Establishment and significance of ancient lake ecosystems in the Mesozoic—evidence from coprolite from the Chang 7 section of the Upper Triassic in the Ordos Basin, China.

The Ordos Basin is a large depression-type lake basin in the late Triassic. Here, we present a study on Yufenites, which were found in the BWZ (BaWangZhuang) section, TNH (TangNiHe) section and Well Z11 in the Tongchuan area on the southern margin of the basin and in the lower shale layer of Chang 7 member in Triassic Yanchang Formation. Yufenites are slightly flattened spindle-shaped fossils, pointed and thin, with obvious spiral patterns on the surface. The spiral starts from the thin part and turns from right to left, and the pitch changes from narrow to wide. The Yufenites contains animal and plant residues that have not been completely digested, and a large number of mineral aggregates that formed around phosphorus-containing dolomite as a growth base. Yufenites had a high viscosity and strength when discharged. Due to the action of microorganisms, the diagenesis of Yufenites occurs earlier and faster than that of the surrounding rocks. Based on this description, we believe that this Yufenites is from a large carnivorous fish. In the Late Triassic, humid and hot climatic conditions, low water kinetic energy, desalinated water, high productivity, magmatic-hydrothermal activity and other factors played an important role in the preservation of Yufenites. [ABSTRACT FROM AUTHOR]