The First Discovery Of Juglans Fruit Fossils And Reconstruction Of The Paleovegetation Of Shengxian Flora In Eastern Zhejiang,Late Miocene

Plants are pioneers of terrestrial ecosystems,as an important part of living nature,mostly playing the role of producers in nature and maintaining the stability of ecosystems.Because specific plants can only survive in specific habitats,the flora is an important indicator of climate and environment.Zonal vegetation is a kind of vegetation distributed in zonal,which is living in a certain area and is compatible with water and heat conditions.It is the regional climax that reflects the natural characteristics of the region.Therefore,the reconstruction of zonal vegetation types is a very critical part of palaeoenvironmental and palaeoclimatic studies.There are abundant exquisite and various plant fossils in Shengxian formation from eastern Zhengjiang in the Late Miocene.In this study,the fossil fruits of Juglans were the main object of study,and the two collected fossil fruits were observed and studied externally and internally using various means such as ultra-deep field microscopy and Micro-CT to summarize and compare their morphological features and analyze their taxonomic position.The fossil fruits were compared with the reported fossil fruits previously and the extant species of the Juglans,and were assigned to a new species as Juglans tiantaiensis sp.nov.L.Xiao et X.Xia.J.tiantaiensis is highly similar to the two extant species J.regia and J.sigillata of sect.Diocaryon.Combined with the phylogenetic evolutionary tree of Juglans studied previously,J.tiantaiensis was identified as the direct ancestor of sect.Diocaryon.In this paper,we integrated the fossils of Juglans from the geological period,discussed the origin and spread of Juglans,and speculate on the origin and spread pathways based on the relevant fossil records of sect.Diocaryon.Juglans originated in North America during the early Eocene,and the ancestors of sect.Diocaryon spread across the North Atlantic land bridge to Europe before the early Oligocene.The closure of the Turgai channel in the early Oligocene allowed the sect.Diocaryon to communicate between Eurasia.The sect.Diocaryon had completed differentiation before the Miocene and spread from Europe to northwest Asia in the late Oligocene and Miocene.Before the Late Miocene,the sect.Diocaryon had arrived in East Asia,however,the cooling climate during the Late Miocene caused the sect.Diocaryon to migrate southward.The Quaternary ice age event left only partial populations in the East Asian Refuge,and the post-ice age sect.Diocaryon continued to spread and evolve to form the present-day geographic distribution pattern.Quaternary ice age events led to the extinction of the populations in sect.Diocaryon of North America and Europe,with only partial populations remaining in the East Asian refuges.Subsequently,the sect.Diocaryon continued to spread and evolve after the ice age,and forming the present-day geographic distribution pattern.In this paper,we counted the plant fossil records from Shengxian Formation in eastern Zhengjiang,Bangmai Formation in Lincang,Yunnan,and Xiaolongtan formation in Kaiyuan,Yunnan during the Late Miocene,with the modern plant taxa from Tiantai,Zhejiang,semiquantitatively reconstructed the zonal vegetation types by using the IPR vegetation analysis method.The results show that the Shengxian,Bangmai,Xiaolongtan Formations and the modern Tiantai area are all subtropical evergreen broadleaf forests.Combining the horizontal comparison of different regions in the Late Miocene period and the vertical comparison of different components in the flora of different eras in Tiantai,eastern Zhejiang,we analyzed the response relationship between the differences among components of zonal vegetation types and climate change.The percentage of evergreen broadleaf woody angiosperm fraction in the IPR vegetation analysis was stable in relation to temperature change,and the increase in temperature increased the percentage of evergreen broadleaf fraction in the zonal vegetation,which are indicators for the recovery and verification of palaeoclimate.