Anthozoans are probably the most famous of the cnidarians [cnidarians are incredibly diverse in form, as evidenced by colonial siphonophores, massive medusae and corals, feathery hydroids, and box jellies with complex eyes]. The anthozoa include the corals that build great reefs in tropical waters, as well as sea anemones, sea fans, and sea pens. They also have a long and diverse fossil record, extending back at least 550 million years. The oldest anthozoans are probably some of the polyp-like and sea pen-like fossils known from the Late Precambrian. A few tens of millions of years later, in the Cambrian period, the first mineralized coral-like organisms appeared. True corals of the kind living today did not appear until the middle Triassic Period.