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The Rugosa or "rugose corals" (referring to their wrinkled appearance), also known as "horn corals" were an important group of organisms. Both solitary and colonial forms are known, but the former are more common. Solitary rugosans usually have a horn shape (hence the alternative term, "horn corals"), while the colonial types commonly have hexagonal . The skeleton is made of 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 (e.g. , which can be observed in intertidal rock pools), the coral animal (or ) fed by using tentacles to capture and sweep organisms into their mouths
As a very general rule, rugose coral have stronger radial 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 are added first, including the and , which are at 180 degrees from one another. After this the major septa () 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 (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 (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 corals can.
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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 and to strengthen the septa and the central axis of the coral.
The simplest and earliest grades of corallite organization appeared during the Middle Ordovician. These had only simple walls, septa and . The polyp lived on top of a tabula in a depression in the top of the coral called the . A little later some types developed a layer (the ) 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 , rugose corals remained small and solitary. The suborders and 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 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 an entire new tabula at once. Nevertheless the rugose corals remained subsidiary components of reefs relative to and . The suborder Columnariina dominated.
Few Silurian forms survived into the . A new adaptive radiation of solitary and colonial rugosans occurred in the Middle Devonian. These were all relatively large forms with wide dissepimentaria.
Coral faunas were seriously affected by the late Devonian mass-extinction events. Then during the early , 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 period. Varieties with prominent prosepta appear. These flourished until the end of the period, when the entire group was exterminated by the huge event.
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Gudo (2002) suggested a division of the large coral group Rugosa into two superorders, and , based on differences in . Unfortunately, no detailed listing of which taxa would fall into which superorder was given, and this classification might be difficult to apply in practice. We use Gudo's division for now, but we may be forced to drop it as this section becomes more complete.
The relationships of the Palaeozoic Rugosa to the post-Palaeozoic are also highly contentious. Some authors believe the two to be unrelated, with Rugosa becoming extinct at the end of the and the Scleractinia evolving indepently from non-sclerotised . Others suggest that the Scleractinia evolved from rugosan ancestors that survived the end of the Palaeozoic. A number of authors (especially in the latter group) also suggest the Rugosa itself to be polyphyletic.
Rugosa [Tetracorallia]
| i. s.: Enterolasma
| Spineria Schouppé & Stacul 1959
|
| Heritschioides
| Craterophyllum
| |--C. eifeliense Fedorowski 1967
| `--C. verticillatum
| Timania rainbowensis
| Neokoninckophyllum kansasense
| Heliophyllum halli
| Tabulophyllum
| |--T. rotundum Spassky 1960
| `--T. schlueteri
| Clisaxophyllum ava
| |--C. a. ava
| `--C. a. atetsuense Minato & Nakazawa 1957
| Bothrophyllum dobroljubovae
| Spirophyllum geminum Fedorowski 1970
| Entelophyllum articulatum
| Kodonophyllum truncatum
|
| Iovaphyllum
| Brachyelasma irregulare He in Kong & Huang 1978
| Tongxinophyllum tongxinensis Gao 1987
| Ipciphyllum arnouldi
| Calophyllum [Plerophyllina]
| `--C. gemmatum Iljina 1984
|--Calceola [Seriales]
| |--C. calceoloides [=Rhizophyllum calceoloides]
| `--C. sandalina
`--Symmetricales
|--Goniophyllum pyramidale
|--Argutastraea quadrigemina
|--Araeopoma
`--Polycoelia
* Type species of genus indicated
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Eckert, J. D., & C. E. Brett. 2001. Early Silurian (Llandovery) crinoids from the Lower Clinton Group, western New York State. Bulletins of American Paleontology 360: 1-88.
Fedorowski, J. 1979. On some aspects of coloniality in Permian corals. In Biology and Systematics of Colonial Organisms (G. Larwood & B. R. Rosen, eds.) pp. 155-171. Academic Press: London.
Gudo, M. 2002. Soft body reconstructions of Palaeozoic corals: Implications for the system of Anthozoa (Coelenterata). Lethaia 35: 328-344.
Nudds, J. R. 1979. Coloniality in the Lithostrotionidae (Rugosa). In Biology and Systematics of Colonial Organisms (G. Larwood & B. R. Rosen, eds.) pp. 173-192. Academic Press: London.
Roniewicz, E., & J. Stolarski. 2001. Triassic roots of the amphiastraeid scleractinian corals. Journal of Paleontology 75 (1): 34-45.
Rong, J.-Y., R.-B. Zhan & J. Jin. 2004. The Late Odovician and Early Silurian pentameride brachiopod Holorhynchus Kiaer, 1902 from north China. Journal of Paleontology 78 (2): 287-299.
Stolarski, J., & A. Russo. 2002. Microstructural diversity of the stylophyllid (Scleractinia) skeleton. Acta Palaeontologica Polonica 47 (4): 651-666.
Thomson, K. S., & K. S. W. Campbell. 1971. The structure and relationships of the primitive Devonian lungfish – Dipnorhynchus sussmilchi (Etheridge). Yale University Peabody Museum of Natural History Bulletin 38: 1-109.
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intro and evolution MAK030502, classification CKT061027