TRIASSIC CORAL GROWTH BANDS AND PHOTOSYMBIOSIS

The first Mesozoic corals of the order Scleractinia, ancestors of all living corals, appeared during Middle Triassic time and became prominent reef builders near the end of the Triassic. Symbiosis between corals and zooxanthellae is a primary driver both on reefs today and in the fossil record. A fundamental question is the “photosymbiosis hypothesis," which attempts to estimate when in the Triassic photosymbiosis began in early scleractinian corals. In living zooxanthellate species photosymbiosis increases metabolism and skeletal growth and these are linked to explain the success story of corals in shallow-water tropical reef complexes and presumably also corals of the past. It has been questioned whether photosymbiosis was present among Middle Triassic corals because they lack evidence of reef construction until much later in the Late Triassic. Most massive reef-building species today form discrete bands that record annual growth rates and azooxanthellate corals lack such bands. We discovered and illustrate here the oldest identified growth bands in Triassic corals. These come from Ceriostella variabilis whose skeletal structure is well-preserved in Middle Triassic biostromes of central Nevada. Quantitative measurement of bands in C. variabilis and assessment of colony growth form allow testing of the photosymbiosis hypothesis. The living zooxanthellate reef coral Montastraea faveolata, morphologically similar to C. variabilis, served as the modern analog for comparison. Analysis of growth bands, comparable in both living and fossil taxa, allowed us to assign a photosymbiotic paleoecology for C. variabilis, which reveals growth bands and colony shapes almost identical to those in M. faveolata. These findings strongly support the hypothesis that photosymbiosis began in the Middle Triassic, in at least one taxon. From this and older Middle Triassic corals, it appears likely that scleractinians were zooxanthellate from their early appearance onwards but an unexplained aspect of Mesozoic coral evolution is the apparent decoupling of photosymbiosis and reef building.