Rugosa

For the rose also sometimes just called "Rugosa", see Rosa rugosa. For the synonym of a genus of insects, see Rugosana.

The Rugosa, also called the Tetracorallia, rugose corals, or horn corals, are an extinct order of solitary and colonial corals that were abundant in Middle Ordovician to Late Permian seas.

Rugosa Temporal range:
Solitary rugose coral Grewingkia canadensis in three views; Ordovician, Indiana
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Hexacorallia
Subclass:	†Rugosa Milne Edwards & Haime 1850
Suborders
†Columnariina †Cystiphyllina †Stauriida †Streptelasmatina

"Tetracorallia" from Ernst Haeckel's Kunstformen der Natur, 1904

Cross-section of Stereolasma rectum, a rugose coral from the Middle Devonian of Erie County, New York

Solitary rugosans (e.g., Caninia, Lophophyllidium, Neozaphrentis, Streptelasma) are often referred to as horn corals because of a unique horn-shaped chamber 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). When radiating septa were present, they were usually in multiples of four, hence Tetracorallia in contrast to modern Hexacorallia, colonial polyps generally with sixfold symmetry.

Rugose corals have a skeleton made of calcite that is often fossilized. 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, especially in the Silurian period. Although there is no direct proof, it is inferred that these Palaeozoic corals possessed stinging cells to capture prey. They also had tentacles to help them catch prey. Technically they were carnivores, but prey-size was so small they are often referred to as microcarnivores.

The stauriid coral Cyathophylloides sp. from the late Ordovician, Georgian Bay Formation, Ontario, Canada

Morphology

Streptelasma divaricans (Nicholson, 1875) from the Liberty Formation (Upper Ordovician) of southern Ohio

Rugose corals always show tabulae, horizontal plates that divide the corallite skeleton. The corallites are usually large relative to different types of coral. Rugose corals will sometimes have dissepiments, which are curved plates connected to septa and tabulae. The symmetry can be distinguished by the orientation of septa in a transverse section of the coral. Rugose corals always display bilateral symmetry whereas tabulate and scleractinian corals show radial symmetry. Initially there are only four major septa; later minor septa are added in the four resulting spaces. The complex arrangement of septa is diagnostic of rugose corals. Rugose corals will also always have a columella, an axial rod which supports the septa running up the center of the corallite. It is present 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.