Horn coral is the common name given to the fossilized remains of an extinct group of marine invertebrates belonging to the order Rugosa. The name “horn coral” comes from the distinct, tapering, conical shape of the solitary forms, which often resemble a bull’s horn. These fossils represent the hard, calcium carbonate skeletons secreted by the individual coral animal, or polyp, that lived inside.
Physical Structure and Anatomy
The skeletal structure of the horn coral, known as the corallum, is characterized by its exterior wall, or epitheca. This wall often has a wrinkled appearance, giving the order its scientific name, Rugosa. The polyp lived in the cup-shaped depression at the wide end of the cone, called the calice.
The interior of the skeleton holds defining anatomical features, visible in cross-section. Vertical, blade-like partitions called septa radiate inward from the outer wall, resembling the spokes of a bicycle wheel. These septa provided internal support for the soft polyp tissues.
The order Rugosa is uniquely identified by the way these septa are inserted, resulting in a four-fold or quadripartite symmetry. The internal cavity is also segmented by horizontal partitions called tabulae, which were secreted as the polyp moved upward, creating a series of flat plates beneath it. These internal structures, the septa and tabulae, distinguish the different species within the Rugosa order.
Geological Time Frame and Extinction
Horn corals first appeared in the fossil record during the Middle Ordovician Period, approximately 470 million years ago. They were prominent organisms throughout the Paleozoic Era, thriving in the warm, shallow seas that covered much of the Earth. During the Devonian Period, in particular, Rugose corals reached their peak diversity and were significant contributors to ancient reef systems.
The existence of the entire Rugosa order came to an abrupt end during the Permian-Triassic extinction event, often referred to as “The Great Dying.” This catastrophic mass extinction occurred about 252 million years ago and wiped out over 90% of all marine species. Environmental changes resulted in the complete disappearance of the Rugose corals from the planet.
The extinction of the horn corals and other Paleozoic coral groups marked a major shift in marine ecosystems. The subsequent Mesozoic Era saw the rise of a completely new group of corals that would eventually become the modern reef-builders. Because of their relatively short, defined existence in geological time, horn coral fossils are sometimes used by paleontologists as index fossils to help date the rock layers in which they are found.
How Horn Coral Differs From Modern Corals
The primary distinction between the extinct horn coral (Rugosa) and modern stony corals (Scleractinia) lies in their fundamental skeletal symmetry. Horn corals are characterized by four-fold symmetry. Conversely, modern corals possess a six-fold, or hexameral, symmetry in their skeletal arrangement.
Modern corals secrete their skeletons using the mineral aragonite, while the horn corals formed their structures from calcite. This difference in mineral composition is a key chemical distinction between the two groups.
While modern reef-building corals are predominantly colonial, the classic horn coral was often a solitary organism, with a single large polyp living in the cone. The solitary horn-shaped individuals are the most recognizable fossils.