Role of photosymbiosis in the Late Triassic expansion of shallow-water Scleractinia

Since their appearance in the Anisian (ca. 240 Ma) scleractinian corals rapidly diversified and expandedacross shallow marine environments, becoming the main builders of reefs. This ecological success coralsowe to their symbiotic relationship with photosynthesizing dinoflagellate algae. Although mechanismsunderlying this relationship have been broadly studied, fundamental questions regarding its origin andevolution remain open. The main obstacle in studies of symbiosis in the past is that coral skeletons do notpreserve zooxanthellae. Conventionally, zooxanthellate corals are regarded as forming highly integratedcolonies with small corallites, whereas the corals lacking zooxanthellae tend to have solitary forms or lessintegrated colonies with relatively large corallites. These features, however, are not exclusive for eitherecological group, pointing to the need for more definitive indicators. Isotopic signatures of the skeletons i.e.,carbonate 13C/12C and 18O/16O, and intracrystalline skeletal organic matter (SOM) 15N/14N have been also usedto distinguish symbiotic from asymbiotic corals. However, their application is restricted only to unalteredskeleton. This feature, although rare in the fossil record, was reported in coral skeletons as old as Triassic.Recent development of two new methods gives opportunity for investigation of symbiosis in fossilscleractinians: microscale growth banding and “persulfate-denitrifier” high precision analysis of nitrogenisotopic composition of intracrystalline SOM. These methods were applied to a set of exceptionallypreserved skeletons of Early Norian (ca. 212 Ma) scleractinian corals from Antalya (Turkey). Selectedskeletons represented both, solitary or poorly integrated (phaceloid) and highly integrated (cerioid,meandroid and thamnasterioid) growth forms. Microscale growth bands of Triassic corals were compared with those of living scleractinians. Theexamined fossil specimens, including forms traditionally considered as asymbiotic, exhibited a highlyregular and continuous banding pattern typical for modern zooxanthellates, contrary to modern asymbioticcorals with irregular, often discontinuous bands. Nitrogen isotopic signatures of intracrystalline SOM inTriassic scleractinians fall below the range measured for modern asymbiotic corals, but overlaps data formmodern zooxanthellates, indicating that each of examined Triassic corals harbored photosymbionts. If theseEarly Norian corals were symbiotic, their δ15N suggests that local source values were similar to those ofmodern oligotrophic (sub)tropical North Atlantic. In fact, similarity between δ15N from Triassic corals andmodern Bermuda specimens suggests that they lived in similarly nutrient-poor waters. Further analyses showthat Triassic specimens with pristine biogenic aragonite exhibit oxygen and carbon isotopic signaturescompatible with those of modern symbiotic corals. The presented combination of microstructural and geochemical indicators, provides a new, powerfultoolkit for assessing symbiosis in well-preserved fossil corals. All criteria support our interpretation thatEarly Norian corals from Antalya lived in symbiosis with dinoflagellates, including small solitary andphaceloid forms, which based on morphology alone would be classified as asymbiotic. Since coral generaexamined herein were widespread in the Late Triassic reefs of NW Tethys, we propose that symbiosis wasthe dominant if not the exclusive lifestyle in shallow-water corals from the Tethyan realm. Theseoligotrophic environments, provided strong impetus for establishing of symbiosis with photosynthesizingalgae. Since the Triassic, the symbiotic relationship of corals and zooxanthellates facilitated the formation ofthe greatest oceanic bioconstructions - coral reefs.