Rugosa - WikiHQ

The Rugosa (rugose corals) are an extinct class of solitary or colonial corals 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 rugose, 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 endobionts of Stromatoporoidea (a type of extinct reef-building sponge), 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 (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.

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.

Colony forms

Colonial rugose corals can show many different growth forms in their corallum (entire colony structure):

Fasciculate corals have their corallites joined at the base, but the edge of each corallite is not connected, leaving a gap of open water between each.
Dendroid corals have a branching bush-like system of corallites.
Phaceloid corals have corallites running in parallel from a common base. Some phaceloid corals have outgrowths bridging between the gaps.
Massive corals have no gaps between their corallites.
Cerioid corals retain walls between adjacent corallites. Petoskey stones (Hexagonaria) are an example of cerioid corals.
Astreoid corals have no walls between adjacent corallites, but each corallite retains its own set of septa which contact the septa of adjacent corallites.
Thamnasteroid corals have no walls, and septa are shared equally between adjacent corallites.
Aphroid corals have no walls, and septa are completely isolated from each other within a colony-wide mesh of dissepiments.

Taxonomy

Taxonomy to the suborder level, mostly based on Treatise on Invertebrate Paleontology (Part F, 1981) (Early Permian – Middle Permian)

Suborder Ketophyllina Zhavoronkova, 1972 (Late Ordovician – Late Devonian)
Suborder Lithostrotionina Spasskiy & Kachanov, 1971 (Mississippian – Late Permian)
Suborder Lonsdaleiina Spasskiy, 1974 (Mississippian – Late Permian)
Suborder Lycophyllina Zhavoronkova, 1972 (Late Ordovician – Middle Devonian)
Suborder Metriophyllina Spasskiy, 1965 (Middle Ordovician – Late Permian)
Suborder Plerophyllina Sokolov, 1960 (Late Silurian – Late Permian)
Suborder Ptenophyllina Wedekind, 1927 (Silurian – Late Devonian)
Suborder Stauriina Verrill, 1865 (Middle Ordovician – Mississippian)
Suborder Stereolasmatina Hill, 1981 (Early Devonian – Late Permian)
Suborder Streptelasmatina Wedekind, 1927 (Middle Ordovician – Late Devonian)
Suborder Tachylasmatina Fedorowski, 1973 (Early Devonian – Late Permian)

Evolutionary history

Ordovician

Rugosans likely originated from a non-skeletal anthozoan similar to modern sea anemones. The oldest rugosan fossils appear near the end of the Middle Ordovician, during the transition from the Darriwilian stage to the Sandbian stage at the start of the Late Ordovician. Lambelasma, a calostyline from Darriwilian-age Iran, is the earliest confirmed example. By the end of the Sandbian stage, rugose corals were common in the shallow seas of North America and Baltoscandia. Five rugosan suborders filled out the reef-building faunas of the Late Ordovician: Cystiphyllina, Calostylina, Stauriina, Streptelasmatina, and Metriophyllina.