| As the major component of modern steppe, the Xerophytic taxa plays an important role in the flora of China. The families or genera of Xerophytic taxa have varying bio-stratigraphic significances in different time ranges. The origination and evolution of Xerophytic taxa underwent a series of development, resulting in the formation of Xeromorphic vegetation. Some of the xerophytes are especially representative, such as Ephedraceae, Nitrariaceae, Chenopodiaceae, Asteraceae, Poaceae, and Caryophyllaceae.Similar to the steppe vegetation in the eastern extent of Europe, the steppe in China is mainly composed of Stipa L., Festuca L., Cleistogenes Keng, Artemisia L. and Sanguisorba L. In the desert regions of Northwest China, which are under control of the East Asian monsoon system (such as Tarim and Qaidam), vegetation is mainly composed of Salsola L., Anabasis L., Suaeda Forssk. ex J. F. Gmel., and Artemisia, with percentages of Nitrariaceae Lindl. (Nitraria) and Ephedraceae Dumort. much lower than other xeromorphic taxa. The extant diversity is suggested to be the result of a radiation initiated in the earliest Oligocene, probably associated with a shift from insect pollination to wind pollination in the genus. However, the palynological fossil record shows that Ephedraceae and Nitrariaceae were widely distributed and diverse in China and were dominating the Late Cretaceous—Palaeogene steppe vegetation.Our study area, Xining, Xunhua and Qaidam basins, situated in the northeast margin of the Tibetan Plateau, China. Those three basins have been influenced by arid climate at least since the Eocene. As such, the available terrestrial sedimentary sequences from these settings provide the ideal material to better understand the evolution of steppe vegetation during the Cretaceous— Eocene.To contribute to a better understanding of the ephedroid evolution, we here focus on the time interval of the Cretaceous to the Eocene transition in NW China, with particular emphasis on the Eocene record in the Xining Basin. By using the single grain technique, we illustrate key taxa with SEM and LM photography and describe key morphological features. Finally, we compare this record with that of the Quaternary of the Qaidam and Xunhua Basin and draw implications for improved understanding of steppe evolution.Based on developmental studies, former researchers have concluded that the equatorial axis in Ephedra pollen is equal to the longest axis and the polar axis is equal to the shortest axis; thus opposite to the original descripition of Ephedripites. Here we recalculate the P/E ratio of each species that have been found in our study area. Besides, "the hyline line" or "the zigzag line" in the orginal descripition has been corrected as the baranched or unbranched sulci, so does the polar area. The definition of Ephedripites subgenus Ephedripites has been amended to polyplicate pollen with unbranched sulci and fused plicae.A total of 30 polyplicate pollen types, belonging to three different genera, was found in the samples, which include one new combined species Ephedripites (Spiralipites) retiformis and one comparative species Steevesipollenites cf. binodosus.We described all the species we’ve found systematically. We consider Ephedripites (Distachyapites) trinata is synonym of Ephedripites (Distachyapites) fusiformis, and Ephedripites (Distachyapites) parafusiformis, Ephedripites (Distachyapites) megatrinata are synonyms of Ephedripites (Distachyapites) megafusiformis. As for the pollen with none fused or partly fused plicae which were assigned to Schizaeisporites. we choose to call them Gnetaceaepollenites, since none of these grains any laesura were found.Based on the morphology character of different genera, we have concluded that: Ephedripites are polyplicate pollen with a few to nurmerous plicae fused at the end, with branched or unbranched sulci; Steevesipollenites are polyplictae pollen with a few to nurmerous plicae fused at the end, and the exine in the end is abnormally thickening; Gnetaceaepollenites are polyplictae pollen with a few to nurmerous plicae, plicae on one side are oblique to those on the other side, forming a "checkered" pattern, and the plicae are none fused or partly fused at the ends.A small amount of Ephedripites subgenus Dischyapites pollen grains have been found within the Cretaceous sediments in East Xining section, Xining Basin. Thus, the first occurance of Ephedripites subgenus Dischyapites has been moved forward to the early Cretaceous, it is clear that the crown-group Ephedra is much older than previously estimated. We hypothesize that the Ephedripites subgenus Dischyapites can adapt to drier climate.Our palynological records show that the morphological changes of ephedroid pollen can be divided into stages as follow:During the late Early Cretaceous, most of the ephedroid pollen predominantly consist of Gnetaceaepollenites, with low percentages of E. subgenus Ephedripites and Steevesipollenites, and rare E. subgenus Distachyapites. In the late Late Cretaceous—early Palaeogene, the ephedroid pollen are dominated by increased percentages of E. subgenus Ephedripites, higher percentages of E. subgenus Distachyapites than the late Early Cretaceous, and slightly higher percentages of Steevesipollenites; notably Gnetaceaepollenites is no longer present.In the late Eocene, two distinct zones mark the Ephedripites composition. Zone I is composed of E. subgenus Distachyapites, E. subgenus Ephedripites and Steevesipollenites. Zone II is dominated by E, subgenus Distachyapites, whereas the percentages of E. subgenus Ephedripites (less than 10%), Steevesipollenites decrease sharply. In the Quaternary, only two types of Ephedra are documented in the samples from the Qaidam Basin; they both belong to the Distachya type, but with poorly developed secondary branches of branched sulci.According to the previous study and our results, we hypothesize that the genus Gnetaceae-pollenites, which is abundant in the Cretaceous, went extinct after the K/T boundary. Ephedripites subgenus Ephedripites, and Steevesipollenites have also been found in the Cretaceous. Both E. subgenus Ephedripites and E. subgenus Distachyapites was highly diverse in Paleocene—middle Eocene, together with small percentages of Steevesipollenites. At around 40 Ma, E. subgenus Distachyapites increased and dominated the steppe vegetation, while E. subgenus Ephedripites and Steevesipollenites decreased. In the late Eocene, the percentage of Ephedripites gradually dropped, and Ephedripites subgenus Ephedripites, and Steevesipollenites went extinct after the E/O boundary.According to the palynological record of Xining Basin, Qaidam Basin and Xunhua Basin, the composition of steppe vegetion in NW China can be divided into 3 types. The late Cretaceous—early Palaeogene pollen assemblages are characterized by ～40% presence of typical steppe taxa, which are mainly composed of Ephedripites. In the early late Eocene (ca.42-40 Ma), the steppe taxa are made up of Ephedripites and Nitrariadites, which account for ～50% of the pollen sum. Later, in the late Eocene, steppe pollen dominates the pollen sum. The base of this time interval is rich in Ephedripites pollen, whereas Nitrariadites becomes dominant at the top. Also pollen of the CAC group appears during this period, although their percentages do not exceed ～5% of the pollen sum. Nowadays, in the Qaidam Basin the steppe taxa account for ～80% of the pollen sum, with the CAC group pollen dominating the pollen assemblages; in contrast, the percentages of Ephedripites pollen are ～12%, whereas Nitrariadites are rare.The early elevation history of Tibetan Plateau has been studied by many researchers. In this study, we interpret that the～40.0 Ma aridification step might be associated with the regional tectonic uplift. The southern-central Tibet which had attainment of high elevations in ～40 Ma, formed an orographic barrier, preventing moisture from the Indian oceans into the studied area at that period. However, previous research shows that the Neotethyan Sea was the main source of moisture for northwest China, thus we cannot exclude the influence of the sea retreat might increase aridity in that region.Our study shows an obvious cooling event in the upper part of Shuiwan section, which were represent as the significant decline of Ephdraceae and the occurance of CAC group. We hypothesis that Ephedraceae was warm-tolerant type vegetation during the Paleogene and adapted to a eurythermic environment during the Neogene. And this pre-Oligocene climate cooling might associate with the regional uplift in the northern Tibetan Plateau. |
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