Close-up of the surface of a polished slab of sedimentary rock containing an unidentified fossil coral. Corals are marine organisms closely related to sea anemones and sea pens. They first appeared at the beginning of the Paleozoic era, some 570 million years ago. They secrete an external skeleton of calcium carbonate, often brightly coloured, which they extract from the surrounding water. Colonies of coral polyps may gradually build up vast reefs of coral, each new inhabitant building on the empty skeletons of its ancestors.
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