Fossil Hunters

Rugose corals

Rugose corals first appear in the geological record in Middle Ordovician rocks from North America. They diversified more slowly than tabulate corals, but their patterns of evolution are similar. They were important members of Palaeozoic reef communities, but their diversity declined during the end-Devonian extinction. At this time, and in the earlier extinction event at the end of the Ordovician, solitary corals and generalist colonies were more likely to survive than highly specialized colonial forms. During the Carboniferous, rugose corals regained some diversity and were the more common of the two orders. Throughout their evolution, it has been suggested that there was a trend towards a more integrated colony form, with more contact between polyps of the same colony. All rugose corals became extinct in the end-Permian mass extinction event.

Rugose skeletons are almost always composed of calcite, though a few late forms may have secreted aragonite corallites. They have a solid structure similar to tabulate corals, though the internal elements of their corallites are distinctively different (Fig. 4.5). These are dominated by septa, vertical plates organized in a radial pattern. Tabulae and dissepiments are also common. In the center of the corallite there is often a central structure, which was produced by the modification of a variety of other internal structures. A coenenchyme, or region of shared tissue between corallites, is sometimes developed in colonial species.

Solitary corals are typically horn shaped; they reclined on soft sediment and grew upwards with time, so bending their corallite. Colonial corals tend to be dome shaped, with a range of corallite shapes, often defined by their proximity to one another. Corallites in direct contact tended to become polygonal in shape, whereas more isolated corallites kept a circular cross-sectional form. Within colonial rugose corals there is often evidence of the colony having fallen over and then regrown. This emphasizes the lack of holdfasts in the group, which precluded them from forming the framework of the reefs they inhabited. This lack of a holdfast also precluded rugose and tabulate corals from living in the high energy environments characteristic of the reef front of a modern reef, which is exposed to breaking waves. Instead they were confined to the deeper, quieter slopes of the fore-reef, or the still lagoons of the back-reef area.

The overall shape of colonial corals is partly determined by their species, and partly by their environment. This is a useful property for interpreting mode of life, but makes taxonomy and classification very difficult. A tall colony can be interpreted as one that lived in an area of high sedimentation rate, but it need not be related to another colony with a similar form.

Calice: convex surface at the top of the corallite, occupied by the polyp. Septa project into the calice, providing a more secure attachment surface for the animal

Fossula: gap in septa, often of taxonomic value

Corallite: often horn shaped in solitary corals. Usually constructed from calcite

Fossula: gap in septa, often of taxonomic value

Central column (columella): common in rugose corals and developed from the modification of a range of other structures including tabulae and septa

Tabulae: horizontal plates dissecting the corallite

Septa: radially arranged, vertical plates, added within the corallite in a characteristic pattern

Dissepiments: small, upwardly convex plates, which are often developed in a marginal zone at the edge of the corallite

Corallite shape: controlled by the overall shape of the colony in many rugose corals. In this example, close contact between adjacent corallites causes them to grow in a polyhedral shape

Corallum: the whole coral colony. Most commonly massive and dome shaped in rugose corals. Its shape was modified by the environment

Fig. 4.5 Major features of the hard-part morphology of rugose corals: (a, b) a generalized solitary coral, and (c) a generalized colonial coral.

Rugose septa

In common with all corals, rugose corallites grew sequentially so that the whole history of their growth and development is preserved within the skeleton. This makes it possible to trace the development of the septa, which are added in a pattern characteristic of the group (Fig. 4.6).

In the early growth stages, the corallite had two opposing septa, known as the cardinal and countercardinal septa, respectively. Next, two alar septa were inserted on either side of the cardinal septum, and then two more septa on either side of the countercardinal, the counterlaterals. These four new septa defined four sections of the corallite, into which new suites of septa were inserted. This pattern of fourfold insertion is characteristic of the Rugosa. It tends to leave gaps, most often in the region of the cardinal septum. These gaps, or fossulae (singular fossula), are taxonomically important.

The addition of septa in four areas of the corallite means that rugose corals secondarily lost their radial symmetry. Any functional reason for this is unclear, although it may have persisted into the soft tissues as they would have been in direct contact with the septa, and the shape of one should mirror the shape of the other.

Mature septa with fossulae

Septa are added on the "cardinal" sides of the alar and counterlateral septa

Counterlateral septa added

Alar septa added

Cardinal and countercardinal septa

Fig. 4.6 The sequential development of septa within a rugose coral.

Mature septa with fossulae

Septa are added on the "cardinal" sides of the alar and counterlateral septa

Counterlateral septa added

Alar septa added

Cardinal and countercardinal septa

Fig. 4.6 The sequential development of septa within a rugose coral.

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