Graptolites and other organisms in a Silurian sea, illustration. The graptolites are the animals attached in a row to the stalk at lower left. Graptolites (class Graptolithina) were hemichordate animals, similar to today's Pterobranchia (sessile worm-shaped animals). They feed 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.