Secular changes in environmental stresses and eukaryotes during the Early Triassic to the early Middle Triassic

The Early Triassic, following the end-Permian mass extinction, was an interval of severe low diversity. Increasing amounts of evidence demonstrate that variable environmental stresses were widespread and intense after the end-Permian mass extinction. Here we report biomarkers from lowest Triassic to lower Middle Triassic strata in South China (Qingyan and Chaohu sections), including biomarkers for environmental stress (2-methyl hopane index) and eukaryotic algae (steranes and C₂₁ n-alkylbenzene ratio). Using the 2-methyl hopane index, we detected the persistence of environmental stress during most of the Early Triassic. Using steranes and the C₂₁ n-alkylbenzene ratio, we found a gradual increase in the biomass of eukaryotic algae during the Early to early Middle Triassic. A decrease in environmental stress in the Qingyan section (Leidapo Member) during the early Middle Triassic was synchronous with the "explosion" of the Qingyan Biota, which is characterized by a high abundance and diversity of invertebrate marine animals. Because the environmental stresses revealed by the 2-methyl hopane index encompass various factors (e.g., pH and temperature), we cannot identify the exact stresses at that time; however, our results reflect the amelioration of harsh environments for life during the interval of complete biotic recovery.