TRANSIENT METAZOAN REEFS IN THE AFTERMATH OF THE END-PERMIAN MASS EXTINCTION

Recovery from the devastating Permian-Triassic mass extinction about 252 myr ago is usually assumed to span ~5 myr, the entire Early Triassic epoch. The post-crisis interval was characterized by large-scale fluctuations of the global carbon cycle and harsh marine conditions, including a combination of ocean acidification, euxinia, and fluctuating productivity. During this interval, metazoan-dominated reefs are thought to have been replaced by various microbial deposits that are considered the hallmark of the Early Triassic. Contrasting with this consensual recovery scenario of a delayed recovery of complex benthic communities, we document metazoan-rich bioaccumulations and reefs from the western USA, formed only 1.5 myr after the extinction. These bioconstructions are comprised of various sponges and serpulids associated with microbialites and diverse eukaryotic organisms. We conclude that the predominance of microbial reefs following the mass extinction is restricted to short intervals during the earliest Triassic. We suggest that metazoan reef building continued throughout the Early Triassic wherever permitted by environmental conditions. However, the absence of corals, or their failure to calcify, remains remarkable for the Early Triassic and contrasts with their flourishing in later Triassic times. The absence could be due to the impossibility for these stenotypic reef builders to cope with intermittent deleterious conditions, especially potential acidification. While present-day reefs are most likely headed towards a severe extinction, these findings remind us that biotic recovery after an extinction event is a rapid phenomenon at the geological time scale, but a very long evolutionary process at the biological (human) time scale.