Corals are part of a group of animals called Cnidaria (nid-AIR-ee-a), also called Coelenterata (sel-EN-ter-AH-ta), which includes sea anemones, corals, jellyfish, and hydras. All of these animals are soft bodied and have multiple arms or tentacles, with which they grab food from the surrounding sea water. All Cnidaria (including corals) live in water and most are marine animals. The soft, jelly-like body of an individual cnidarian animal is called a polyp.
Coral polyps secrete a rock-like skeleton of calcium carbonate around them. Calcium carbonate is also the dominant consituent of the rock, limestone. Because modern corals live in large colonies, these skeletons can become quite large, sometimes forming reefs. When the polyp dies, its soft tissue decays, but the hard skeleton is left behind. The hard skeleton of ancient corals is what is preserved as a fossil.
All modern corals belong to the order Scleractinia (SCLER-ac-TIN-ee-a). In the past, fossils indicate that there were two other orders of corals, which are now extinct. The order Rugosa was dominated by solitary corals in which each coral polyp had its own skeleton. Rugose means wrinkled or rough, and the outer surfaces of most rugose coral skeletons has a wrinkled appearance. Because some of the solitary rugose corals formed horn-shaped skeletons, they are called horn corals. Some Rugose corals also formed colonies and coral heads. The order Tabulata consisted entirely of groups of coral animals that lived in large colonies with a shared skeleton. These colonial corals grew in mounds, branching shapes, as chain-like shapes, and even in shapes resembling pipe organs.
Both types of corals are abundant in Ordovician, Silurian, Devonian, and Mississippian rocks in Kentucky, and are rarely found in Pennsylvanian rocks in Kentucky.
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Perhaps the most famous exposures of fossil corals in the world are the Falls of the Ohio, near Louisville, Kentucky. Millions of solitary and colonial coral fossils can be seen in the rocks exposed in this protected area. The fossil beds are part of Falls of the Ohio State Park in Indiana, although most of the fossil beds are actually in Kentucky, as the Kentucky border extends to the north shore of the Ohio River. Collecting is not allowed at the park, which is also a World Heritage Site and Federally Protected Waterway. The illustration to the left is the cover of a popular publication concerning the fossil beds at the Falls of the Ohio, which can be ordered through the Kentucky Geological Survey. Many of the examples used in the types of corals section on this web site are also illustrated in this publication.
References for corals:
- Conkin, J.E., and Conkin, B.M.,1980, Handbook to strata and fossils at the Falls of the Ohio: University of Louisville Reproducation Services, 27 p.
- Davis, W.J., 1885, Kentucky fossil corals , a monograph of the fossil corals of the Silurian and Devonian rocks of Kentucky, part 2: Kentucky Geological Survey, ser. 2, Monograph 2, 139 plates.
- Greb, S.F., 1989, Guide to "Progression of Life," with notes on the history of life in Kentucky: Kentucky Geological Survey, ser. 11, Special Publication 13, 44 p.
- Greb, S.F., Hendricks, R.T., Chesnut, D.R., Jr., 1993, Fossil beds of the Falls of the Ohio: Kentucky Geological Survey, Special Publication 19, 39 p.
- Perkins, R.D., 1963, Petrology of the Jeffersonville Limestone (Middle Devonian) of southeastern Indiana: Geological Society of America Bulletin, v. 74, p. 157-174.
- Powell, R.L., 1970, Geology of the Falls of the Ohio River: Indiana Geological Survey Circular 10, 45 p.
- Stumm, E.C., 1964, Silurian and Devonian corals of the Falls of the Ohio: Geological Society of America Memoirs, v. 93, 184 p.