Early Triassic trace fossils from the Three Gorges area of South China: implications for the recovery of benthic ecosystems following the Permian-Triassic extinction

The Lower Triassic Daye and Jialingjiang formations of the Three Gorges area (South China) record the recovery interval of benthic tracemaking invertebrates following the P-Tr mass extinction. A total of 17 ichnospecies in 14 ichnogenera are documented from Smithian and Spathian strata. Our trace fossil data, in combination with previously published studies, show that ichnodiversity in the Middle Yangtze region increased markedly in the early Spathian. Trace fossils in the Smithian are dominated by simple, small, horizontal burrows, including Didymaulichnus and Planolites, whereas Spathian trace fossils are diverse and abundant with moderate-high bioturbation levels and complex burrow networks, such as Thalassinoides. Both burrow sizes and penetration depths increased gradually from the early Spathian to the middle-late Spathian, implying a gradual recovery pattern for benthic ecosystems. Early Triassic ichnofossils are characterised by aspects of opportunistic behaviour (e.g., low-to-moderate ichnodiversity, low-to-moderate bioturbation, small burrow widths, and shallow tiering), suggesting stressed environmental conditions. The recovery tempo and pattern of ichnocoenoses in South China is likely structured by temporal and spatial changes of the refuge zone in the Early Triassic.