Didymograptus

Graptolites, especially (Didymograptus) artus/murchisoni., are important extinct marine animals used as key index fossils for dating rocks from the Ordovician and Silurian periods. Because they evolved rapidly and were widely distributed in ancient oceans, they help geologists correlate rock layers across different regions. Didymograptus is particularly useful due to its distinctive double-branched shape and abundance in marine shale. Studying these fossils also provides insights into ancient ocean conditions and the early evolution of colonial animals related to modern hemichordates.