Corals are the skeletons of some marine animals belonging to the Phylum Cnidaria (NÎ-dÂ'-ri-a), a medium-sized phylum that contains a large variety of solitary and colonial invertebrates including sea anemones, hydroids, sea pens, jellyfish, siphonophores, and millepores, in addition to corals. Two body forms, polyp and medusa, are characteristic of the phylum and commonly occur as alternating stages in the development of individuals of the Classes Hydrozoa and Scyphozoa (see Table 1, classification chart). A third class, Anthozoa, has only the polyp form and includes the true corals. Current molecular studies suggest that Anthozoa is the most primitive of the three classes. Cnidarians are relatively simple; they differ from other multi-celled animals in having only two body layers, in appearing to have a radial symmetry, and in the possession of stinging cells. They may have no skeleton, an organic skeleton, or a mineralized one. Only those with mineralized skeletons, principally the stony corals, are common as fossils. Stony corals belong to the anthozoan Subclass Zoantharia. Their skeletons are external, secreted by the outer tissue layer, and calcareous (composed of CaCO3), but may be formed of either calcite or aragonite.
The oldest known fossil zoantharian corals are from the Lower Cambrian; they are common fossils in rocks of every subsequent geologic period because of their easily preserved skeletons. They were limited to the marine environment and are most common in limestones and calcareous shales, representing relatively clear and moderately shallow water conditions. Coral skeletons are major components of many living and fossil coral reefs such as the Great Barrier Reef that extends for 1200 miles along the northeast coast of Australia. Among the most notable fossil reefs are those in the Triassic of the Alps and the Devonian of western Canada. The latter is an important petroleum source because of the porosity that resulted from reef building.
The known geologic record of corals is outlined in Table 1. There are many orders of Paleozoic corals, but the overall complexity and adaptive success of the Mesozoic-Cenozoic Order Scleractinia far exceeds that of the Paleozoic groups. The principal Paleozoic corals, Tabulata and Rugosa, built their skeletons of calcite, whereas the Mesozoic-Cenozoic Scleractinia built their of aragonite. This compositional difference, plus morphologic similarities and differences, is the basis for dividing the Zoantharia into Groups 1 and 2.
Biostratigraphy - Corals are very sensitive to environment, and their mobility is limited to a floating larval stage that may live for a very brief period of time. For these general reasons, most individual species are not widespread and are useful for correlation in only limited areas or special situations. Most species, however, lived for only a short period of geologic time, and many are used as index fossils within their areas of distribution.
Biogeography and Paleoecology - Environmental sensitivity and lack of mobility, the factors that limit the use of corals for stratigraphic correlations, make them excellent tools for paleogeographic and paleoecologic analyses. Their limited distributions help define marine biogeographic provinces throughout the Phanerozoic, while the ecologic limitations aid environmental interpretations.
Humanistic Studies - The radiating plates seen in many corals, and the honeycomb pattern of coral colonies, are intrinsically appealing and serve many decorative and artistic functions. The esthetic appeal of living coral reefs is evident from the popularity of the numerous coral reef nature parks and preserves that have been established in many parts of the world.