Fossil Leaves Of Podocarpaceae And Cephalotaxaceae From The Pliocene Of Western Yunnan And Their Biogeography

Leaf is a sensitive organ to climate change in the process of plant evolution,and its structural characteristics can best reflect the coevolution of environmental factors and plants.The characteristics of leaf epidermis are of great value to the classification of gymnosperms,and they can provide basis for plant identification,such as the size and shape of epidermal cells,the fluctuation of anticlinal wall,the shape of stomata and so on.The well preserved epidermal structure of leaf fossils not only provides a systematic connection with the morphological characteristics of extant species,but also has a certain impact on the classification and definition of extant species.Angiosperms occupied a dominant position in the Cenozoic,but gymnosperms still exist in large quantities,among which conifers are the most prosperous.Because their leaves are mostly needle shaped,they are called conifers.However,there are also some special conifers whose leaves are flat,such as Podocarpus and Nageia in Podocarpaceae,Cephalotaxus in Cephalotaxaceae,and these two families are closely related.In view of the close systematic relationship between them,we focus on the study of the fossil leaves of Podocarpaceae and Cephalotaxaceae from the Pliocene of Tengchong,Yunnan Province.There are 173 species of 18 genera in the family Podocarpaceae,14 species in China,and they are distributed in the south of China.Because the leaves of plants can reflect the changes of habitat conditions,there are many studies on leaf epidermis structure of the family Podocarpaceae,but they are mainly concentrated on the existing plants,but few studies on fossil leaf epidermis.We described the leaf fossils of four species belonging to two genera of Podocarpaceae,namely Podocarpus pliocarphyllus sp.nov.,P.yunnanensis,P.cf.forrestii and Nageia cf.Nagi.Cephalotaxaceae contains only one genus Cephalotaxus,with 9 species and 3varieties,mainly distributed in southern East Asia and Northern Indochina Peninsula.Because most species of Cephalotaxaceae are endemic to China,China is considered as the center of its diversity distribution,but there are few fossils of Cephalotaxaceae in China.Cephalotaxus tengchongensis sp.nov.,a new fossil species of Cephalotaxus Cephalotaxaceae,is described in this paper.It is very similar to the macro morphology and micro structural characteristics of the leaves of Cephalotaxus fortunei.The discovery of the fossils of Podocarpaceae and Cephalotaxaceae not only enriches the fossil records of these two families,but also provides evidence for the biogeographical history of ancient plants.Based on the global fossil records,the palaeobiogeography of Podocarpus,Nageia and Cephalotaxus is analyzed,and the origin and spread of the three genera are discussed.It is inferred from fossil records that the genus Podocarpus originated from South America in West Gondwana.The subgenus Podocarpus differentiated in the early Paleogene and spread to Australasia,Africa and North America through multiple routes.The subgenus Foliolatus is younger than the subgenus Podocarpus and originated from Australasia.In the Oligocene,it spread from Australasia to northeast India through the Sunda Islands.Glaciation and geographical barriers promote the distribution modernization of subgenus Foliolatus.Fossil evidence suggests that the genus Nageia originated from Northeast Asia in the early Cretaceous.It spread to South China at least in the Eocene,and spread to Western Yunnan in the early Pliocene.The genus Cephalotaxus originated in the Cretaceous.It was distributed in Eurasia and North America during the Cretaceous.Later,due to the cold climate,the distribution of this genus gradually moved southward and narrowed to today's range,making most of its species endemic to China.Neogene is the key period of vegetation evolution.The global climate has changed dramatically since Neogene.Due to the uplift of the Qinghai Tibet Plateau,the climate became cold.The plant fossils of Podocarpaceae and Cephalotaxaceae studied in this paper are collected from the Pliocene Mangbang Formation in Tuantian Township,Tengchong City,Yunnan Province.There are abundant plant fossils in this area.Based on the characteristics of the identified fossil species and genera,we analyzed the paleoenvironment and paleoclimate of Tengchong,Yunnan Province by using the principle of the nearest living relative.It is believed that Tengchong still had a warm and humid climate in the early Pliocene.According to the climatic and environmental characteristics of the distribution areas of the nearest living relatives of the five fossil species,quantitative analysis shows that the altitude of Western Yunnan is 730–1200 m in the Pliocene,the mean annual temperature is 14.0–19.0?,the mean temperature of the coldest month is 6.3–13.7?,the mean temperature of the warmest month is 19.5–26.0?,and the annual precipitation is 1138–1991 mm.