Trilobite

Isotelus maximus is Ohio's official state fossil invertebrate, designated in 1985. It was one of the largest trilobites that ever lived — a marine arthropod that crawled the shallow seafloor of what is now southwestern Ohio during the Ordovician period. The species name maximus, meaning largest, was given because Ohio specimens are among the biggest trilobites known anywhere in the world.

Isotelus maximus had a smooth, oval body divided into three sections: the head shield (cephalon), a segmented middle section (thorax), and a fused tail plate (pygidium). Large specimens reached up to 24 inches in length, making them striking by any standard — most trilobites are a few inches long at most. Like all trilobites, Isotelus had compound eyes and could roll into a ball for protection.

Trilobites were marine arthropods, distant relatives of modern horseshoe crabs and insects. Isotelus crawled along the muddy seafloor of the shallow Ordovician sea that covered Ohio, feeding on organic material and small organisms. Trilobites as a group survived until the end of the Permian, about 252 million years ago, when they went extinct in the largest mass extinction in Earth's history.

Ohio designated Isotelus maximus as its state fossil invertebrate in 1985. The choice reflected southwestern Ohio's extraordinary Ordovician fossil record, which has been studied since the 19th century. The Cincinnati region sits atop the Cincinnati Arch, a geological structure that brings Upper Ordovician limestone to the surface across Hamilton, Butler, and Warren counties, exposing some of the richest shallow-marine fossil beds in North America.

Isotelus fossils have been collected from Ohio quarries, road cuts, and creek beds for over 150 years. The species was well known to scientists long before the state designation, and its exceptional size made it a natural symbol of Ohio's paleontological heritage.

Isotelus maximus fossils come primarily from the Cincinnatian Series of Upper Ordovician limestone in southwestern Ohio. Hamilton, Butler, Warren, and Clermont counties have the highest density of outcrops. Road cuts, quarry walls, and creek beds throughout the Cincinnati region regularly expose trilobite material.

The Cincinnati Museum Center's Museum of Natural History and Science holds a strong collection of Ohio Ordovician fossils including Isotelus specimens. Trilobite fossils are common enough in southwestern Ohio that they are familiar to local rock collectors and school groups.