpu County,Fujian Province in this thesis.The macro-and microstructures of plant fossils were comprehensive described and taxonomic positions of these fossils were confirmed by comparising with extant species and fossil species using cluster analysis.These angiosperm fossils were classified into 27 species of 16 genera from 14families(17 new species included).Among them,there are 2 species of 2 genera from2 families as monocotyledons,and 25 species of 14 genera from 12 families as dicotyledons.The Miocene Fotan flora was a zonal vegetation type and was dominated by evergreen broad-leaved plants according to the growth and life-type characteristics of fossil assemblages.Diverse life forms of plants,including shrubs,vines,climbers,trees and herbs indicate that the Fotan forest vegetation had a multilayered structure.The habitats of plant assemblages also indicate that the sedimentary environment might be a humid lake or marsh basin during the Miocene.There is a negative correlation between stomatal densities of plant fossils and the atmospheric CO₂ concentration,which supports the reconstruction for pCO₂concentration in geological history.In this thesis,three fossil species,Liquidambar fujianensis,Calophyllum zhangpuensis,Syzygium fujianensis sp.nov.were selected as the proxy sources.Combining stomatal parameters and carbon isotope data from three fossil species,we reconstructed the paleoatmospheric CO₂ concentration of the middle Miocene based on the mechanistic-theoretical model including the Konrad model and the Franks model.The results show that the range of the CO₂ concentration during the middle Miocene Zhangpu,Fujian was 394–743 ppmv.The estimated values we reconstructed are relatively higher than the values obtained with marine geochemical and terrestrial proxies during the Miocene period.Our data coincide with the climatic optimum during the Miocene and point to a coupling between atmospheric CO₂ and temperature.In addition,it also proves that the gas exchange model can maintain high sensitivity in the case of high CO₂ concentration.The diversity and frequency of insect herbivory on 3420 fossil leaves were analysed from two plant-bearing layer exposed at the same locality.Five principal damage categories were identified including external foliage feeding,piercing and sucking,oviposition,mining and galling.Besides,we scored 85 distinctive insect damage types(DTs),which provided insights into the diverse interactions between plants and insects.Furthermore,high diversity and frequency of insect herbivory demonstrated a warm climate and high pCO₂ concentration during the Miocene,which coincided to the results of paleoclimate reconstruction.These suggest that insect herbivory can be used as an evidence to estimate the paleoclimate information and climate fluctuation.On the basis of the comparison with extant taxa,we infer that the possible phytophagous insects as herbivore culprits from the middle Miocene Fotan flora belong to the insect orders Lepidoptera,Coleoptera,Hemiptera,Orthoptera,Thysanoptera,Diptera,Hymenoptera,and Isoptera etc.,and may include Cecidomyiidae,Eulophidae,Miridae,Cynipidae,Tenthredinidae,Agaoninae,Months and Aphids.Finally,insect herbivories within two layers of the Fotan Group were respectively assessed and compared.The significant difference of the insect-mediated damage diversity and frequency between the two layers demonstrate precipitation,plant diversity,insect composition and density and plant defense strategies have changed over time.