Middle Triassic Coral Growth Bands and Their Implication for Photosymbiosis

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.

Evidence of photosymbiosis in Palaeozoic tabulate corals

“…As this phenomenon is often considered to be related to photosymbiosis [12,44], it therefore supports the results inferred from stable isotope analysis. Growth bands may occur in non-photosymbiotic corals but they are very thin, discontinuous and usually not visible to the naked eye [45], therefore very different from those of tabulates.…”

Corals on the slope (Aptian, Maestrat Basin, Spain)

“…In this respect, Stanley and Helmle (2010) reported the presence of growth bands in large and massive Middle Triassic colonial corals that suggest light-induced responses and, thus, the probable existence of zooxanthellate species already in the early Mesozoic. The results of stable isotope analyses performed on Late Triassic coral skeletons by Stanley and Swart (1995) and on the organic matrix from specimens of the same age by Muscatine et al (2005) also support this hypothesis.…”

From Fore Reef to Lagoon: Evolution of the Upper Triassic Dachstein Carbonate Platform on the Tennengebirge (Salzburg, Austria)

“…There is a long history of discussion about whether Late Triassic corals had a symbiotic relationship with photoautotrophs like Symbiodinium, the photosynthesizing endosymbiotic dinoflagellates (zooxanthellae) that live in coral polyps today (see Stanley and Swart, 1995;Stanton, 2006;Kiessling, 2010;Stanley and Helmle, 2010). This is a complicated issue, since, to date, no fossil zooxanthellae (or other photosymbionts) have been discovered.…”

“…This is a complicated issue, since, to date, no fossil zooxanthellae (or other photosymbionts) have been discovered. Nevertheless, other criteria can be used to hypothesize photosymbiotic association with corals (discussed in Leinfelder et al, 2002, andStanton, 2006), such as massive skeleton size, skeletal growth banding (e.g., Stanley and Helmle, 2010), restriction of corals to environments within the euphotic zone, horizontal flattening of growth morphologies with depth (e.g., Martindale et al, 2012a), predominance of highly integrated coral taxa (e.g., thamnasterioid and meandroid forms), and isotopic evidence of photosynthesis (e.g., Stanley and Swart, 1995). The aforementioned criteria are by no means absolute evidence of photosymbionts and the strength of some lines of evidence is questionable (e.g., Stanton, 2006); however, with enough criteria satisfied, there are cases where photosymbiosis can be the most parsimonious explanation.…”

“…The reef recovery in the Middle and Late Triassic occurred in four evolutionary stages (Flügel, 2002); the first stage is represented by the middle Anisian to early Ladinian reefs which are very similar to Permian reefs, with the addition of the first scleractinian corals (the modern order of stony corals). The second stage is the late Ladinian to early Carnian (first) reef optimum (Cordevolian), which saw the rise of reefs built by Permian holdover taxa (e.g., ''Tubiphytes''), baffling and encrusting sponges, and some corals that are thought to have incorporated photosymbiotic zooxanthellae around the Middle to Late Triassic (Stanley and Swart, 1995;Kiessling, 2010;Stanley and Helmle, 2010). The third reef stage is the late Carnian and early Norian faunal turnover from baffling and encrusting sponges, ''Tubiphytes'', and some corals to reefs dominated by large corals.…”