Graptolites, the group of fossils to which Isograptus victoriae belongs, are a group of colonial marine animals that lived in the oceans from about 510 to 320 million years ago, from the middle of Cambrian to the middle of the Carboniferous. Their twig-like or net-like colonies, made up of interconnected tubes, floated in the water or were attached to the sea floor or a hard surface, and had a world-wide distribution. Though the individual animals in a colony were very small, the colonies could grow to half a metre or more in size. Because of their world-wide distribution and relatively rapid rates of evolution through time, graptolites are one of the most important groups of fossils for dating rocks deposited during the Ordovician, Silurian and Early Devonian periods (485 to around 405 million years ago). The name 'graptolite' means 'written on rock', describing the appearance of their fossilised remains. Graptolites are extinct but are related to a little-known group of small, colonial, usually encrusting deep-sea animals living today, called pterobranchs.

No other group of fossils has been as important scientifically in Victoria as graptolites. Their abundant and diverse remains in rocks of Ordovician age (485-444 million years old) in the state have been used to subdivide the rock sequences into nine stages and 31 zones, the most detailed subdivision of the Ordovician anywhere in the world. This subdivision is used around Australia and is also recognised in a number of other countries. The name of one of the Victorian stages, the Darriwilian (467-458 million years ago), has been endorsed as the standard international name for a stage in the Middle Ordovician, the only international stage name of Australian origin. Graptolites were also used in mapping the geological structure of the Victorian goldfields, a role that was crucial because of the uniformity of the rock types which were otherwise very hard to distinguish.

Isograptus victoriae was named in 1933 by Dr William John Harris, one of the most prolific workers on Victorian graptolites and at that time a headmaster at Echuca. He named it after the state because it was first found here, in the Castlemaine district. It occurs throughout Australia and is the index fossil (the fossil that characterises a geological time unit) of the Castlemainian stage (470-468 million years ago). It is also recognised world-wide as a key index fossil of the lower Middle Ordovician, including in New Zealand, China, North and South America, the United Kingdom and elsewhere in Europe. First recognised in Victoria, it is a fossil of international scientific importance.

Isograptus victoriae is one of eight candidates for Victoria's fossil emblem.

More Information