Novel reef coral communities emerging after deep-time hyperthermal events

Modern coral reefs are among the most vulnerable ecosystems to climate change. Accordingly, we hypothesize that past hyperthermal events had lasting impacts on reef coral communities. Specifically, novel communities are expected to emerge after ancient warming events, where novel communities are those that document a rapid and irreversible shift into a new state that differs in composition and/or function from past systems. To test our hypothesis, we used a global compilation of reef coral occurrences from the middle Triassic to modern times (244.08 Ma) and applied a rigorous novel community detection framework at 0.1 Myr time bins. Novelty is quantified based on two components – cumulative novelty (i.e., the deviation from historical baselines and instantaneous novelty (i.e., the magnitude of change relative to the previous state). A novel community state is identified when both cumulative and instantaneous novelty match in a time series of ecological change. Surprisingly, over the entire evolutionary history of scleractinian corals, there were only two novelty events at global scales, and they both occurred in the aftermath of hyperthermal events: The first in the Hettangian stage and the second in the Toarcian. Our results underscore the hypothesis that profound global warming can have lasting consequences on coral reef ecosystems.