Graptolites

Isograptus victoriae from the Lower Ordovician of Castlemaine, Victoria. Source: Museum Victoria.

The word 'graptolite' means 'writing in rock', and that is what the fossils of these extinct animals look like.

Each graptolite consisted of a stick- or twig-like colony of tiny animals that either floated in the sea or was attached to the sea floor like a tiny, up-right shrub. The graptolite colony consisted of one or many branches that were straight, curved or spiral. The individual animals lived in a series of tiny, cup-like structures organised along the length of the graptolite skeleton.

Climacograptus bicornis, from the Upper Ordovician of the Wellington River, Victoria. Source: Museum Victoria.

Most graptolite fossils are preserved flattened in shale (a sedimentary rock) with one or both edges having the appearance of a tiny saw blade. A magnifying glass will show quite a lot of detail, but the rarer three-dimensional fossils show much more.

There were hundreds of different species, and scientists are able to tell the ages of rocks by identifying the type of graptolite fossils present. Graptolites are particularly important for dating rocks deposited during the Silurian and Ordovician periods (410 to 490 million years ago).

Graptolites arose in the Cambrian Period (490 to 545 million years ago) and finally became extinct in the Carboniferous Period (298 to 354 million years ago).

Graptolite fossils are found in many areas of Victoria, including the Bendigo, Castlemaine and Gisborne districts.

Tetragraptus fruticosus from the Lower Ordovician of Campbelltown, Victoria. Source: Museum Victoria.

Rhabdinopora scitulum from the Lower Ordovician of the Romsey district, Victoria. Source: Museum Victoria.

Dicranograptus hians from the Upper Ordovician of the Wellington River, Victoria. Source: Museum Victoria.