Stony Corals (Scleractinia)

Scleractinian corals, whose name means hard-rayed, first appeared in the Middle Triassic and refilled the ecological niche once held by tabulate and rugose corals, to which they are probably not closely related. Their septa are basically six-rayed, which is why they are sometimes called hexacorals. Colonial scleractinians from modern tropical seas are now the world's primary reef formers, and a colony consists of large numbers of polyps cemented together by the calcium carbonate they secrete. Some scleractinians are solitary instead, and some of those live in temperate waters or below the photic zone.