Geology & Fossils
Ohio is best known for fossils of marine animals that lived during the Paleozoic Era, long before dinosaurs evolved. (All maps used below courtesy of the Ohio Geological Survey)
Ordovician Period: State Fossil Territory
The best known fossils in southwestern Ohio are marine invertebrates. And its best known marine invertebrates are the state’s oldest fossils. These are entombed in rocks from the Ordovician Period (435-500 million years ago), which are found near the Cincinnati Dome — an enormous area in southwestern Ohio where the Earth has been raised up. Ohio’s state fossil, a trilobite, is among the abundant and well-preserved fossils found here.
Ordovician life was probably pretty boring on land, for there were no land-dwelling animals. There weren’t even any plants, aside from algae. But the Ordovician seas that may have covered all of Ohio at times teamed with life. Trilobites and ostracods were perhaps the most abundant animals. But clam-like brachiopods and graptolites were also abundant, as were colonial corals and bryozoans. Nautiloids resembled snails with tentacles.
Vertebrates — animals with backbones — made their first appearance during the Ordovician. Small, jawless fishes left tiny pieces of bony armor and spines which offered protection against their predators.
Silurian Period
Living things began to colonize land during the Silurian Period, which last from 438 to 408 million years ago. Ohio was located in tropical latitudes south of the Equator during this period. Much of the bedrock in western Ohio dates to the Silurian. However thick glacial sediments from the Ice Age cover all but the southernmost portion of Ohio’s Silurian rocks.
Devonian Period: Monster Fishes!
Two of Ohio’s best known fossil graveyards lie in the Central Lowlands, in opposite corners of the state. Near Lake Erie, in northeastern Ohio, is one of the world’s most famous hunting grounds for a fish that was truly monstrous.
One of Ohio’s most sensational fossil discoveries was made near Cleveland in the 1880s when someone found a shark. Though shark teeth are very common fossils around the world, this discovery included fossilized cartilage — a tough but soft tissue sharks have instead of bones. They even found impressions of the skin and body shape. In addition, this animal, which was named Cladoselache, is the earliest known shark.
Cladoselache hunted in Devonian seas more than 350 million years ago. The shark reconstructed from the ancient mud was 4-5 feet long, big for the Devonian, but not a monster.
Ohio’s monster fish belonged to a group of ancient fishes known as arthrodires. Unlike sharks, arthrodires were protected by armor on their heads and flanks. During the Devonian period, arthrodires may have been Earth’s biggest predators.
Fossils of a Devonian arthrodire called Dunkleosteus have been found near Cleveland. With its long, tapering tail, it is thought to have lurked on the bottom of seas and large lakes and rivers. Imagine seeing a pair of enormous eyes staring at you through mud-filled water. Suddenly, they rush towards you, followed by nearly forty feet of fish!
In place of teeth, enormous bones protruding from Dunkleosteus ’ jaws were sculpted into stabbing weapons and tools for shearing and slicing. You would think that a forty-foot fish could swallow any Devonian creature it wanted to. Yet arthrodires, or “jointed-necked fishes,” are named for joints that enabled them to open their mouths exceptionally wide. What on earth did Dunkleosteus eat?
The best known Ohio trilobite, and the best known trilobite of the Devonian Period, is Phacops rana, which was adopted as the state fossil by neighboring Pennsylvania. From the 1920’s to the 1970’s, specimens of this species were collected by the thousands from a quarry near Sylvania, west of Toledo.
Mississippian Period
Lush forests of tree-sized plants and giant ferns covered much of the earth in the Mississippian and Pennsylvanian periods. They produced much of the world’s coal. Today, eastern Ohio’s best known prehistoric souvenirs are its fossil fuels. Major deposits of coal — Ohio’s most valuable mineral resource — and natural gas occur mainly in the east, petroleum in the southeast.
Pennsylvanian Period
The portion of Ohio drained by these rivers is called the Allegheny Plateau. It belongs to the Appalachian Highlands, which are even higher in Pennsylvania and West Virginia, Ohio’s neighbors to the east.
The Allegheny Plateau largely escaped Ice Age glaciers. Today, southeastern Ohio remains a hilly area with many steep valleys and outcroppings of bedrock. In contrast to glaciated areas, soils are generally shallow here. However, glacial silt carried by rivers enriches the lower valleys of some rivers.
Buckeyes and other hardwoods dominated the forests that once covered nearly all of Ohio, split here and there by prairies. Today, Ohio’s prairies are gone, and only about one quarter of the state is forested, with the most heavily wooded area in southeast Ohio.
Remains of amphibians and reptiles that inhabited fabulous coal forests during the Pennsylvanian Period (280-310 million years ago) have been found in eastern Ohio.
Permian Period
In southeast Ohio are the state’s youngest rocks, dating to the Permian Period (230-280 million years ago). Notice that Ohio’s Permian rocks and Pennsylvanian rocks (see picture above) are located in the region that wasn’t covered by Ice Age glaciers (see picture below).
Ice Age Ohio
During the Ice Age, glaciers covered about three quarters of Ohio. The great ice sheets rounded hills and filled valleys with fertile soils. They also dammed streams and created lakes.
These lakes became shallower as they filled with silt and plants took root in the shallows. Eventually, many became swamps, especially in northern Ohio. Pioneers drained swamps in northwestern Ohio, which was once under the waters of Lake Erie at the end of the Ice Age. Today, fertile farms are found here.
(All maps used above courtesy of the Ohio Geological Survey)
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