Corals

Tabulate coral Pleurodictyum megastoma from the Lower Devonian of the Heathcote district, Victoria. Source: Museum Victoria.

Corals are tiny marine animals (polyps) which live in small cone-like cells, commonly in warm, tropical waters.

The animals have tentacles to assist feeding, and may seal the end of their cells with an operculum (lid). They often live in colonies, behaving either independently as individuals or with a degree of specialisation of function so that the whole colony operates, to some extent, as an organism. Their skeletons often accumulate in vast quantities, sometimes as reefs, which may become consolidated as various types of limestone.

There are many hundreds of different living species-700 alone in the Indo-Pacific region, and similar numbers of extinct species.

Two extinct types of corals which are frequently preserved in limestones are the rugose and the tabulate corals, both of which arose in the Ordovician Period (434 to 490 million years ago) and became extinct at the end of the Permian Period (251 million years ago).

Scientists have used the daily, monthly and yearly growth bands of rugose coral to establish that the length of a year during the Devonian Period (354 to 410 million years ago) was 399 days (but each day was shorter than at present).

Fossil corals are found in various parts of Victoria, including Lilydale (in limestone), Mornington and Torquay (in clays).

Scleractinian coral Septastrea. Source: Unknown.

Compound rugose coral Martinophyllum approximans, from the Lower Devonian of Coopers Creek, Victoria. Source: Museum Victoria.

Tabulate coral Pleurodictyum megastoma from the Lower Devonian of the Heathcote district, Victoria. Source: Museum Victoria.