Early Triassic trace fossils from South China marginal-marine settings: Implications for biotic recovery following the end-Permian mass extinction

The Longmendong section is one of the few Early Triassic successions containing fluvial to marine deposits in South China. No trace fossils have been recorded in the fluvial deposits, but a total of 26 ichnospecies are reported from the Lower Triassic Dongchuan, Feixianguan and Jialingjiang formations in this section. This ichnofauna represents brackish-water conditions during the late Die-nerian to Smithian, and fully shallow-marine settings in the Spathian. Several ichnologic metrics, such as ichnodiversity, ichnodisparity, ichnoabundance, bioturbation intensity, burrow size, and depth of bioturbation, have been analyzed in order to evaluate the role of environmental and evolutionary factors. Although the upward increases in ichnodiversity, ichnodisparity, burrow size, and bioturbation intensity may be linked to the biotic recovery after the end-Permian mass extinction, environmental controls also play an important role in this case. Transgressive-estuarine successions typically show a vertical decrease in the salinity stress, showing the passage of brackish-water ichnofaunas to more diverse associations showing more marine affinities. However, the vertical increase in depth of bioturbation cannot be explained by environmental controls alone, instead most likely reflecting the phase of biotic recovery. Overall, ichnologic data suggest that the brackish-water ecosystem was less impacted by the end-Permian mass extinction than the fully marine realm. The shallow, fully marine benthos completely recovered in the Spathian as is the case for other areas in South China. This study underscores the importance of a careful evaluation of sedimentary facies and environmental conditions as a prerequisite for interpreting evolutionary mechanisms of biotic recovery.