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Graptolites are an extinct group of marine animals that lived worldwide in the oceans during the Palaeozoic Era, first appearing about 500 million years ago (mya) and becoming extinct about 315 mya.
Graptolites were colonial organisms. Each graptolite was a twig-like colony composed of one or more branches that were straight, curved or spiral. Tiny cup-like structures along these branches housed the many tiny individual animals of the colony. Most graptolites floated freely in the sea, although some were attached to the sea floor and resembled small shrubs.
The name 'graptolite' comes from the Greek words meaning 'writing on stone', which refers to the way graptolite fossils typically resemble pencil marks on the rock. Graptolite fossils are commonly preserved in shale or slate, rocks that readily split into thin layers. Dead graptolites and other marine species accumulated on the sea floor where they were buried by layers of mud, sand or silt. Over time these sediments became rock with the graptolite remains encased within it. These fossils have been formed by carbonisation, a process by which the graptolite remains are reduced to carbon, resulting in the pencil-like markings.
Fossils are the remains, moulds or traces of dead plants or animals preserved in rock, generally sedimentary rock such as sandstones, siltstones, shales and limestones. Fossils provide invaluable information about the Earth's past, including the evolution of plants and animals, the age and formation of rocks, and the former positions of the continents.
Fossils that are particularly useful for dating rocks are called index or zone fossils. Index fossils are generally common fossils, easily preserved and identifiable, and widely distributed. Organisms chosen as index fossils need to be species that lived for a relatively short period, for instance a few hundred thousand years, so that dating can be narrowed to a limited time span. Index fossils assist scientists to date rock sequences as the age of the rock correlates to the age of the fossils encased in them. This information can then be applied to rocks found in other regions that contain the same fossilised species. Graptolites are one of the most important index fossils.
The graptolites of Victoria are significant as important index fossils for dating early Palaeozoic rocks around the world. The rocks of central and eastern Victoria contain one of the richest and most diverse groups of graptolites and have been used to date rock sequences in this region. The information has then been transposed to rock sequences in New Zealand, Asia, Europe and North America, where the graptolites found in Victoria also occur in rocks of the same age.