Corals are among the animals that got their start in the Cambrian explosion and expanded during the Ordovician diversification. During the Silurian, corals began to make reefs.
There are reefs and reefs, of course – a small hill built up by the calcareous homes of corals and other animals might only be a few meters or tens of meters high. Not the Great Barrier Reef, for sure. But by Silurian time, these constructs were becoming larger and larger.
Corals are filter feeders, and many of them are colonial, meaning that the individual polyps can’t live on their own, but survive only as part of the colony. They secreted calcium carbonate, the mineral calcite, to make little cavities in which they lived. The individuals, called corallites, might only be a fraction of a centimeter across, but many thousands of them would form a single colony. Add a few thousand colonies together, and you start to have a reef. Corals weren’t the only inhabitants, or even the only ones helping to construct the reef. Bryozoans, brachiopods, crinoids, sponges, even calcareous algae and many more critters were there, contributing to the structure. Just as today, a reef was a complex, diverse, and highly supportive environment for life. Things like trilobites were not contributing directly to the reef edifice, but they were abundant enough that when they died, their bodies certainly contributed to the mass of the reef.
Also as today, Silurian reefs were located in the tropics. Because the continents were in different positions then, that means that most of the tropical Silurian reefs are in what is now North America and Europe, as well as parts of China and elsewhere in Asia. The U.S. Midwest has some of the biggest Silurian reefs.
Two varieties of coral developed during the Silurian. Tabulate corals are extinct, but they were plentiful then. They are colonies of many individuals, each of which builds its own little cell upward by creating a tabula – a table – upon which the animal sits. The lower segments are abandoned as the colony grows up and up. Tabulates are the most common coral in Silurian reefs.
Another coral that flourished during the Silurian was the rugose corals. Rugose means wrinkled, a good description of the outer walls of these corals. They were mostly solitary, with individuals getting up to 10 centimeters long, and rarely up to a meter. The fossil skeletons of rugose corals typically have a tapering horn-like shape, giving the common name horn coral to this group. The pointed end was down, attached to the sea floor, and the animal’s tentacles extended out of the wider, open end. They were not colonial in the way tabulate corals were, but they often grew close together enough to contribute to reef-making. They often had vertical segmentations, called septae, that divided the internal space into four chambers. The individual animals, the corallites, were often quite a bit larger than those of the tabulate colonies. Rugose corals are also extinct.
We’ll talk about a special kind of reef – pinnacle reefs – later in the Silurian.
—Richard I. Gibson
Image of rugose corals from Kunstformen der Natur (1904; public domain)
Stephen Hui Geological Museum