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An improvement of this: A Completely Different Kind of Reef Which I couldn't delete because it already had answers, and deleting an answered question will dock me some reputation.

Today's reefs are built primarily by Anthozoa, the corals and sea anemones. Alongside them are Porifera, the sponges, and Crinoidea, the sea lilies. But in this scenario, the corals, anemones, sponges and crinoids have been extinct for 65 million years, leaving the role of reefbuilders to go to the following:

Ascidiacea

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Cirripedia

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Bryozoa

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Bivalvia

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Canalipalpata

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If our 21st-century aquatic reefs consist only of the listed animals, would they be more extensive and common than the coral reefs? Or will they still be limited to the tropical shallows?

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Believe it or not, but reefs are even now not just limited to shallow tropical water. You can for example find cold reefs at relatively large depths in the North sea and other places. However, I do not think a removal of the largest class of corals will make reefs more common and widespread than today. After all, all of the listed species exist today too, and if they are not thriving in a region now, there is no reason to believe they would do better if Anthozoa was completely lacking. They are tougher, though, especially Cirripedia. I can even find it at the shore here where I live at 64° north.

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