What are graptolites?
Graptolites (formally known by their scientific name Graptolithina) are extinct marine creatures that formed twig-like or net-like colonies composed of one or more branches. These colonies mostly floated freely in the sea but some may have been attached to the sea floor. Because of their branching form they may have superficially resembled seaweed but they were in fact animals. Each colony was made up of a large number of individual microscopic animals that lived in a series of tiny cup-like structures arranged along the branches of the colony. The individual animals were all joined together by a type of nerve cord.
Graptolite, Pendeograptus fruticosus, Early Ordovician (c. 482 my old), Castlemaine district, Victoria
Source: Museum Victoria
The graptolite colonies were originally three-dimensional but during fossilization they usually became completely flattened. After flattening the cup-like structures that housed the individual animals appear like serrations along one or both edges of the colony. The flattened remains of the colonies preserved in slates or shales tend to resemble pencil marks on the rock, and the name ‘graptolite’ is derived from Greek words meaning ‘writing on stone’.
Where are graptolites found?
Graptolites first appeared in the Cambrian Period, 545–490 million years ago (mya), and evolved rapidly during the following Ordovician Period (490–434 mya) when the greatest variety of different forms lived. They suffered a major decline in the Silurian (434–410 mya) and only a few forms lasted into the Devonian. They finally became extinct during the Carboniferous Period, about 315 my ago.
Hundreds of different species of graptolites are known and many of them were widely distributed around the world because they drifted in the surface waters of the oceans. Graptolites are therefore one of the most important groups of fossils for dating rocks of Ordovician to early Devonian age. Ordovician rocks occur over wide areas in central and eastern Victoria, and contain one of the most abundant and varied assemblages of graptolites in the world.
These assemblages have been used to subdivide the Victorian Ordovician sequences into 30 intervals and to date these intervals accurately with other rock sequences in New Zealand, Asia, Europe and North America. Graptolites were also used to work out the structure and sequence of Ordovician rocks in the central Victorian goldfields, as the strata themselves are too uniform in appearance to allow this to be done on the basis of rock type.
Graptolite, Rhabdinopora scitulum, Early Ordovician (c. 485 my old), Romsey district, Victoria
Photographer: Rodney Start / Source: Museum Victoria
Further Reading
Palmer, D. and Rickards, R. B. (eds) 1991. Graptolites: writing in the rocks. Fossils illustrated, volume 1, Boydell Press, Woodbridge, Suffolk, UK. [Detailed information and photographs on graptolites].