Abstract

In living zooxanthellate corals, photosymbiosis explains increased metabolism and accelerated skeletal growth, accounting for the success of these corals in shallow-water tropical reefs. Mesozoic corals of the order Scleractinia appeared in the geologic record during the Middle Triassic, but it was not until the Late Triassic that these corals became prominent reef builders—a change hypothesized to coincide with the advent of photosymbiosis. There is considerable discussion, however, concerning algal symbiosis and the timing of their co-evolution with corals. Thus, the beginning of photosymbiosis in the earliest corals of the Middle Triassic has not been established, nor whether their paleoecology was similar to that of modern corals. Many massive colonial reef-building corals lay down thick, discrete bands in their skeletons that record annual growth. We discovered and illustrate here growth bands in Middle Triassic corals from central Nevada, in particular Ceriostella variabilis, whose skeletal structure and bands are well-preserved in Middle Triassic biostromes of central Nevada. To test the photosymbiosis hypothesis we studied colony growth forms in these fossil corals and performed a quantitative analysis of the bands, both in C. variabilis and in a morphologically similar living zooxanthellate reef coral, Montastraea faveolata. Results of these analyses revealed growth bands and colony shapes almost identical in both living and fossil corals. These findings suggest that photosymbiosis was present in Middle Triassic corals at a very early stage in their Mesozoic history. Scleractinians were also likely zooxanthellate from the onset of their Middle Triassic occurrence but for unexplained reasons were not as efficient as modern corals in building reefs.

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