Graptolithina are an extinct group of marine animals that lived during the Paleozoic era, primarily from the Cambrian to the Devonian periods. They are known for their distinctive, tree-like structures called graptolites, which were composed of tiny, colonial organisms. These structures are often found in sedimentary rock and are important for biostratigraphy, helping scientists date and correlate rock layers. Graptolites were primarily planktonic, drifting in the ocean, and they played a significant role in the marine ecosystem of their time. Their fossils provide valuable insights into ancient environments and the evolution of early life on Earth.