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Corals evolved to survive within only a very narrow temperature window.
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I picked only one paper to scan, so this is cherry-picking, but:
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Scleractinia stands out as one of the few orders of calcified metazoans that arose in Triassic time, long after a greater proliferation of calcified metazoan orders in the Paleozoic. The origin of this coral group, so important in reefs of today, has remained an unsolved problem in paleontology. The idea that Scleractinia evolved from older Paleozoic rugose corals that somehow survived the Permian mass extinction persists among some schools of thought. Paleozoic scleractiniamorphs also have been presented as possible ancestors. The paleontological record shows the first appearance of fossils currently classified within the order Scleractinia to be in the Middle Triassic. These earliest Scleractinia provide a picture of unexpectedly robust taxonomic diversity and high colony integration. Results from molecular biology support a polyphyletic evolution for living Scleractinia and the molecular clock, calibrated against the fossil record, suggests that two major groups of ancestors could extend back to late Paleozoic time. The idea that Scleractinia were derived from soft-bodied, “anemone-like” ancestors that survived the Permian mass extinction, has become a widely considered hypothesis.
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The evolution of modern corals and their early history
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Reconstruction of the past 5 million years of climate history, based on oxygen isotope fractionation in deep sea sediment cores (serving as a proxy for the total global mass of glacial ice sheets), fitted to a model of orbital forcing (Lisiecki and Raymo 2005)[2] and to the temperature scale derived from Vostok ice cores following Petit et al. (1999).[3]
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en.wikipedia.org/wiki/Global_temperature_record
It appears that coral revert to a soft-bodied form at times, and the reef is actually built by algae in the coral's epidermis.
Apparently... from a 10 minute investigation...
(the term aragonitic biomineralization reminds me of an
old inactive forum member)