Distribution of Paleozoic Corals in the United States
by Alan Goldstein Interpretive Naturalist Falls of the Ohio State Park
Originally published in the Mid-America Paleontological Society Digest, March 1985, this is an updated version (2011).
Paleozoic corals are widely distributed throughout the world. They can be found in sedimentary rock layers where environmental conditions were favorable. Based on evidence, most corals thrived in warm, tropical marine conditions comparable to modern scleractinian corals (of which they were not related).
Like many forms of life in the ocean, some genera had world-wide (cosmopolitan) distribution, while others lived within a very restrictive environment. Within a small geographic area, some Paleozoic corals served as index fossils (i.e. for correlation of specific strata). However, over large areas they have limited value as compared to other phyla. Corals have been used to recreate landmass distribution and the location of channels or seaways connecting otherwise separated seas during the Paleozoic (see Oliver, 1976).
The Devonian Period was the ‘age of corals’ because the continents were located within +/- 30 degrees of the equator. Large areas of the present continents were submerged under shallow seas (comparable to Philippines / New Guinea area today) – ideal for the development of coral ecosystems. True reefs where corals grew on top of one another (like a barrier reef) were few and far between. Most formed patch reefs – bioherms or biostromes – with ecosystems comparable to the shallow waters of Florida Bay.
In North America, major Devonian coral deposits can be found in Nevada (Lower Devonian), Indiana, Kentucky, Ohio, New York and Ontario (Middle Devonian) and Iowa and Alberta (Upper Devonian). There are other states with Devonian corals (Missouri, Tennessee, etc.), but exposures are much more limited.
Environmental conditions for maximum coral diversity required shallow, clear seas where sunlight could penetrate. An abundance of plankton (a primary food source) was also essential. Peak species diversity is associated with limestone deposits, indicating minimal sources of terrestrial sediments (like shale-forming clays). Species diversity and maximum physical size drops off significantly in Devonian shale deposits.
The richest coral deposit of the Devonian Period in North America is the “coral zone” of the Jeffersonville Limestone found at the Falls of the Ohio and its neighborhood. Here corals are found atop corals, some in growth position, but most are preserved in a random, storm-tossed, orientation. Indiana and Kentucky (which share most species) have over 100 genera. This is more than the second and third on the list (Nevada and Ontario) combined!
Corals are typically identified by slice them and studying their internal structure. Many corals were named in the 19th century by external characteristics. While many corals have been studied and re-named, not every old name is invalid. Sometimes the genus identification is possible, even when the species is not. It takes a number of characteristics to identify any species of any type of organism, because of variability of traits. What holds true today is also true in the fossil record.
Among those amateurs fascinated in paleontology, corals always rank near the bottom of the list. While it is true that they are more difficult than brachiopods and trilobites, they offer a tremendous variation of growth habits, symmetry, and overall beauty. For fossil collectors, this is lack of interest is a mixed blessing. Really nice coral specimens are inexpensive compared to equally well-preserved trilobites or crinoids. However, corals are not widely available for collectors and there is not a lot of diversity in the paleontology market. On occasion, one might find an uncommon coral priced less than a common trilobite!
Corals have more distinct external features than bryozoans and sponges. The process of silicification (replacement of calcite by quartz) may bring our external features in exquisite detail. This increases their beauty because corals have great three-dimensional structure, but the process that exposes the fine structure of the coral exterior hollows out the interior, destroying critical diagnostic microstructure.
Growth bands on Devonian corals indicate a year was roughly 400 days (Wells, 1963). This matches the prediction that the Earth’s orbit is slowly decreasing its distance from the sun. The same gravitation force is also pulling the Moon further and further from the Earth. The Moon - and its tidal influence - was stronger 390 million years ago than today.
The following list of coral genera was pulled from the Treatise on Invertebrate Paleontology (volume F, Coelenterata, Supplement 1, Rugosa and Tabulata, (Hill, 1981). It is not a perfect list, and additional scientific work has added and possibly deleted the names on the list. In addition, typos can creep in and spell check is of no help with Latin names. Additions and corrections by readers are welcome.
Bibliography
Bassler, R.S., 1950. Faunal lists and descriptions of Paleozoic corals, Geological Society of America Memoir 44, 315 p., 20 pl. Hill, D. 1981. Rugosa and Tabulata. In: Teichert, C. editor. Treatise on invertebrate paleontology, Part F, Coelenterata, Supplement 1. Boulder, CO. and Lawrence, KS: Geological society of America and Univeristy of Kansas Press. 762 p., 462 fig. Oliver, W.A., Jr., 1976. Biogeography of Devonian rugose corals, Journal of Paleontology, v. 50, p. 365 – 373. Wells, J. W., 1963. Coral growth and geochronology, Nature, v 197, no. 4871, p. 948-950, 2 fig. List of Genera Alabama - Kentucky Maine - New York Ohio - Wyoming Created February 22, 2011, Updated May 15, 2012.
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