Rugose corals - mound shapes
Although technically all rugose corals were solitary animals, some grew in groups, such that their skeletons were touching. These groups of rugose corals formed mound-shaped fossils that can be difficult to differentiate from colonial or tabulate corals. In rugose mounds, each tube or corallite skeleton has its own skeletal wall, while corallums in tabulate colonies shared walls. Also, septae in rugose corals are longer and generally more complex then those in tabulate corals. In Kentucky, these types of corals are common in Silurian, Devonian, and Mississipian strata.
Entelophyllum are a type of rugose coral that occurs in groups and form mound shapes in Silurian limestones. You can see that each corallum (tube) is separate from it's neighbor. This is especially obvious when looked at from the top (see detail). Individual calices are relatively small, each generally less than 1 cm across. This specimen is 28 cm in diameter. It was collected by R. Todd Hendricks and donated to the Kentucky Geological Survey.
Eridophyllum is another good example of a rugose (horn) coral that lived in groups or mounds. These fossils are common in the Jeffersonville Limestone (Devonian) The example are from the Falls of the Ohio. The example on the left shows what the mounds look like in the bedrock. The detail on the right, is from another example at the Falls and hows the individual corallums of the group, which look like fingers in the rock matrix. Photographs by R. Todd Hendricks.
Prismatophyllum is another type of colonial rugose coral, which commonly occurs as mound-shaped masses in the Jeffersonville Limestone (Devonian). It differs from other rugose colonial mounds like Entelophyllum and Eridophyllum by having each corallum cemented together in a solid mound, rather than closely-spaced corallums growing in a mound shape. In fact, this is the type of rugose coral that is easy to confuse with a colonial tabulate coral, because the individual corallum tubes are not easily distinguishable in most specimens. Prismatophyllum is classified as a rugose coral because the walls of individual chambers are not shared. Rather each calyx (hole) is surrounded by it's own corallum chamber wall, which is cemented to corallum chamber wall of its neighbor. This is why there is a relatively wide distance between the calices. If the chambers were shared, the calices would be closer together. Another way to distinguish this coral from a tabulate is that the calices are large, as much as 2 cm across; tabulate calices are usually smaller. Prismatophyllum calices also have well-developed septa (grooves), which tabulate corals generally lack. Also, the calices are slightly sunken, such that the outer corallum walls tend to form octagonal to pentagonal shapes around the calyx. This specimen is 30 cm in diameter. It was collected by R. Todd Hendricks and donated to the Kentucky Geological Survey.
Acrocyathus is a type of rugose coral mound. It was previously known as Lithostrotion. In this fossil you can see that the corallums (tubular chambers) between each calyx are very thick. That's because each corallum is cemented to the next, rather than having shared walls as occurs in tabulate colonial corals. Acrocyathus (Lithostrotian) corals are common in the Mississippian-age St. Louis Limestone. Often the fossils are silicified, which means the original limestone was replaced with silica (quartz). Because quartz is more resistant to weathering than the surrounding limestone rock, fossils of these corals commonly weather out of natural outcroppings, or are eroded in stream banks. Some may even be replaced with agate, which is a variety of quartz.
Heliophyllum is a type of rugose colonial, mound-shaped coral found in Devonian strata. The living coral polyps would have lived in each of the holes (called a calyx). Specimen donated to the Kentucky Geological Survey collection by R. Todd Hendricks. Identification by Alan Goldstein.