Rugose corals belong to the phylum Cnidaria, a phylum of organisms that makes microscopic intracellular stinging capsules known as nematocysts or cnidae. Phylum Cnidaria includes other corals, jellyfish and sea anemones. Rugosans, an extinct group of corals, were suspension feeders that used their tentacles to capture small particles from the water column. A single coral polyp bears close resemblance to a sea anemone within a hard skeleton.
The vertical parts of a rugose coral skeleton are the septa, columella, and epitheca, all precipitated by the coral from calcium carbonate in the sea water. In cross section, these parts may resemble a bike tire. The septa make up the spokes of the wheel, while the columella sits at the center of the tire. The epitheca is the outermost wall of the coral, or the rubber of a tire. Rugose corals have a fossula, which is a gap in their septa created by the order in which septa are added during the coral’s growth. As septal insertion patterns vary among other subclasses of coral, the fossula is an important identifying feature for rugose corals. Longitudinally, rugose coral contain septa, dissepiments, and tabulae. The septa are in a vertical formation like table legs, while tabulae and dissepiments are horizontal, like a tabletop. Tabulae lay across the width of the skeleton's center, while dissepiments lay across smaller portions.
Rugose corals can be solitary or colonial. Solitary rugose corals consist of one coral polyp living within a hard, calcium carbonate or calcite skeleton. Solitary rugose corals are commonly called ‘horn corals’ because of their conical horn or tooth-like appearance, though they are highly variable morphology and size.
Colonial rugose corals consist of many coral polyps, each living inside a portion of the skeleton called a corallite. A single coral polyp is surrounded by similar polyps growing together to form a colonial structure. Rugose coral colonies may take a few different forms. For example, fasciculate colonies look like bundles of sticks, whereas massive forms may resemble honeycombs.
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no audioThe external skeletons of rugose corals are made of the mineral calcite, allowing them to be preserved over geologic time. The high abundance of rugose corals in the Paleozoic Era also increases the chances of preservation. However, sediment often fills the corals’ inner cavities, resulting in little to no preservation of the internal structures such as the septa and tabulae. In cases of exceptional preservation, the internal features of a coral skeleton may be preserved while the outer epitheca is stripped away.
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no audioAfter their first appearance in the Middle Ordovician period 470 million years ago, rugose generic diversity peaked in the Devonian when global climate created widespread carbonate shelf environments. Rugose diversity then decreased during the Devonian mass extinction 359 million years ago, but the cooler climates of these time periods allowed other shallow marine species to dominate the shelf environments that were once the domain of the rugosans. Rugosans died off during the End-Permian mass extinction 250 million years ago due to warming climate conditions and changing ocean chemistry.
Figure 7: A graph highlighting rugose coral diversity of genera for the Paleozoic Era. Cm= Cambrian, O= Ordovician, S= Silurian, D= Devonian, C= Carboniferous, P= Permian, Tr= Triassic, J= Jurassic, K= Cretaceous. Notice the data line stops at the end of the Permian – this indicates the corals’ extinction at the end of the Paleozoic Era. Dinosaurs did not appear within the fossil record until the Late Triassic, meaning that all rugose corals are older than all dinosaurs! Rugose coral occurrences were downloaded from the Paleobiology Database on November 11, 2024, with the following parameters: time intervals = Paleozoic, environment = marine, taxon = Rugosa.
In Indiana, rugose corals are preserved in Ordovician to Pennsylvanian rocks. Specimens that are representative of these time periods are housed in our IU Paleontology Collections, but most of our rugose corals in the collection were extracted from rocks of the Mississippian Period, between 358 and 323 million years ago. Major collectors of Mississippian rugose corals in the IU Paleontology Collection include Jesse James Galloway, Thomas G. Perry. Alan Horowitz, Don L. Kissling, Arthur C. Brookley, Joaquin E. Rodriguez, Don Hattin, Clyde A. Malott, and S. Vincent.
Figure 8: A map of bedrock ages and rugose coral locations in Indiana. Base map from Indiana Geological and Water Survey. Rugose coral occurrences were downloaded from the Paleobiology Database on November 8, 2024, with the following parameters: time intervals = Paleozoic, continent = North America, environment = marine, taxon = Rugosa.
Undergraduate Co-authors: Melissa Humbarger, Matthew Sullivan, and Abigail Smith
Graduate Author: Samantha Hartzell
CBRC Research Assistant: CJ Salcido
CBRC Director: Claudia Johnson
Hendricks H., Stigall, A., and Lieberman, B. (2015). The Digital Atlas of Ancient Life: Delivering information on paleontology and biogeography via the web. Palaeontologia Electronica. DOI
Moore, R.C., Robinson, R.A., Curt, T., Ashlock, V., Keim, Jack D., McCormick, L., and Williams, R.B. (1981). Treatise on Invertebrate Paleontology Supplement 1.1: Rugosa and Tabulata (Vol. 1). Geological Society of America; University of Kansas. 1-94.
Moore, R.C., and Teichert, C. (Eds.). (1953). Treatise on invertebrate paleontology. Part F. Geological Society of America; University of Kansas Paleontological Institute. 233-256.
