From Ohio History Central
Horsetails are tall, comparatively thin spore-producing plants that are characterized by a stem that is divided into segments by periodic constrictions from which whorls of leaves emerged. They appeared in the Devonian and survive to the modern day but became greatly reduced after the Late Paleozoic. Many people may be familiar with the modern horsetail, Equisetum, which is sometimes called a scouring rush. Some Paleozoic species grew as large as 60 feet high. Portions of the stem, with several segments, are not uncommon in Pennsylvanian rocks. These segments have a series of closely spaced vertical ribs and grooves that are divided by a horizontal groove. Horsetails lived in the luxuriant coal swamps. Calamites is a common genus.