Paleoenvironments of the Lower Triassic Chaohu Fauna, South China

The Lower Triassic ichthyosaurs from the Chaohu area (Anhui Province, China) testify for a major recovery pulse of marine ecosystems after the Permian-Triassic mass extinction. However, the paleoenvironment that favored the preservation of the Chaohu ichthyosaurs is poorly understood. We performed high-resolution analyses of carbonate rare earth elements (REE), total organic carbon (TOC), total sulfur (TS) and sedimentary microfacies around the occurrence horizon of ichthyosaur fossils within the Nanlinghu Formation of the South Majiashan section (Chaohu), to understand seawater redox conditions at the time of the marine recovery pulse. The preservation of the Chaohu Fauna is well correlated with redox environments. Prior to the ichthyosaur horizon, a negative Ce anomaly (Ce/Ce*) (-0.88) and high TOC/TS (mostly 4) indicate well-oxygenated condition (Stage A), corresponding to the lower part of the middle Nanlinghu Formation. Stage B is characterized by high Ce/Ce* (mostly >1.1) and low TOC/TS (<2) that suggest an anoxic environment, coincident with the major horizon yielding ichthyosaur fossils. The decrease occurrence of reptiles was synchronous with the development of oxic-dysoxic environments inferred from a decrease in Ce/Ce* (to-0.94) in Stage C. The ichthyosaurs likely evolved much earlier before the first fossils occurrences in the Nanlinghu Formation because of the ameliorated living environment and abundant food in the early Spathian; however, the development of anoxic conditions favored their preservation at Chaohu.