From Ohio History Central
Exceptional Utah Ordovician Graptolites Assemblage
Graptolites (Phylum Hemichordata, Class Graptolithina) were colonial marine organisms that are most common in rocks that represent deposition in deep water. Most graptolite colonies were floating forms. Although some of these colonies may have reached nearly three feet in diameter, most graptolite fossils are represented by fragmentary, black-to-gray pieces that look like small saw blades or pencil marks on bedding planes. Although they range from Cambrian to Pennsylvanian, they are found in Ohio in Ordovician through Devonian rocks and are very rare, with a few exceptions.