Fossil Equisetum from the Lower Cretaceous in Jiuquan Basin, Gansu, Northwest China and its paleoclimatic significance

Tubers of Equisetum are described for the first time from the Lower Cretaceous Zhonggou Formation, Jiuquan Basin, Gansu Province, Northwest China. The fossils are preserved as rows of 2–3 tubers. The tubers are elliptical, ovate and more or less rounded, the most distal one being mucronate, with some ridges on the surface of the tubers. According to the similarity of the morphology and arrangement with extant Equisetum subgenus Equisetum, the present fossils are assigned to a new species of Equisetum. Furthermore, the cellular details of the tubers are clearly revealed for the first time. And the cellular morphology of extant Equisetum tubers can be compared to those of the fossil species. The occurrence of Equisetum would suggest a wet habitat, and the presence of tubers implies that burial occurred during the late fall or winter. Based on the palynological assemblages and mega-fossil assemblages, it had been thought that in the Early Cretaceous the Jiuquan Basin was arid, but the living habitat of Equisetum indicates a wet, riparian environment. Therefore, the occurrence of fossil Equisetum associated with fishes and ostracodes in the lower sequence would suggest a locally wet environment, probably a lake-shore in an arid region during the late Early Cretaceous in the Jiuquan Basin. The study is of great significance for further interpretation of the paleoclimate and environment in the Early Cretaceous of Northwest China.