Additional records of ichnogenus Rhizocorallium from the Lower and Middle Triassic, South China: Implications for biotic recovery after the end-Permian mass extinction

The ichnogenus Rhizocorallium is represented by complex burrow networks that have been interpreted as an indication of biotic recovery following major mass extinctions. Thousands of specimens of this ichnogenus are observed from Lower-Middle Triassic successions in 10 sections of South China, and they are identified to eight known or uncertain ichnospecies. Globally, Rhizocorallium was sporadically present in the Griesbachian-Smithian interval, underwent specific proliferation and spatial expansion in the Spathian, and became very abundant during the Anisian. In general, the Spathian and Pelsonian (middle-late Anisian) saw ichnospecies richness peaks and morphologic complexity. Burrow sizes of Rhizocorallium generally all increase from the Griesbachian to Spathian, followed by a relatively steeper increase from the Spathian to Pelsonian. Rhizocorallium possesses the shallowest penetration depths of burrows prior to the Spathian and follows a pattern of occupying stepwise-increasing tiers from the Spathian to Pelsonian. The Rhizocorallium-bearing strata possess fairly high bioturbation levels in the Spathian and Anisian. The global Early-Middle Triassic Rhizocorallium ichno-taxa were distributed among seven habitat types in shallow seas. They, however, had an environmental preference for carbonate lagoon settings. Pre-Spathian Rhizocorallium possessed a relatively smaller size, shallower depth penetration, few ichnospecies, and limited ecologic/environmental and geo-graphic distributions. Their occurrences in that time indicate early stages of recovery. This is consistent with other biota, which still suffered postextinction biotic depletion and environmental stresses. Although Spathian Rhizocorallium possessed elevated ichnospecies numbers and morphologic complexity, as well as environmental and geographic expansion, most ichnotaxa remained relatively small and thus may indicate stages 2-3 of ecologic recovery. In comparison, middle- late Anisian Rhizocorallium not only showed the highest ichnospecies richness and deepest penetration depth, but it also exhibited the greatest morphologic complexity and largest burrow sizes, with some forms having tube diameters >20 mm. This indicates the final stage (4) of ecologic recovery following the end-Permian crisis. Measured by all ichno-ecologic proxies, Early-Middle Triassic Rhizocorallium ichnotaxa show a stepwise recovery pattern: unexpected rebound in the Griesbachian-Smithian, an increased ecologic recovery in the Spathian, and a full recovery in the middle-late Anisian (Middle Triassic).