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Ohio's State Invertebrate Fossil

Ohio's State Invertebrate Fossil

Isotelus: Giant Trilobite of the Ordovician Period

Isotelus (pronounced eye-so-TEE-luss) is a fossil genus of large trilobites, an extinct group of arthropods related to insects, spiders, and horseshoe crabs, among others.  The name trilobite means “three-lobed animal,” from the three longwise divisions, or lobes, of the body. The genus name “Isotelus” derives from the Greek isos (equal) and telos (end) and refers to the fact that the tail shield is approximately equal in size to the head shield.

Trilobites lived in marine environments, which are not found in Ohio today. But 445 million years ago, land that is now Ohio was covered with a shallow sea with many varieties of trilobites. Isotelus fossils occur in rocks outcropping near Cincinnati and Dayton in southwestern Ohio. The largest species in Ohio, with the scientific name Isotelus maximus, grew to a length that is estimated at more than 50 centimeters (more than 20 inches). The largest “complete” specimens of Isotelus from Ohio are somewhat smaller, at just over 38 centimeters (15 inches). Most body fossils of Isotelus consist of the dorsal (top) shell, or exoskeleton, which was fortified with the mineral calcite (CaCO3), making it more likely to be preserved as a fossil. The many pairs of legs and the antennae typically are not preserved; however, rare impressions of the legs are known.

After learning about a large Isotelus discovered in 1919 at Huffman Dam, near Dayton, two classes of third- and fourth-grade students and their teachers lobbied their state representatives to sponsor a bill to make Isotelus our State Invertebrate Fossil. In 1985, their bill became law, making Isotelus Ohio’s official state invertebrate (animal without a backbone) fossil.

Fun Facts

HOW TO SAY IT: eye-so-TEE-luss
AGE: Late Ordovician Period (Ohio specimens)
TIME: About 445 million years ago
LENGTH: Estimated at up to 50 centimeters/20 inches (I. maximus)
STATUS: Extinct (none are living today)

Isotelus as Predator & Prey

Isotelus was probably a predator that hunted worms and other invertebrates on the seafloor or within the seafloor sediment. Some trace fossils (traces left by organisms, such as tracks, trails, and burrows, which indicate behavior) matching the shape and size of Isotelus fossils have been interpreted as hunting burrows. Some of these burrows intersect smaller trace fossils that were probably made by worms.

Although they were predators, Isotelus was also prey for large orthoconic (straight-shelled) cephalopods—animals related to modern squids, octopuses, cuttlefishes, and nautiluses. Some specimens of Isotelus have triangular bitemarks that are consistent with the shape of a cephalopod’s beak. The large eurypterid, or sea scorpion, Megalograptus also may have preyed upon Isotelus.

Like most trilobites, Isotelus could enroll into a ball, like a pill bug (or “roly-poly”). Trilobites enrolled as a defensive measure to protect their more vulnerable undersides from predators or adverse environmental conditions. Even larger Isotelus trilobites were capable of enrollment.

How big? How do we know?

The genus Isotelus includes the world’s largest trilobite: the species Isotelus rex, first described in 2003 by David Rudkin and others. A specimen of I. rex was discovered on the shore of Hudson Bay in northeastern Manitoba, Canada, that was just over 70 centimeters (almost 28 inches) long. Its total length was easy to estimate, because the specimen was nearly complete.

Although the two Isotelus species that occur in Ohio, I. gigas and I. maximus1 are very large trilobites, they are considerably smaller than I. rex. The larger of these, I. maximus, has a broader outline than I. gigas, has cheek spines, and is known from complete specimens exceeding 38 centimeters (15 inches). However, I. maximus has been estimated to reach lengths of more than 50 centimeters (more than 20 inches), based on some large partial specimens. By making reasonable assumptions about the proportions of certain elements of the exoskeleton relative to its total length, paleontologists can use partial remains (such as an isolated tail shield) to estimate how long the complete trilobite would have been.

Isotelus size comparison

1Two additional Ohio Isotelus species, I. megistos and I. brachycephalus, are junior synonyms of I. maximus.

Molting to Grow

Only fully-grown Isotelus trilobites were large—juveniles started quite small. In order to grow, trilobites, like modern insects, had to periodically molt their hard dorsal (top) “shells” or exoskeletons. Isotelus maximus may have used its cheek spines as part of a molting process, by first walking backwards and digging the spines into the seafloor muds and then using the spines as leverage to escape from its old exoskeleton.

Isotelus exploded diagram

Sometimes individual sclerites, or separate elements of the exoskeleton, would become disarticulated as the old exoskeleton was shed, or shortly thereafter as the soft connective tissue decayed. Limestones in the Cincinnati area often contain a jumble of Isotelus sclerites (see schematic diagram).

Some complete, outstretched trilobites likely represent molted exoskeletons, rather than carcasses (dead trilobites). Perfectly enrolled trilobites, with the end of the tail tucked against the underside of the head, are most likely carcasses.

Collecting Isotelus Trilobites

A great place to find fossils of Isotelus trilobites is at Caesar Creek State Park in Warren County. Collectors will first need to ask for a free fossil-collecting permit, available at the Army Corps of Engineers Visitor Center.

Pieces of Isotelus exoskeletons are common fossils and easy to spot because of their brown color. Large, complete Isotelus fossils are rare, but smaller dime- to nickel-sized, enrolled Isotelus often can be found if you are persistent—or lucky.

Fossil collecting at Caesar Creek


Division of Geological Survey paleontologist Mark Peter explains where you can find Isotelus fossils today and how some of Ohio’s youngest residents were behind its status as State Invertebrate Fossil.