Graptolites are found throughout the fossil record: from the Cambrian through to the Permian. They are very diverse and some types of graptolite only lived during very particular geological time periods. This makes them good index fossils: if you can identify the graptolites in a rock, you can make a good guess about the age of the rest of the fossils in the bed.
Graptolites are a little mysterious. They belong to the hemichordates and have a stomachord, which may have some similarities to the notochord found in vertebrates. They were colonial and each zooid lived inside little cups, which were connected to each other by long tube like strips. The structures that lived in may have been able to move within the water, although pterabranchia, their modern relative is sessile.