CORALLITE SIZES AND THEIR LINK TO EXTINCTION RISK OF SCLERACTINIAN CORALS ACROSS THE TRIASSIC-JURASSIC BOUNDARY

The end-Triassic mass extinction and reef crisis was a significant crisis in the evolution of scleractinian corals. The crisis is thought to have been caused by volcanically induced global warming. Accordingly, we would expect photosymbiotic corals to be more strongly affected than non-symbiotic corals. However, although corallite integration has decreased across the Triassic-Jurassic boundary, the extinction of scleractinian corals seems to be non-selective with regards to their inferred photosymbiotic mode. Using our new Reef Traits Database (RTD) we test how corallite sizes may have changed across the Triassic-Jurassic boundary. Corallite sizes have been used as a proxy for the efficacy of photosymbiosis with smaller corallite sizes associated with higher photosymbiotic autotrophy in colonial corals. As such, we use maximum corallite diameter to investigate how the dependency on a photosynthetic diet may have increased the extinction risk of certain corals at the end-Triassic mass extinction. Based on a survey of nearly 2000 measurements we establish trends in corallite sizes across the Triassic-Jurassic. We find that there was a significant increase of corallite diameter at the Triassic-Jurassic boundary suggesting stronger heterotrophy. Corallite sizes were 1.8 times larger after the mass extinction, which is the strongest change across the entire 100 myr interval. Generalized linear modeling suggests that the increase of corallite diameters was due to the selective extinction of corals with smaller corallites.