What are corals?
Corals belong to a group of animals that also includes sea anemones and jellyfish, all of which have a radially symmetrical body, like the spokes of a wheel. In corals this body plan is in the form of a more or less cylindrical polyp having a mouth surrounded by feeding tentacles on its upper surface, just like a sea anemone. However, unlike anemones, corals form a hard skeleton which is commonly fossilized. Coral polyps may be either solitary or joined together to form large colonies. Colonial corals form reefs, such as the massive Great Barrier Reef of Queensland. Extensive deposits of fossil corals indicate that large coral reefs existed at many times during the history of life on Earth, although the earliest fossil reefs were built by corals that were very different from those living today.
Fossil coral colony, Lithostrotion sp.
Artist: Kate Nolan / Source: Museum Victoria
Where are fossil corals found?
Corals were abundant throughout the Palaeozoic Era, between 545 and 251 million years ago (mya). In eastern and central Victoria both colonial and solitary corals are found in Palaeozoic sedimentary rocks, especially limestones, that were deposited during the late Silurian and early Devonian periods (420 to 400 mya). Fossils of small solitary corals occur in much younger Tertiary clays and marls between 30 and 5 million years old, especially in the Torquay, Geelong and Mornington districts.
Fossil solitary coral, Placotrochus magnus
Source: Museum Victoria
Fossil and living corals
Two groups of corals are prominent fossils in Palaeozoic sedimentary rocks: rugose corals, which included forms that constructed colonies as well as solitary forms; and the tabulate corals, which were exclusively colonial. Both groups were involved in the formation of reefs. Rugose and tabulate corals became extinct at the end of the Palaeozoic (251 mya), after which the modern reef-building corals, or scleractinians, arose early in the Mesozoic Era. Rugose and tabulate corals are believed to have lived in warm, clear, shallow seas, as do the scleractinians that build reefs at the present day.
Although the tropical waters of Queensland and other parts of the world are famous for their vast coral reefs, corals also inhabit cold seas. Living examples of solitary coral polyps and also small coral colonies can be found on reefs in southern Australia, including Port Phillip Bay.
Edgar, G. J. 1997. Australian marine life. Reed Books, Kew, Victoria. [Photographs of six species of living corals from southern Australia]