Trilobites were an exclusively marine group of arthropods that appeared in the Cambrian Period and persisted throughout the Paleozoic, eventually becoming eztinct in the Permian Period. Most were small, bottom-dwelling organisms that fed on organic matter in sediment on the sea bottom. The body is divided in three segments: head (cephalon), middle (thorax), and tail (pygidium). Many species had large, compound eyes.
Trilobites are among the most desirable of fossils for the fossil collector and Ohio has an abundance of trilobites in the Paleozoic rocks of the state. This importance was recognized in 1985 when an Ordovician trilobite, Isotelus, was named the official state fossil of Ohio. Specimens of Isotelus have been found that are nearly two feet in length. Although no Cambrian rocks crop out at the surface in Ohio, Trilobite fossils are particularly abundant and well-preserved in Ordovician rocks that crop out in southwestern Ohio. Common species in these rocks are Cryptolithus, Flexicalymene, and Isotelus.
Trilobites were abundant and diverse in the seas that covered Ohio during the Silurian Period but many of the specimens have been destroyed or altered by the process of limestone changing to dolomite. However, excellent examples of Silurian trilobites are occasionally found. Common species are Bumastus, Calymene, and Dalmanites.
Devonian rocks in Ohio are well known for trilobite fossils. The Columbus Limestone yields several species, the best known and most common of which is Coronura, a large species that reached nearly a foot in length. Most specimens are not complete. The best-known trilobites from Ohio Devonian rocks come from the Middle Devonian Silica Shale of northwestern Ohio, where Phacops rana is exceedingly abundant and well preserved. Phacops specimens from the Silica Shale are known worldwide. They are only a few inches in length and are noted for their large, frog-like compound eyes. The Silica Shale is exposed only in quarries, which are now closed to collecting.
A major, worldwide extinction event near the end of the Devonian Period significantly reduced the abundance and diversity of Late Paleozoic trilobites. Although several species are known from Mississippian rocks in Ohio, they are small and uncommon. By Pennsylvanian time, only three species are known from marine rocks in Ohio and they are small and uncommon. No marine rocks of Permian age are present in Ohio to mark the final extinction of the trilobites.
- Babcock, L. E., 1996. "Phylum Arthropoda, Class Trilobita," in Fossils of Ohio, edited by R. M. Feldmann and Merrianne Hackathorn. Ohio Division of Geological Survey Bulletin 70, p. 90-113.
- Smith, A. J., and Tillman, J. R., 1996. "Phylum Arthropoda, Class Crustacea, Subclass Ostracoda," in Fossils of Ohio, edited by R. M. Feldmann and Merrianne Hackathorn. Ohio Division of Geological Survey Bulletin 70, p. 114-123.