Arthopods (Phylum Arthropoda) are perhaps the most successful group of multicellular animals and include insects, spiders and scorpions, centipedes and millipedes, lobsters, crabs, crayfish, ostracodes and barnacles, and trilobites. Arthropods are characterized by jointed legs, a body divided into segments, and a hard external covering (exoskeleton), that is periodically shed to accommodate the growth of the organism. They are an ancient group, dating to the late Precambrian and all of the above groups are known from the fossil record.
Trilobites were an exclusively marine group of arthropods that appeared in the Cambrian Period and persisted throughout the Paleozoic, eventually becoming extinct during 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, a Cambrian trilobite, Cedaria, was found in a drill core from Warren County. 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 Eldredgeops rana (formerly known as Phacops rana) is exceedingly abundant and well preserved. Eldredgeops 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. Many are found rolled into a ball shape, a protective behavior employed by a number of trilobite species. 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.
OTHER ARTHROPOD FOSSILS
Ohio rocks have produced fossils of crustaceans, including ostracodes, millipedes, spiders and eurypterids, and insects. Ostracodes are microscopic crustaceans that have a two-part shell (carapace) made of calcium carbonate. In general, the shell looks like a tiny bean and many of them have intricate ornamentation on the shell surface. They are known from the Cambrian Period to the Recent and can be found in modern freshwater lakes and ponds. Most fossil species were marine and they are exceedingly abundant in some Paleozoic rocks in Ohio, ranging from Ordovician through Pennsylvanian. A microscope is needed to collect and study them.
Phyllocarids are crustaceans with a two-part shell and a tail. Some reached two or more inches in length and they are found in marine rocks in Ohio ranging from Devonian through Pennsylvanian age. The most abundant genus is Echinocaris, known from Devonian rocks. A decopod crustacean, Palaeopalaemon, related to shrimps and lobsters, is known from Devonian rocks in northeastern Ohio. Eurypterids were aquatic predatory arthropods that looked superficially like scorpions. Most had large pincers and, although most were small, some reached lengths of several feet. They were Paleozoic creatures and became extinct by the end of the Permian. They are rare fossils in Ohio being known from Ordovician, Silurian, and Devonian rocks.
Millipedes are segmented, terrestrial arthropods with a long, cylindrical body. Each body segment has two pairs of legs, thus leading to the name "thousand-leggers" for modern species. Rare specimens are known from shales associated with coal beds in Pennsylvanian rocks of eastern Ohio.
Fossil insect remains are known from Pennsylvanian rocks in eastern Ohio. Although they are rare fossils, some of them are spectacular because of their large size and preservation. Cockroaches nearly four inches in length have been found. Insect wings found in these rocks are commonly confused with fossil leaves. Perhaps the rarest arthropod fossils are those of spiders. A few specimens are known from Pennsylvanian rocks in eastern Ohio. The preservation of these specimens is remarkable and they look superficially similar to some modern spiders.
- Feldmann, R. M., 1996. "Introduction," in Fossils of Ohio, edited by R. M. Feldmann and Merrianne Hackathorn. Ohio Division of Geological Survey Bulletin 70, p. 1-25.
- Hannibal, J. T., 1996. "Phylum Arthropoda: Phyllocarids, Millipedes, Insects, and Other Less Common Forms," in Fossils of Ohio, edited by R. M. Feldmann and Merrianne Hackathorn. Ohio Division of Geological Survey Bulletin 70, p. 124-131.