Graptolites are extinct marine animals that lived from the Cambrian to the Devonian periods, approximately 500 to 300 million years ago. They are primarily known from their fossilized remains, which often appear as thin, flat, and branching structures. These organisms are classified as colonial animals, meaning they lived in groups, and they are believed to have been related to modern hemichordates. Graptolites thrived in oceanic environments and are important for understanding the geological history of Earth. Their fossils are commonly used as index fossils, helping geologists date rock layers and understand the relative ages of different geological formations.