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Originally Posted by cran
For a long time, geologists and palaeontologists have used fossil corals and reefs as an indicator of the environmental setting ... it was believed that all corals existed within a fairly narrow range of temperature and depth limits ... so that a fossil coral community indicated a warm shallow marine environment ...
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By and large this is still true. Fossil coral reefs are in almost all cases shallow water features, from multiple lines of evidence.
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Recent living coral discoveries must challenge that view somewhat ... cold deepwater corals have been found off Norway and in other parts of the Atlantic ... and some of these appear to be closely related (think identical) to some of the fossil forms ... one conclusion proposed is that these ancient corals did not die out but migrated to the deepwater environments and the more recent forms filled the niches thus created ...
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Not that recent. Deep water coral build ups have been known for many decades. While some ancient deep water builp ups may have been mistaken for shallow water reefs, in general deep and shallow water build ups have very different characteristics. Depwater corals are still "modern" at the taxonomic level, being all scleractinians. No rugose or tabulates have been found in deep water, unfortunately.
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but I wonder if these ancient corals were always deepwater forms, and the modern corals expanded into the shallow regions? ... of course, to consider that means overturning some longstanding beliefs about a number of geological settings ... and finding valid explanations for the underlying and overlying strata ... plus a plausible mechanism for vertical transport without destroying the fossil structures ...
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It's probably the other way round, corals first appeared in shallow water and only later moved into deep water enironments. The earliest corals in the Cambrian are in shallow water environments.
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so, a third possibility is that the ancient corals were pandemic, and occupied all marine settings ... and lost the warm shallow regions to modern forms, leaving only the deepwater settings where modern corals cannot survive ...
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Something like this appears to have happened with brachipods, stalked crinoids, and bryozoans, which are less common in shallow tropical environmnets than they were previously.
But palaeoenvironmnetal interpretation is probably a subject for another thread!
Jon