Earliest Triassic microbial mounds indicate an ‘oasis’ for recovery of life following the end-Permian extinction

Metazoan assemblages are rare in Early Triassic successions because of stressful environments in shallow seas after the end-Permian mass extinction (EPME). There is active debate about whether life in the oceans recovered rapidly or was delayed by up to 6–7 Myr. Here we present a new seabed assemblage showing evidence for relatively diverse animals in the middle Griesbachian, less than 1 Myr after the EPME crisis. The fossils are in microbial mounds from Chongqing, in the Upper Yangtze region, SW China, primarily composed of calcified filamentous cyanobacteria, including Cyanonema, Rivulariaceae, and Girvanellaceae, as well as planktonic calcareous microorganisms (calicispheres). The animals of the microbial mound include relatively common microconchids, sponges, gastropods and bivalves, and less abundant echinoderms and brachiopods. These metazoan communities differ among sub-facies of the microbial mound deposits, implying diverse ecosystem structures during the growth phase of the mounds. This kind of microbial mound might not only be typical of the aftermath of the mass extinction, but also have served as an ‘oasis’ for animals in the otherwise inhospitable environments of the Early Triassic seas.