Trace fossils as proxy for biotic recovery after the end-Permian mass extinction: A critical review

Trace fossils represent records of the activity of both epifaunal and infaunal animals, providing significant information for a deeper understanding of Earth’s past environments and ecosystems. Increasingly, more and more ichno-metrics (quantitative ichnological indicators) have been proposed and applied to critical geological intervals as a methodology to assess the mechanisms and timing of biotic recoveries following mass extinctions. However, detailed assessment of the robustness and the scope of their applications is needed before we place them on more solid theoretical grounds. This paper presents a critical review of a range of popularly used ichnological parameters, including ichnodiversity, bioturbation index, ichnofabric index, bedding plane bioturbation index, burrow size, complexity, tiering, key ichnotaxa and some new parameters (e.g., ichnodisparity) with respect to their applicability and relative robustness as proxies for assessing the pacing of marine ecosystem recovery following major biotic perturbations, with a particular reference to the end-Permian mass extinction event. A detailed summary of the significance and caveats of each parameter is also provided. We suggest that bedding plane bioturbation index remains to be explored to indicate recovery while ichnodisparity holds potential to assess biotic recovery in future studies. Key ichnotaxa (e.g., Rhizocorallium and Thalassinoides) that are produced by malacostracan crustaceans, among other organisms, can be reliable indicators of environmental stability and ecosystem recovery. Further, we propose that the overall low bioturbation intensities may have substantially influenced marine elemental cycling during the Permian–Triassic transition that led to a possible drawdown of sulfate concentration in the Early Triassic oceans through enhanced pyrite burial.