The Rugosa or "rugose corals" (referring to their wrinkled appearance), also known as "horn corals" were an important group of Paleozoic organisms. Both solitary and colonial forms are known, but the former are more common. Solitary rugosans usually have a horn shaped (hence the alternative term, "horn corals"), while the colonial types commonly have hexagonal corallites. The skeleton is made of calcite and is generally quite massive. Solitary rugose corals range in size from a few millimeters in diameter and in length to 14 centimeters in diameter and a height of close to one meter. Some colonies may be 4 meters in diameter. Like modern corals (e.g. sea anemones, which can be observed in intertidal rock pools), the coral animal (or polyp) fed by using tentacles to capture and sweep organisms into their mouths
As a very general rule, rugose coral have stronger radial septa than they do transverse platforms. The septa radiate out from the center. Rugose corals have both major and minor septa. Rugose corals differ from other corals by the pattern by which they add septa through their ontogeny (development and growth). Both solitary and colonial rugosans have this distinctive septal insertion pattern, which gives most rugose corals bilateral symmetry. The six prosepta are added first, including the cardinal septum and counter septum, which are at 180 degrees from one another. . After this the major septa (metasepta) are inserted serially in four positions; minor septa short and inserted between major septa, probably serially also. It is this four-fold developmental pattern that gives rugose corals their alternative name of tetracorals (tetra meaning four).
New corallites may bud asexually, although they also reproduced sexually. The buds have four septa. As the corallite grows, the septa begin to spread further apart, and new septa are added, generally, four septa at a time are added to maintain a rigid structure.
The growth lines on the coral span its length from the calice (top) to the base. Rugose corals generally added a new layer of growth each day (a new wrinkle), and the days in the Paleozoic year have been determined through counting growth rings on rugose corals. It is now known for example that the Devonian period had a year of 400 days (in the past the Earth rotated more quickly around it's axis; this rotation is being gradually but continually slowed by the tidal "brake" exerted by the moon's gravity).
It is not known whether rugose corals had symbiotic photosymbiotic zooxanthellae (algae) as modern corals do. Some have suggested not, but personally I see no reason why they would not have.
They were rarely reef-builders as modern corals are. The reason being they were not able to attach themselves to the bottom the way modern scleractinian corals can.
Classification of the Rugose corals is provisional and will no doubt be revised as more detailed investigation of the microstructure of the different groups is made. The present arrangement (from the Treatise of Invertebrate Paleontology) has three suborders:
suborder Streptelasmatina (Ordovician to Late Permian) - includes both solitary and colonial types. The tabulae are domed (convex upward). The periphery of the corallum has either a stereozone or a dissepimentarium. Examples include Streptelasma, Heliophyllum.
suborder Columnariina (Ordovician—Permian). Usually colonial, rarely solitary forms. Examples: include Spongophyllum and Lonsdaleia. The septa are thin, the tabulae flat, depressed or downwardly convex
suborder Cystiphyllina (Ordovician—Devonian). Solitary or colonial forms. The septa are large with complex microstructure. A wide dissepimentarium or a stereozone is present. Examples include Cystiphyllum, Calceola, and Goniophyllum.
An alternative classification has instead two orders, and elevates the Rugosa from ordinal to subclass ranking:
Order Stauriida (Mid Ordovician - Late Permian)
Order Cystiphyllida (Mid Ordovician - Mid Devonian)
The general trend among rugose corals was to evolve a strong skeleton. Several different lineages show convergent trends toward similar morphologies. For example, several lineages developed carinae and columella to strengthen the septa and the central axis of the coral.
Lambeophyllum - Sandbian (Ordovician)
|The simplest and earliest grades of corallite organization appeared during the Middle Ordovician (left). These had only simple walls, septa and tabulae. The polyp lived on top of a tabula in a depression in the top of the coral called the calyx. A little later some types developed a layer (the marginal stereozone) of thickened calcite around the periphery of the corallite. This would have doubtless served a strengthening function.|
For this whole period, and until the early Silurian, rugose corals remained small and solitary. The suborders Streptelasmatina and Columnariina were dominant.
The period from the early Silurian onwards saw the emergence of colonial forms, and an adaptive radiation and exploitation of the reef habitat by both solitary and colonial types. During this time small horizontal internal blister-like plates called dissepiments appeared in several lineages. These have a strengthening function and also make incremental growth of the corallite possible, as there is no need to lift the whole polyp and create entire new tabula at once. Nevertheless the rugose corals remained subsidiary components of reefs relative to tabulate corals and stromatoporoids. The suborder Columnariina dominated.
Few Silurian forms survived into the Devonian. A new adaptive radiation of solitary and colonial rugosans occurred in the Middle Devonian. These were all relatively large forms with wide dissepimentaria.
|above: A Middle Devonian (Eifelian) reef featuring the crinoid Dolatocrinus, a tabulate coral (Favosites) and a trilobite (lower right corner), and a number of species of Rugose corals. The giant one in the center is probably Siphonophrentis gigantea. The squat white ones at the middle right would be Heterophrentis prolifica. The cluster of yellow ones in the middle foreground and right middle background are a colonial corals of the genus Eridophyllum.|
Coral faunas were seriously affected by the late Devonian mass-extinction events. Then during the early Carboniferous, a new adaptive radiation occurred in both solitary and colonial forms. At this time a number of types developed a column through the middle of the corallite (through a thickening of the end of the counter septum) called the columella. It is assumed this would also aid in strengthening and growth of the structure. In addition the microstructure of the skeletons became quite complex. The rugose corals of this period were the most advanced types that evolved. The organizational grades would seem to represent a tendency towards greater efficiency, like the three grades of sponge organization. Yet paradoxically the more primitive types continue to co-exist alongside the more advanced forms.
The final radiation of rugose corals occurred during the Permian period. Varieties with prominent prosepta appear. These flourished until the end of the period, when the entire group was exterminated by the huge Permo-Triassic extinction event.
References & Links
Rugose Corals (in notational form, with a few photos) and Rugose Corals (notational form)
Introduction to the Rugosa: only a brief mention