THE ECOLOGICAL COMPOSITION OF MICROBIAL-METAZOAN REEF ECOSYSTEMS FOLLOWING THE LATEST PERMIAN MASS EXTINCTION

The latest Permian mass extinction event was a critical event in the evolution of modern reef ecosystems, with Paleozoic corals going extinct and Scleractinian corals evolving and dominating reefs later in the Triassic. Prior to the evolution of Scleractinian reefs, reef ecosystems record major changes in their ecological composition, with microbial reefs filling the niche previously occupied by rich metazoan reefs, resulting in widespread microbial bioherms during the Early Triassic. Here, we find that the microbial buildups from Çürük Dag (Turkey) and Kuh e Surmeh (Iran) contain abundant lipid biomarkers representing input from cyanobacteria, anoxygenic phototrophic bacteria, sulfate-reducing bacteria, and halophilic archaea. The composition of lipid biomarkers suggests that the buildups were constructed by cyanobacteria-dominated microbial mats in an at least episodically and locally hypersaline environment. We also demonstrate for the first time that microbes, keratose sponges, and microconchids grew synergistically to form microbial-metazoan reefs in the immediate aftermath of the latest Permian mass extinction along the western margin of the Neotethys. Abundant fossils of oxygen-dependent invertebrates (i.e. microconchids, bivalves, gastropods, brachiopods, foraminifera, and ostracods) were also found within the bioherms. The presence of invertebrates in conjunction with abundant molecular fossils of cyanobacteria indicates local oxygenation. The deeper layers of the microbial mat would, however, have been anoxic and oxygen levels would have varied diurnally. The new finding of reefal structures built at least in part by metazoans suggests that reef ecosystems were not driven to extinction by the latest Permian mass extinction and were able to persist in oxygenated settings, albeit in an impoverished state.