Fossil Morphology And Evolutionof Of Polypod Ferns In Myanmar Amber

A series of global land and ocean changes occurred in the Cretaceous period,which had a significant impact on the origin and diffusion of paleoplants.Cretaceous Burmese amber is a kind of resin fossil that exists in the coastal area and is wrapped with insects,plants and other paleontology secreted by pine and fir plants,which is then buried on the seabed or underground near the coast and formed after tens of millions of years of mineralization.Myanmar amber contains a large number of terrestrial arthropods,angiosperms and marine paleontological fossils,which provides direct fossil evidence for the study of paleogeography,paleoenvironment and paleoclimate in the offshore area during the Mesozoic era.Previously,more than 2200 fossil groups have been reported in Myanmar amber,but most of them are animal(especially arthropod)fossils,and there are relatively few plant fossils.Due to the lack of Mesozoic fossil records,the origin and radiation evolution of many groups of ferns,especially polypod ferns,have been controversial.This paper studies the leaf fossils of the order Polypodiales from northern Myanmar in the mid-Cretaceous,and discusses the origin and evolution of this group.Through the fine morphological study of fossils,this paper identifies 3 families(including 1 new family),3 genera(2 extant genera and 1 fossil genus)and 3 species of polypods fossils,which are Cyrtomium glabrum Long,Wang and Engel,sp.nov.,Microlepia burmasium Long,Wang and Shi sp.nov.,Coniopteris suspendisse Long,Wang,Feng and Shi sp.nov.Cyrtomium(Dryopteridaceae)fossil and Microlepia(Dennstaedtiaceae)fossil are the first discovery of those two extant genera in Mesozoic,and both are the oldest fossil records.This study shows that the extant genus Cyrtomium and Microlepia have appeared in the mid-Cretaceous,and originated in the mid-Cretaceous,which is about 60 to 70 million years earlier than the differentiation time inferred based on molecular research(45-20Ma),providing fossil evidence for the early origin and evolution of Dryopteridaceae and Dennstaedtiaceae.In addition,combined with the fossil morphology of the genus Coniopteris,this study established it into a new family,Coniopteridaceae Long,Wang and Shi,fam.nov.,and classified it into the basal group of the order Polypodiales.According to other fossil records of the genus Coniopteris previously found in the early Jurassic,it is inferred that the origin time of the order Polypodiales is at least the early Jurassic.Based on the in-depth morphological comparison between the fossils of this genus and the tree fern groups with controversial classification,it is speculated that the genus Coniopteris may be a transitional fossil group for the evolution of the Order Polypodiales and the Order Cyatheales,and its extinction may be related to the sudden climate change(the climate becomes drier and hotter)caused by frequent fires in the Late Cretaceous.Based on the Myanmar amber fossils of 100 million years ago collected by the research group in the early stage,this paper makes a morphological comparison and identification of the fossils of polypod fern,which not only provides new fossil evidence for the origin and evolution of this group,but also puts forward new opinions on the fossil attribution of specific groups,and provides fossil support for the subsequent evolutionary research of the order Polypodiales.