Study On Fern Fossils From The Late Miocene Of Lincang,Yunnan,China

Located at the southeast edge of Tibetan Plateau,Yunnan experienced huge geomorphological changes during the Neogene.Its climate was also largely influenced by the establishment of the East Asian Monsoon system,thus promoting the fast speciation of local plants and the changes of the forest structure.Pteridophytes are important components of vascular plants with over 10,000 species in total and respond to climatic and environmental shifts rapidly.Some well-preserved fern fossils were found from the Miocene strata of Lincang which is one of the best Neogene fossil site in Yunnan and they play a significant role in extending our understanding of the explosive diversification of ferns in western Yunnan.By means of comparison of macroscopic morphology,some fern fossils were identified from the upper Miocene sendiment of the Bangmai Formation in Lincang city,Yunnan,China.They could be assigned into five species,included in five genus of four families.In Dennstaedtiaceae,the species Microlepia cf.khasyana was identified and in Thelypteridaceae Macrothelypteris cf.oligophlebia,in Davalliaceae Humata cf.henryana.It is noteworthy that Microlepia cf.khasyana is the first Dennstaedtioid fossil in Yunnan.In Polypodiaceae,two species in two genera were recognized,i.e.Neolepisorus cf.fortunei and Drynaria diplosticha.Drynaria diplosticha has an unique pattern that its sori form two regular rows at either side of the midrib,which is rare in the extant species of Drynaria.These ferns also have climatic significance: they indicate that the climate in Lincang during the late Miocene was moderately warm and humid.It was not as hot as tropical monsoonal climate or rainforest climate,but rather,it may be similar to the current climate in southwestern China.During that time,Lincang was rather mountainous and quite humid with developed drainage networks and multilayered forests.Taking as well some other fern fossils into account,a brief summary of the Cenozoic distribution of either the family or the genera to which each of our fossils belongs was made.Dennstaedtioid ferns may be quite widely distributed in Yunnan during the Cenozoic Era.Huge amount of Thelypteridoid fossils have been unearthed worldwide in the Cenozoic and together with other imporant fossil records from the southeastern Asia,our fossil may imply that this region played a decisive role in its early evolution.Other than long distance dispersal in late Cenozoic,dispersal at their early evolutionary stage and parallel evolution afterwards may also contribute to their current distributional range.Davallia constituted a rather large part of the fern population in the Lincang flora and their fast diversification and habitat expension might occur mainly in Miocene.Western yunnan is a plant biodiversity hotspot.The last part of my thesis is a review about reasons of some rapid diversification events of plants,as well as discussion about the drivers of spiciation in western Yunnan with the help of former studies of fern fossils and topographical background.Western Yunnan might be the speciation center of ferns,driven by the rapid topological changes,the development of multi-layered forests and the monsoonal climate.This can be best respresented by the radiation of Drynaria in Asia during its early evolution although its origin is still left open for discussion.