Devonian Corals of the Falls of the Ohio
and Surrounding Areas
by Alan Goldstein
Figure 1: Siphonophrentis elongata (Rafinesque & Clifford)
The fossil beds at the Falls of the Ohio, located between Louisville, Kentucky and Clarksville, Indiana, are world-renowned. First described by European settlers paddling down the Ohio River over 250 years ago, the abundance of corals is still striking today. Are these fossil corals related those living in today's oceans? In a word, "no."
Modern (scleractinian) corals first appear in the fossil record in the Early Triassic period, about 241 million years ago. The corals from the Falls of the Ohio are much older, about 390 million years old! Some varieties superficially resemble living corals, because they lived in a similar habitat - shallow, warm, tropical seas.
Two orders of coral dominated the Devonian seas: rugosa and tabulata. Both became extinct at or near the end of the Permian period, about 240 million years ago. Both orders appeared in the early Ordovician period and their diversity peaked during the Devonian.
The Rugose Corals
Rugose corals get their name because the exterior of many forms have a wrinkly appearance. They are often called "horn corals" because their form may resemble the horn of a cow or goat. In fact, the largest horn coral (Siphonophrentis elongata, figure 1) was referred to as a "petrified buffalo horn" by settlers due to its shape.
Figure 2: Horn Coral Heliophyllum showing septa Figure 3: Horn Coral Cystiphylloides showing dissepimentaria
Horn corals show a wide variation in form, although external form is not a distinguishing feature for identification. Most rugose corals have septae radiating from the center (like bicycle spokes) when observed in cross-section (figure 2). Cystiphylloides is characterized by dissepimentaria, bubble-like structures forming layers (figure 3). Some have a both as internal structures.
Looking closely at the overall shape and internal structure, one can appreciate the enormous variety of the 212 species that have been recorded at the Falls of the Ohio. It is easy to understand why identifying new species is so difficult.
Figure 4: Eridophyllum seriale Edwards & Haime Figure 5: Prismatophyllum ovoideum (Davis)
Further "complicating" factors - many rugose corals are not solitary, but colonial - and some may be either form! Individual corallites in a colonial coral may grow nearly parallel to neighbors, but occasionally contact with them, such as Eridophyllum (figure 4). If the rugose coral is massive and the individuals are in full-contact with one another, it forms beautiful geometric patterns, like Prismatophyllum prisma (figure 5). The largest colonial rugose coral on the Indiana shore (directly below the Interpretive Center) is a Prismatophyllum colony 11 feet (3.3 m) across. A 30 foot (10 m) colony is reported on the Kentucky side!
The Tabulate Corals
Tabulate corals are always colonial. Their name is derived from a flat shelf (tabula) that separates individuals vertically in the colony. The individual corallites that make up the colony are typically 0.5 - 8 mm wide (figure 6). Despite the small individuals, these corals can be quite large. The most common coral is a form called Favosites (Emmonsia). Emmonsia was given its own genus name, but some specialists consider it to be a sub-genus of Favosites. These corals vary from a few inches (or centimeters) to 15 feet (4.5 m) across. The larger colonies probably had hundreds of thousands (or millions) of small corallites living at one time.
Figure 6: Favosites (Emmonsia) emmonsi Figure 7: Favosites (Emmonsia) ramosa
The forms of tabulate corals reflect the environment. As in a modern reef, colonial corals can grow in mounds, bushes, sheets and plates. The first two forms are dominant, typical of high energy conditions, where there is strong wave action and currents. Favosites (Emmonsia) grows in mounds and very thick branching forms (figures 7). The largest [F. (E.) ramosa] form colonies over 50 feet (15 m) across with other corals filling in the space between branches. Alveolites and Thamnopora grow in bushes with branches from 1/4" to one inch thick (figure 8). The latter occurs in colonies as large as 8 feet (2.4 m) across. Platyaxum is an uncommon coral, growing in a plate-like form (figure 9).
Figure 8: Thamnopora limitaris (Rominger) Figure 9: Platyaxum orthesoleniskum (Werner)
Two other corals deserve particular mention because they are unlike other tabulate corals. Aulocystis is a dendroid (branching), often encrusting coral with a tight bush-like form (figure 10). Syringopora grows in mound or bush-like form consisting of curved, straight or gently undulating tubes (figure 11).
Figure 10: Aulocystis(?) procumbens Davis Figure 11: Syingopora hisingeri Billings
A Devonian Coral Garden
There are 212 species of corals known from the fossil beds at the Falls of the Ohio. Not all occur in the "coral beds;" some may be found in the slightly higher (younger) rock layers. Additional studies may reveal more or fewer species. It takes experience to identify corals with accuracy. The random orientation of many of the solitary rugose corals can make identification even more difficult.
Figure 12: Inverted Favosites on fossil beds Figure 13: Coral that fell (twice) and changed
orientation to grow towards the sun (twice)
Look for evidence of Devonian hurricanes. Can you find inverted colonies? Corals should radiate from the center outward, as they grow toward the sun. Many corals radiate toward a center point, indicating they are upside-down (figure 12). Some of these colonies were more than 1000 pounds! Others are on their side. Many colonial corals show rhythmic banding, whether these are daily seasonal or lunar cycles are unknown. Horn corals that are "J" or "L" shaped were also jostled by storms, and bent back towards sunlight (see figure 1, 13).
The coral beds are best exposed late summer through the fall. Visitors have the rare opportunity to walk on an ancient sea floor to get a good idea how corals were distributed if one could go back in time with snorkeling gear.
References & Suggested Reading
Davis, W. J., 1885, Kentucky Fossil Corals, Kentucky Geological Survey.
Greb, Stephen F., et al, 1993, Fossil Beds of the Falls of the Ohio, Kentucky Geological Survey.
Oliver, W. A., Jr., 1976, Devonian Noncystimorph Colonial Rugose Corals in the New York Area,
U. S. Geological Survey, Professional Paper 869.
Stumm, E.C., 1964, Silurian and Devonian corals of the Falls of the Ohio: Geological Society of America Memoir 93.
Species Lists (with photos)
Horn Corals Colonial Rugose Corals Tabulate Corals
Created February 17, 2010, Updated March 24, 2015