The survival, recovery, and diversification of metazoan reef ecosystems following the end-Permian mass extinction event

Abstract

The Triassic Period records important ecological transitions in the aftermath of the end-Permian mass extinction and is a key interval in the evolution of modern coral reefs. There have been several critical developments in our understanding of Triassic reef evolution over the past decade: the timing of events and duration of stages have changed dramatically; the discovery of metazoan reefs in the Early Triassic; details about the environmental perturbations that drove the extinction; the relationship between tectonic activity and platform margin reef proliferation; and additional proxy evidence for the co-evolution of coral reef-builders and their photosymbionts. Here, we provide an up-to-date synthesis of reef collapse and recovery dynamics following the end-Permian extinction, specifically integrating recent discoveries. The evolution of reef ecosystems can be divided into five phases based on their composition. 1) Microbial-metazoan reefs represent survival communities that characterize the immediate extinction aftermath. 2) The re-establishment of reefs built by metazoans (small sponge biostromes and bivalve buildups) is observed in oxygenated settings in the Olenekian (Early Triassic). 3) Towards the end of the Olenekian and into the Anisian (Middle Triassic) low-diversity, “Tubiphytes”-dominated reefs formed, which represent the first Triassic platform-margin reefs; platform-margin reefs, however, are not widespread until the late Anisian. 4) Late Anisian reefs also record a composition change and increase in species richness with sponges and “Tubiphytes” as the main reef builders. 5) The first scleractinian corals (which are the main reef builder in modern marine reef ecosystems) evolved during the Anisian but are not reported as dominant reef builders until the Late Triassic. The radiation of coral reefs is posited to be coupled to the acquisition of photosymbionts (e.g., zooxanthellae). There is clearly a stepwise evolution of reef types during the Triassic; however, once each reef type appears it persists throughout the remainder of the Triassic. The survival, recovery, and diversification of reef ecosystems is, therefore, more complex than previously outlined, particularly with respect to the earliest post-extinction ecosystems. These recent advances highlight the need to thoroughly document the faunal compositions of understudied reef systems as well as to continue the exploration of Triassic ecosystems in underrepresented regions.