Fossil Researches On Tree Fern Groups In Mid-cretaceous Amber Of Myanmar

The transition of sea and land and plate drift not only continuously changed the appearance of globe,but also had a great impact on the dispersal and evolution of paleontology.The tree fern groups Cyatheales was a relatively ancient group in the modern fern lineages(known as living fossil),and its dispersal and evolution was also considered to be related to plate drift by molecular biology,while the clear fossil evidence was lack for supporting it.Although many tree fern fossils have been identified and described before,frond fossils are still relatively scarce,and their origin and diffusion are rarely discussed.The mid-Cretaceous Myanmar amber(?100 Ma)deposited in the Cretaceous Myanmar amber forest from Northeast coastal zone of the Indian plate,and drift north to today's northern Myanmar.Myanmar amber is famous for high species diversity,which has attracted the attention of scholars.However,tree fern fossils are rarely reported in amber,so that it is impossible to fully understand the species diversity,evolution and dispersal of tree ferns in Cretaceous amber forest.Therefore,several pieces of amber containing tree fern pinnae were found and identified with detailed morphological description from tens of thousands of amber fossils collected in our laboratory,including the crown group of Cyatheaceae,fossil species Thyrsopteris cyathindusia sp.nov.in Thyrsopteridaceae and extinct linage Pteresinia pecupsule sp.nov.of tree ferns.On this basis,combined with the previously published fossil records of tree ferns,present study deeply discusses the origin,diffusion and evolution of tree fern groups.1.Discovery and significance of Cyatheaceae(core tree ferns)in Myanmar amber.The fertile pinnule contained in the amber shows the typical morphology of the extant Cyatheaceae that can be clearly the crown group of Cyatheaceae,it indicated the crown group has appeared in the mid-Cretaceous period which proved molecular date estimation of crown group of Cyatheaceae.The discovery of Cyatheaceae further supported that Cretaceous amber forest was developed under tropical climate conditions.Molecular biology studies suggested that the extant distribution of Cyatheaceae was related to Gondwana separation.Therefore,the discovery of Cyatheaceae in Cretaceous amber provided direct fossil evidence for the dispersal pattern of Cyatheaceae.It indicated that Cyatheaceae colonized to Asia along with Indian plate drift,which is conformed to the dispersal route under the“out-of-India”hypothesis.2.Discovery and significance of fossil species in Thyrsopteridaceae,named as Thyrsopteris cyathindusia sp.nov.Thyrsopteridaceae possessed only an extant species Thyrsopteris elegans,T.Cyathindusia retained more laminar tissue than the extant species,which indicated different evolutionary clades were existed in the family.The extant species and fossil species of the genus Thyrsopteris were all distributed on the land plate produced by Gondwana separation.Therefore,the discovery of T.cyathindusia indicated that the genus Thyrsopteris of Thyrsopteridaceae might evolve to two routes along with Gondwana separation: one clade spread under the northward drift of Indian plate,and its fertile pinnae retained a small amount of laminar tissue to adapt to the tropical climate conditions,including T.cyathindusia;another clade spread to the Juan Fernández Islands along the Antarctica-South America route,and its fertile pinnae showed “skeletonized”characteristic under warm climate conditions,including the single extant species T.elegans.3.Discovery and significance of extinct group in tree ferns,named as Pteresinia pecupsule sp.nov.The fertile pinnule of P.pecupsule show a unique and compound semi capsule-double lobed indusia,which has not been found in tree ferns before,it further showed the species diversity and morphological evolution diversity of tree ferns in Cretaceous period.This study conducted a detailed study of the fossil morphologies of the two major clades of the tree ferns,the Cyatheaceae and the Thyrsopteridaceae.Based on this,it was confirmed that these two families spread with Gondwana separation under the molecular biology hypothesis.In addition,the discovery of the extinct tree fern species P.pecupsule in mid-Cretaceous amber also supported the hypothesis of the origin of the tree ferns group in the ancient Gondwana.In conclusion,this study provides conclusive fossil evidence for the route framework of the tree ferns spreading to various continental plates with the Gondwana separation.