Rugosa


The Rugosa are an extinct class of solitary or colonial corals that were abundant in Middle Ordovician to Late Permian seas.
Solitary rugosans are often referred to as horn corals because of their unique horn-shaped skeleton with a wrinkled, or rugose, wall. Some solitary rugosans reached nearly a meter in length. However, some species of rugose corals could form large colonies.
Rugose corals are known from their fossilized skeleton, made of calcite. Like modern corals, rugose corals were invariably benthic, living on the sea floor or in a reef-framework. Some symbiotic rugose corals were endobionts of Stromatoporoidea, especially in the Silurian period. As with other cnidarians, it is presumed that these Palaeozoic corals possessed tentacles with stinging cells to capture prey. Technically they were carnivores, but prey-size was so small they are often referred to as microcarnivores.
When radiating septa were present, they were usually in multiples of four, so rugose corals were historically known as Tetracorallia in contrast to modern Hexacorallia, where the colonial polyps generally have sixfold symmetry.

Morphology

Each polyp of a rugose coral is hosted on a corallite, the fundamental skeletal structure of the coral. Unlike most living corals, many rugosan species live a solitary life, with a relatively large polyp surviving on its own. These horn-shaped solitary species develop from a pointed tip up towards the polyp-bearing cup. Other rugosan species have polyps which grow together as a colony, with the entire colony's skeleton known as a corallum. The Petoskey stone, the state stone of Michigan, is actually a corallum of the colonial rugose coral ''Hexagonaria.''

Septa

Rugosans are most easily identifiable by their prominent septa, longitudinal plates directed inward to form a radial pattern in transverse section. A fine level of magnification will reveal that each septum is actually a series of crystalline tubes with bristle-like calcite fibers.
Rugose corals have a unique arrangement of septa which preserves bilateral symmetry, unlike the fully radial symmetry of tabulate and scleractinian corals. The first septum to develop is the cardinal septum, marked by a furrow known as the fossula. Next, the counter septum develops on the opposite end of the rim. The straight symmetrical axis between the cardinal and counter septum is bilateral in basic form even if the rest of the coral shows radial symmetry.
Following those first two septa are a pair of widely-spaced alar septa and a pair of more narrowly-spaced counter-lateral septa. Finally, many more major and minor septa arise in the vicinity of the alar and counter-lateral septa, filling in the rest of the corallite.

Tabulae and dissepiments

Rugose corals also show tabulae, horizontal plates that slice through the corallite skeleton in a top-to-bottom series. Tabulae tend to be concentrated near the center of the corallite in a region known as the tabularium. Rugose corals almost always have dissepiments: curved, overlapping plates interlinking between the various septa and tabulae. Dissepiments are concentrated near the wall of the corallite in a mesh-like region known as the marginarium or dissepimentarium.
Many shallow-water solitary rugosans have a dense marginarium which may help protect the coral against waves. In some rugosans, the dissepiments and the bases of the septa are so densely packed that the outer region is a solid layer known as the stereozone. Conversely, many deep-water colonial rugosans have reduced septa and no distinct boundary between individual corallites, relying on a broad, low-density marginarium to spread out the coral's weight akin to a snowshoe.

Axial structures

Rugose corals often have a columella, a dense rod running up the center of the corallite. It is common in rugose corals because they were mainly solitary and so required the extra support. Tabulate corals have no columella because they were always colonial and relied on the support of neighboring corallites. Alternatively, the center of the corallite may be supported by the aulos, a tube filled with tabulae.

Epitheca

The epitheca is roughly textured, with concentric growth rings and longitudinal ridges and furrows. The costae tend to line up precisely with the internal septa. There are multiple orders of growth rings, with the thinnest developing daily and the thickest developing yearly. By counting the ratio of daily to yearly rings in rugosan fossils, paleontologist John W. Wells determined that there were over 400 days per year in the Middle Devonian, with each day shorter than a modern 24-hour day.

Taxonomy

Taxonomy to the suborder level, mostly based on Treatise on Invertebrate Paleontology and Scrutton :
  • Order Cystiphyllida Nicholson, 1889
  • Order Stauriida Verrill, 1865
  • * Suborder Arachnophyllina Zhavoronkova, 1972
  • * Suborder Aulophyllina Hill, 1981
  • * Suborder Calostylina Prant, 1957
  • * Suborder Caniniina Wang, 1950
  • * Suborder Columnariina Rominger, 1876
  • * Suborder Cyathophyllina Nicholson, 1889
  • * Suborder Diffingiina Fedorowski, 1985
  • * Suborder Ketophyllina Zhavoronkova, 1972
  • * Suborder Lithostrotionina Spasskiy & Kachanov, 1971
  • * Suborder Lonsdaleiina Spasskiy, 1974
  • * Suborder Lycophyllina Zhavoronkova, 1972
  • * Suborder Metriophyllina Spasskiy, 1965
  • * Suborder Plerophyllina Sokolov, 1960
  • * Suborder Ptenophyllina Wedekind, 1927
  • * Suborder Stauriina Verrill, 1865
  • * Suborder Stereolasmatina Hill, 1981
  • * Suborder Streptelasmatina Wedekind, 1927
  • * Suborder Tachylasmatina Fedorowski, 1973