Rugosa

Septa

Rugosans are most easily identifiable by their prominent septa (singular: septum), plates which run up the longitudinal axis of the corallite and are directed inward to form a radial (spoke-shaped) pattern in transverse cross-section. A fine level of magnification will reveal that each septum is actually a series of crystalline tubes (trabeculae) with bristle-like calcite fibers.^[1]

Rugose corals have a unique arrangement of septa which preserves bilateral symmetry, unlike the fully radial symmetry of tabulate and modern 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.^[1]

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.^[1]

Tabulae and dissepiments

Rugose corals also show tabulae (singular: tabula), 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.^[1]

Many shallow-water solitary rugosans have a dense marginarium which may help protect the coral against waves. In some rugosans, the dissepiments and/or the bases of the septa are so densely packed that the outer region of the corallite is a solid layer known as the stereozone. On the other hand, colonial rugosans living in turbid waters tend to 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.^[6]

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.^[1][7] Alternatively, the center of the corallite may be supported by the aulos, a tube filled with tabulae.^[1]

Epitheca

The epitheca (exterior layer of the corallite) is roughly textured, with concentric growth rings (rugae) as well as 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.^[1][8][9]

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.

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.^[11] 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.^[1][3] Cystiphyllines, streptelasmatines, and metriophyllines were usually solitary, stauriines were colonial, and calostylines included many species of both solitary and colonial corals.

Some early rugosans were very simple, such as Primitophyllum (a cystiphylline) and Lambeophyllum (a calostyline). These simple forms resembled tiny cones with rudimentary septa and no tabulae, and they may be a good estimate for an ancestral rugosan. Others were more typical, such as the solitary Streptelasma (a streptelasmatine) and colonial Favistina (a stauriine).^[3][11]

Silurian

The Ordovician suborders all persisted into the Silurian, though newer groups achieved even greater abundance. The Arachnophyllina, Ketophyllina, and Lycophyllina diversify quickly, while others (the Columnariina, Cyathophyllina, and Ptenophyllina) slowly take hold. Calostylines diminish and go extinct near the Silurian-Devonian boundary.^[1][3] Lycophyllines were usually solitary, while arachnophyllines, columnariines, and ptenophyllines were usually colonial. Cyathophyllines and ketophyllines included many solitary and colonial species.

Devonian

The Early Devonian was the apex of diversity for rugosans and many other marine animals. The Middle to Late Devonian, however, saw a series of extinction events which profoundly reshaped rugosan faunas. Arachnophyllines, cystiphyllines, lycophyllines, and streptelasmatines all die out near the start of the Late Devonian. Others (ketophyllines, columnariines, and ptenophyllines) last longer, though they too would vanish by the end of the period. However, the Devonian was also the origin of a few more rugosan suborders: Plerophyllina and Stereolasmatina, both of which were small and solitary.^[1][3]

Carboniferous-Permian

Rugosans recovered from extinction quickly in the Mississippian (early Carboniferous). Plerophyllines and stereolasmatines lead the charge, alongside a substantial diversification of four new suborders: Aulophyllina, Caniniina, Lithostrotionina, and Lonsdaleiina. Aulophyllines and caniniines were usually large and solitary, while lithostrotionines and lonsdaleiines were usually colonial.

An extinction at the end of the Mississippian eliminated the last stauriines and cyathophyllines, while the other suborders survived at a slightly reduced capacity through the Pennsylvanian (late Carboniferous). Rugosans experienced a few small extinctions within the Permian period, but on the whole they were still abundant reef builders until the entire class died out during the Permian-Triassic mass extinction.^[1][3]