Early Triassic trace fossils from South China: implications for biotic recovery from the end-Permian mass extinction

The end-Permian Mass Extinction (EPME) caused the largest biodiversity loss and had the most severe impact on both marine and continental ecosystems in the Phanerozoic. Trace fossil analysis has proven useful for deciphering patterns of Early Triassic biotic recovery worldwide. In this study, a review of global Permian–Triassic trace fossil data from pre- and post-extinction intervals indicates that habitable zones preferentially developed in siliciclastic settings in high-latitude regions and mixed carbonate-siliciclastic settings in low latitude regions during the Griesbachian. These zones expanded greatly during the Spathian. The review also highlighted diachronous recovery of infaunal organisms between low- and high-latitude regions potentially reflecting greater stress from elevated Late Permian temperatures on equatorial to low-latitude oceans.

Trace fossil and facies analysis was undertaken on two Lower–Middle Triassic stratigraphic sections in South China. The Susong section (Lower Yangtze Sedimentary Province) comprises a range of carbonate and mudstone facies that record overall shallowing from offshore to intertidal settings. The Tianshengqiao section (Upper Yangtze Sedimentary Province) consists of mixed carbonate and siliciclastic facies which were deposited in shallow marine to offshore settings. Griesbachian to Dienerian ichnological records in both sections are characterized by low ichnodiversity, low ichnofabric indices (1–2) and low bedding plane bioturbation indices (1–2). Higher ichnofabric indices (3 and 4), corresponding to a dense population of diminutive ichnotaxon, in the Tianshengqiao section suggests opportunistic infaunal biotic activity during the earliest Triassic. Ichnological data from the Susong section show an increase in ichnodiversity during the late Smithian with 12 ichnogenera identified including Arenicolites, Chondrites, Didymaulichnus, Laevicyclus, Monocraterion, Palaeophycus, Phycodes, Plaolites, Rhizocorallium, Thalassinoides, Treptichnus, and Trichichnus. This is matched by increased ichnofabric indices of 4–5 and bedding plane bioturbation indices of 3–5. Although complex traces such as Rhizocorallium are present in Spathian-aged strata in this section, low ichnodiversity and ichnofabric indices and diminutive Planolites suggest a decline in recovery. In the Tianshengqiao section, ichnofabric indices are moderate to high (3–5) although only six ichnogenera are present and Planolites burrows are consistently small in Smithian strata. These stressed ichnological parameters remain unchanged during the Spathian. Complex traces such as large Rhizocorallium and Thalassinoides, and large Planolites, did not appear until the Anisian. Ichnological results from both sections record the response of organisms to unfavourable environmental conditions although the Susong section shows earlier recovery during the Smithian prior to latest Smithian–Spathian decline. This decline may have resulted from a temperature spike at the Smithian/Spathian boundary recognised in South China and elsewhere.

Ichnological data from the Tianshengqiao section indicate protracted recovery throughout the Early Triassic. This is similar to previous studies in other parts of South China. Comparison of the South China trace fossil records with ichnological data from elsewhere highlights a diachronous pattern of recovery of trace makers and the influence of regional factors (such as paleogeography, tectonic and depositional history including location with respect to siliciclastic influx) on the rate of recovery.