The Rugosa ( rugose corals) are an extinct class of solitary or colonial that were abundant in Middle Ordovician to Late Permian seas.
Solitary rugosans (e.g., Caninia, Lophophyllidium, Neozaphrentis, Streptelasma) are often referred to as horn corals because of their unique horn-shaped skeleton with a wrinkled, or , wall. Some solitary rugosans reached nearly a meter (3 ft 3 in) in length. However, some species of rugose corals could form large colonies (e.g., Lithostrotion).
Rugose corals are known from their fossilized skeleton, made of calcite. Like modern corals (Scleractinia), rugose corals were invariably benthic, living on the sea floor or in a reef-framework. Some symbiotic rugose corals were of Stromatoporoidea (a type of extinct reef-building sponge), especially in the Silurian period.
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 (and corresponding corallite) surviving on its own. These horn-shaped solitary species develop from a pointed tip (apex) up towards the polyp-bearing cup (calice). 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
Septum, longitudinal plates directed inward to form a radial (
spoke-shaped) pattern in
Transverse plane. A fine level of magnification will reveal that each septum is actually a series of crystalline tubes (trabeculae) 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 (in the cardinal septum's half of the corallite) and a pair of more narrowly-spaced counter-lateral septa (in the counter septum's half). 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.[Sorauf, J.E., 2007: The function of dissepiments and marginaria in the Rugosa (Cnidaria, Zoantharia). - In: Hubmann, B. & Piller, W. E. (Eds.): Fossil Corals and Sponges. Proceedings of the 9th International Symposium on Fossil Cnidaria and Porifera. - Österr. Akad. Wiss., Schriftenr. Erdwiss. Komm. 17: 11-29, 4 Figs., 2 Pis., Wien.]
Axial structures
Rugose corals often have a columella, a dense rod running up the center (axis) 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 (exterior layer of the corallite) is roughly textured, with concentric growth rings (rugae) and longitudinal ridges (costae) 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 (Part F, 1981)
and Scrutton (1997):
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Order Cystiphyllida Nicholson, 1889 (Middle Ordovician – Middle Devonian)
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Order Stauriida Verrill, 1865 (Middle Ordovician – Late Permian)
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Suborder Arachnophyllina Zhavoronkova, 1972 (Silurian – Middle Devonian)
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Suborder Aulophyllina Hill, 1981 (Late Devonian – Middle Permian)
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Suborder Calostylina Prant, 1957 (Middle Ordovician – Early Devonian)
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Suborder Caniniina Wang, 1950 (Mississippian – Late Permian)
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Suborder Columnariina Rominger, 1876 (Silurian – Late Devonian)
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Suborder Cyathophyllina Nicholson, 1889 (Silurian – Mississippian)
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Suborder Diffingiina Fedorowski, 1985
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Suborder Ketophyllina Zhavoronkova, 1972 (Late Ordovician – Late Devonian)
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Suborder Lithostrotionina Spasskiy & Kachanov, 1971 (Mississippian – Late Permian)
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Suborder Lonsdaleiina Spasskiy, 1974 (Mississippian – Late Permian)
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Suborder Lycophyllina Zhavoronkova, 1972 (Late Ordovician – Middle Devonian)
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Suborder Metriophyllina Spasskiy, 1965 (Middle Ordovician – Late Permian)
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Suborder Plerophyllina Sokolov, 1960 (Late Silurian – Late Permian)
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Suborder Ptenophyllina Wedekind, 1927 (Silurian – Late Devonian)
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Suborder Stauriina Verrill, 1865 (Middle Ordovician – Mississippian)
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Suborder Stereolasmatina Hill, 1981 (Early Devonian – Late Permian)
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Suborder Streptelasmatina Wedekind, 1927 (Middle Ordovician – Late Devonian)
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Suborder Tachylasmatina Fedorowski, 1973 (Early Devonian – Late Permian)
Gallery
File:RugosaOrdovician.jpg|Solitary rugose coral Grewingkia in three views; Ordovician, Indiana
File:Stereolasma cross section.jpg|Cross-section of Stereolasma, a rugose coral from the Devonian of Erie County, New York
File:Cyathophylloides sp. (colonial coral).jpg|The Stauriidae coral Cyathophylloides from the late Ordovician, Georgian Bay Formation, Ontario, Canada
File:Streptelasma divaricans (Nicholson, 1875).jpg| Streptelasma (Nicholson, 1875) from the Liberty Formation (Upper Ordovician) of southern Ohio
External links
