Cupressaceae From The Oligocene Of Northwestern Qaidam Basin, China, And Its Geological Significance

The impacts of the Qinghai-Tibet Plateau uplift on the environment of China and even Asia have always been a geological research focus.The Qaidam Basin,the largest terrestrial sedimentary basin in the Plateau,formed in the Cenozoic.Since the late Cenozoic,it has experienced strong tectonic uplift and aridification process.Paleobotanical investigation of the basin is not well.In this paper,the current fossil Cupressaceae were studied in morphology and cuticle experiment is carried out,aimed for its geological significance.Through morphological research and comparison,the studied fossils were identified as 3 genera and 5 species: Cupressus funebris Endl.,Cupressus sp.,Chamaecyparis cf.lawsoniana Parl.,Chamaecyparis sp.,Thuja cf.koraiensis Nakai.The main features of cuticle of one fossil Cupressus funebris are as follows: the epidermal cells are oblong,about 21-36 ?m long and 11-15 ?m wide,and the cell boundaries are obvious,some epidermal cells have papillas on the surface,and the pores are mostly irregular ellipses,about 10-12 ?m long and 4-6 ?m wide.The main features of the cuticle from fossil Thuja cf.koraiensis are as following: the epidermal cells are oblong,about 7.9-12.7 ?m long and 4-4.5 ?m wide,and the stomatal apparatus is elliptic,about 6.7-8.5 ?m long and 4-5 ?m wide,the stomatal apparatus is of the annular type,has 2 guard cells and 5-6 subsidiary cells,and sometimes adjacent stomatal apparatus are close to each other or even share subsidiary cells.According to the fossil records and the discovery of the current fossils of Cupressaceae,it shows that the distribution of Cupressaceae in the Oligocene period was more extensive than that of now.The migration route of Cupressus ?Chamaecyparis and Thuja were further discussed.It is speculated that Cupressus,originated in the Mediterranean region,and spread to the Qaidam Basin and southwest China during the Oligocene.The Chamaecyparis,originated in east Asia inthe early Cretaceous,at least it spread to Qaidam Basin in Oligocene,and then might have spread to Europe.The Thuja,originated in high latitudes region of North America in the Paleocene or before the Paleocene,spread to east Asia through the Bering Land Bridge in the Oligocene.Tectonic movement and climate change led to the disappearance of all three genera in the current study area.The Coexistence Approach was used to deduce paleoenvironment of the northwestern Qaidam Basin,by using ofthe closest living relatives of Cupressus?Chamaecyparis and Thuja fossils current studied.It shows that the northwestern Qaidam Basin might be higher temperature and precipitation during the Oligocene,the mean annual temperature is 10-16.7 ?,and the mean annual precipitation is600-1500 mm.By studying the altitude range of the closest living relatives of the current fossils,it is speculated that the altitude of the study area during the Oligocene period may between 630-1930 m.At present,the mean annual temperature in the study area is about-1.03 ?,the mean annual precipitation is about 60 mm,and the elevation is about 3400 m,indicating that the climate of the Oligocene in the study area is quite different from that of present.