Corals are marine animals with simple body structures. The mouth of a coral’s sac-like body is surrounded by a ring of tentacles. The living coral animal, the polyp, secretes a cup-like skeleton called the corallite. Many corallites cemented together make up the entire skeleton, or corallum. Inside the corallite, a radial divider, called a septum (plural, septa) grows vertically from the attachment base and helps support the soft tissues. Many coral polyps contain algal cells, which use photosynthesis to produce food for themselves and the coral.
Corals can live together in large colonies, or reefs, which can be hundreds of miles across. Coral reefs are among the most complex ecosystems on Earth because many thousands of species other than corals make the reef their home. Corals themselves require specific living conditions, so fossil coral reefs tell us a great deal about the environmental conditions at the time of reef formation. Living coral reefs are confined to subtropical regions in shallow waters that are warm and clear. Thus, Wisconsin’s Silurian reefs provide evidence that the state was once covered by a warm, subtropical sea. Although ancient corals formed the reef itself, many other organisms flourished in the small habitats the reef provided. Brachiopods covered the reef structure, gastropods fed on the abundant algae and detritus, cephalopods hunted for prey, and crinoids swayed in the agitated waters.
Two groups of reef-forming fossils are found in Wisconsin: the tabulates and rugosans.
Tabulate corals grow upward, depositing horizontal plates known as tabulae. Tabulates formed mounds that appear similar to honeycombs.
Tabulate corals are the most abundant coral fossils in the Silurian rock of Wisconsin and are usually the largest reef corals. They form massive colonies, about 0.3 meter (1 foot) wide or larger. They can be identified by the presence of tabulae. Fieldstones, commonly found in fencerows along the edges of farm fields in eastern Wisconsin, often contain well-preserved tabulate corals. Beach pebbles and gravel along the Lake Michigan shoreline also abound with coral fragments.
Favositid tabulates: Honeycomb corals
The favositid corals are quite common. They usually formed large colonies. The corallite is prismatic in shape, resembling honeycombs. Favositids have mural pores, tiny holes in the wall of the skeleton, which connect different corallites. These pores are distributed in characteristic patterns and numbers, which are useful for distinguishing the various types of favositids. Favositids lived from the Ordovician to the Permian, at which time they became extinct. They are most abundant in middle Silurian to lower Devonian rock. Favosites is the most common fossil coral in Wisconsin.
Halysitid tabulates: Chain corals
Halysitids resemble interlocking strings of delicate chains. Halysites is a common chain coral in Wisconsin, and it is used worldwide as an indicator of Silurian rock. Halysites is best seen in weathered rock because the rock between the chains dissolves, leaving the chains beautifully exposed.
Syringoporid tabulates: Tube corals
Syringoporids are easy to identify because of the presence of distinct tubes connecting the corallites. They are quite delicate, but in many places they have been preserved in astonishing detail. Syringopora is a common syringoporid in Wisconsin; it lived from the Silurian to the Permian.
Rugosans are cone-shaped corals that resemble cow horns. The polyp lived in a space in the center of the cone, known as the calice. Rugosan corals first appeared in the Ordovician and are the second most common type of coral in Wisconsin. Because most horn corals appear to be similar, they can be difficult to identify. Rugose corals can be colonial or solitary. Solitary and colonial rugosans are characterized by external growth bands, which formed much like tree rings. Rugose corals declined after the Silurian and eventually died out at the end of the Paleozoic Era.
These corals formed bunches and were attached to one another.
Solitary rugose corals
Solitary rugose corals commonly were dislodged and then tipped over. If they survived, in many cases they grew upward again. Such rugosans have geniculations (contortions) caused by the change in growth direction.
(Photos courtesy the Milwaukee Public Museum.)