The exceptionally preserved Early Cretaceous Moqi Fauna from eastern Inner Mongolia, China, and its age relationship with the Jehol Biota

New fossil-bearing horizons at the Gezidong and Jiaxikou localities, eastern Inner Mongolia, referred to as the Moqi fossil bed, yield a diverse fossil assemblage coined herein as Moqi Fauna. The Moqi Fauna provides important insights into the evolution of some vertebrate clades, such as frogs and salamanders, and their Early Cretaceous diversification. In this paper, we report an improved chronology of the Moqi fossil bed based on SIMS and high-precision CA-ID-IRMS U-Pb zircon analyses of three tuff samples from horizons that are interstratified with the fossil-bearing layer at two localities. The SIMS U-Pb dating method applied on zircons from three samples gave dates of 117.8 +/- 0.9/1.5 Ma, 117.7 +/- 1.0/1.5 Ma, and 118.3 +/- 1.2/1.7 Ma, respectively. Two high-precision CA-ID-IRMS U-Pb weighted mean Pb-206/U-238 ages of, 119.20 +/- 0.38/0.38/0.72 Ma and 118.67 +/- 0.13/0.14/0.28 Ma were also obtained. These findings indicate the age of the Moqi Fauna is ca. 119.20 Ma to 118.67 Ma. Comparison of the Moqi Fauna with the well-known Jehol Biota, as well as the Fuxin Biota of a slightly younger age, suggests that the Moqi Fauna was a distinct fauna which shows a potential link with the Jehol Biota.

Automated summary

New fossil-bearing horizons at the Moqi fossil bed in eastern Inner Mongolia provide insights into the evolution and diversification of vertebrate clades, such as frogs and salamanders, during the Early Cretaceous. A chronology of the Moqi fossil bed was established using SIMS and high-precision CA-ID-IRMS U-Pb zircon analyses, indicating an age range of approximately 119.20 Ma to 118.67 Ma for the Moqi Fauna. Comparison with other well-known biotas suggests a potential link between the Moqi Fauna and the Jehol Biota.