Rugose coral found a namesake in the Latin word for wrinkled (rug- or rugo-), due to their characteristically ridged outer surface. However, the distinct shape of their coral structure gave rise to a more commonly used name of “horn coral.” Older paleontology articles, especially from the late 1800s, often cite these organisms as tetracoralla in reference to the corals’ tendency to insert septa in groups of four.
Rugosa first appeared in the mid-Ordovician (about 465 Ma) and disappeared at the end of the Permian (about 250 Ma). Because of the occurrence of the mass Permian-Triassic extinction, there are no known direct descendants of rugose corals. However, it is thought that rugose polyps had arms full of stinging cells that helped to catch prey, which is a common characteristic found in modern corals. Rugosa’s relatively short existence, combined with their distinct characteristics and worldwide geographic spread makes them excellent index fossils.
All horn corals live inside of a “cup” called the calyx, a basin-shaped depression within a hard outer sleeve (or theca). The polyp builds these encasings by secreting calcite to add a new layer of growth each day that it is alive. A distinct characteristic of Rugosa is found inside the calyx, where radially-aligned septa extend inward from the outer wall. These features serve as skeletal support plates for the polyp.
The theca is built in (what humans would call) an inverse fashion, with the point of the horn at the bottom and the wide, open end at the top. Inside the base of the theca, transverse partitions form shelf-like structures called tabulae. They form one by one as the polyp pulls itself upward, until it rests on top of the skeleton with its arms pointing toward the open ocean. This growth mechanism means they can get to be a wide range of sizes, from a few millimeters long to over one full meter.
The arrangement of a rugose coral’s septa is very important for classifying its type. While solitary horns will nearly always have septa in groups of four, the colonial corals usually develop in multiples of 6 – all of which grow in varying patterns depending on what stage of life the coral is at.
Rugosa were usually solitary creatures, though some could be found in colonial masses with a more hexagonal coral structure. They attached themselves to the seafloor with rudder-like talons while the polyp (sitting atop the horn) used tentacles that swept in the ocean currents to catch organisms for feeding. Although some horn corals reproduced sexually, it was more common for budding to occur, a type of asexual reproduction. The buds consist of four septa that spread apart as they grew, until new septa were added to maintain the coral’s rigid structure. The parent polyp then split off new polyps to create separate corallites.
Colonial masses would grow as individuals banded together. However, even then rugose corals were rarely reef-builders like the corals known today. Standing up on end meant that it was relatively easy for ocean currents to knock the animal over, and they would have to build a new calyx within the old one to “upright” itself. This resulted in fossils with an awkwardly twisted, cup-in-cup shape. To accommodate for this inconvenience, it’s likely that solitary corals colonized soft bottom environments by sinking their points into the soft sediment.
Rugosa fossils have been found in all four types of reef areas (fore-reef, reef, back-reef, and bioherms), usually in carbonate rocks. Studies indicate that they lived in tropical shallow seas, generally very close to the continental shelf, and preferred clear, high-energy waters that were well oxygenated.
- Carlson, R. and Poor, J. (2005). Historical Geology: Interpretations and Applications. Upper Saddle River: Pearson Education, Inc.
- Clarkson, E. N. K. (1998). Invertebrate Paleontology and Evolution. Malden: Blackwell Science Ltd.
- Doyle, P. (1996). Understanding Fossils: An Introduction to Invertebrate Paleontology. West Sussex: John Wiley and Sons Ltd.
- Haeckel, E. (1998). Art Forms in Nature. New York: Prestel.
- Kentucky Geological Survey. (2006). Rugose Corals. Retrieved May 3, 2011 from http://www.uky.edu/KGS/fossils/rugosecorals.htm
- Levin, H. (1999). Ancient Invertebrates and their Living Relatives. Upper Saddle River: Prentice-Hall, Inc.
- White, T. et. al. (2010). Rugosa. Retrieved May 7, 2011 from http://www.palaeos.org/Rugos