Microstructures Of Several Gymnosperms From The Early Cretaceous Of Central And East Inner Mongolia And Their Significances

Gymnosperms are of huge ecological and economic value,and are important parts in forest ecosystem.Gymnosperms were extremely prosperous in Mesozoic,the study of the Mesozoic gymnosperms plant fossils are helpful in understanding the vegetation landscape,the plant diversity and the palaeogeographic distribution of plants in geological history.Studying on the Mesozoic gymnosperms plant fossils has important significance for researches on the global climate change and plate tectonic activity in geological period.At the same time,it also provides important fossil evidence for the origin,evolution and geographic distribution of gymnosperms.In this thesis,some gymnosperms plant fossils with reproductive organs are studied systematically.These fossils were collected from the Lower Cretaceous Huolinhe Formation of Huolinhe Basin and from the Lower Cretaceous Guyang Formation of Guyang Basin in Inner Mongolia.Based on the morphological features and microstructures of vegetative and reproductive organs,these gymnosperms fossils are classified and identified,and are assigned to 4 genus and 9 species,including 5 new species.The new species are Ginkgo neimengensis sp.nov.,Ginkgo guyangensis sp.nov.(Ginkgoaceae),Chamaecyparis chinense sp.nov.(Cupressaceae s.s.),Taxus guyangensis sp.nov.(Taxaceae)and the undefined conifers Schizolepis longipetiolus sp.nov.The two new species,Ginkgo neimengensis sp.nov.and Ginkgo guyangensis sp.nov.are established on the bases of well-preserved Ginkgo ovulate organ fossils from the Huolinhe Basin and the Guyang Basin of Inner Mongolia,respectively.They are also the first fossil record of Ginkgo ovulate organs from the Huolinhe Basin and the Guyang Basin of Inner Mongolia.Five ovules bear at the apex of peduncle of Ginkgo neimengensis sp.nov.Two ovules bear at the apex of peduncle of Ginkgo guyangensis sp.nov.The ovules are seated each in a cup-shaped collar,but attached directly to the peduncle without pedicel.The microstructures of ovulate organ of Ginkgo neimengensis sp.nov.and associated Ginkgo leaves are studied detailedly.The occurrence of the two new species are greatly enriched the fossil materials of Ginkgo ovulate organ in the world.Ginkgo guyangensis sp.nov.is morphologically intermediate between the early Early Cretaceous species and the Paleogene species.This discovery make up for the missing link of the evolution of Ginkgo ovulate organs between the early Early Cretaceous species and the Paleogene.The occurrence of Ginkgo guyangensis sp.nov.is further confirmed the previous inference that the ovulate organs of Ginkgo biloba could have originated from the Jurassic Ginkgo yimaensis type.Chamaecyparis chinense sp.nov.is described as a new species on the basis of well preserved branches and female cone from the Lower Cretaceous Guyang Formation.It is the first fossil record of Chamaecyparis with reproductive organ from China,and is also the earliest and the most reliable fossil record of the genus in the world.The detailed comparisons indicate that the new species probable represent the ancestral taxa of the genus,and has the closest affinity with the extant species Chamaecyparis pisifera.The new species combined with reliable fossil record of Chamaecyparis and molecular biology suggests that the genus Chamaecyparis was originated in East Asian in the Early Cretaceous.During the Paleogene(no later than early Eocene),Chamaecyparis extended its range eastward to North America via the Bering land bridge,then it reached Europe from East Asian or North America.The fossil evidence shows that this genus has richer species than that of today,and widely distributed in East Asian,North America and Europe.The cooling and drying global climates during the Miocene and alternating glacial and inter-glacial periods during the Quaternary might have resulted in the remarkable diminishing of Chamaecyparis in East Asian and North America,and resulted in the extinction from Europe.Thus,the global climate changes led to the distinctive disjunction of Chamaecyparis between East Asia and North America.Taxus guyangensis sp.nov.is from the Lower Cretaceous Guyang Formation of the Guyang Basin of Inner Mongolia in northern China,based on an excellently preserved leafy branch with attached leaves and seed-bearing structures.Three ovules occur on the leafy branch.A pair of ovules is borne on the terminal ovuliferous shoot and another one appears on the terminal of a lateral shoot.The aril is macroscopic on a mature ovule,which enclosed the middle and lower part of the ovule.This fossil is the most complete evidence of this genus currently known.This discovery also indicates that the multi-ovulate shoot of Taxus existed in the Early Cretaceous.Comparisons of Taxus guyangensis sp.nov.with living and previously published reliable fossil species of Taxus and Taxus-like fossils reveal that it has a close resemblance to western North America extant species Taxus brevifolia and might have represented the ancestral taxa of extant Taxus brevifolia.The reliable fossil record of Taxus is summarized in detail.The new species together with other known fossil record of Taxus and the molecular clock evidence show that the age of Taxus dates back to the Early Cretaceous,and it probable originated in East Asian during the Early Cretaceous.During the Eocene,the Taxus extended its range to northwestern North America across the Bering land bridge.During the late Oligocene,it might has reached Europe,and then dispersed into northeastern North America via North Atlantic land bridges.The morphological features and microstructures of Schizolepis longipetiolus sp.nov.are examined.The characteristics of Schizolepis are analyzed in detail,and the result demonstrates that it has significant difference between Schizolepis and Pinaceae.Therefore,it is inappropriate to assigned Schizolepis into Pinaceae,in this paper,this genus is temporarily treated as conifers incertae sedis.The fossil record of Schizolepis together with the plate tectonic pattern in geological history show that Schizolepis might originate in Europe during the Late Triassic,and then spread from Europe to Asia between the Late Triassic and the Late Jurassic.Species of Schizolepis were especially abundant in the Early Cretaceous,and almost of them distributed in East Asian,particularly in Northern China.After the Early Cretaceous,it became extinct,but the occurrence of Schizolepis jeholensis from the Lower Cretaceous Guyang Formation of Guyang Basin suggests that Schizolepis has been existed until the late Early Cretaceous.