Graptolites from the Silurian, illustration. These specimens are Stimulgraptus graptolites. Graptolites (class Graptolithina) were hemichordate animals, similar to today's Pterobranchia (sessile worm-shaped animals). They fed by filtering plankton out of the surrounding water. Graptolites existed from the Cambrian to the Carboniferous, from around 500 to 325 million years ago. Graptolites were abundant in the oceans, and had a worldwide distribution, acting as index fossils for dating Palaeozoic rocks.