Region 3: Fossils of the Inland Basin

Much of the Inland Basin is located in the Northeastern United States, but the parts extending into the Midwest span nearly 100 million years of the Paleozoic Era. A band of rocks running from extreme northeastern to central southern Ohio are from the Devonian period, while most of the Inland Basin, including eastern Ohio, and the southern portions of Indiana and Illinois, is composed of younger Mississippian and Pennsylvanian rocks. The youngest bedrock of the region, from the Permian period, is located in the southeasternmost part of Ohio.

When the rocks of this region were formed, much of central North America was covered by a relatively shallow, tropical, inland sea. The late Devonian- and early Carboniferous-aged rocks of the Inland Basin contain the fossils of a diverse marine ecosystem. Younger Carboniferous rocks show the environment transitioning from the shallow sea to an environment of extensive deltas and swamps. These rocks contain the fossils of organisms from freshwater and forest ecosystems.

Marine Environments in the Devonian

During the Devonian period, communities of corals, crinoids, bryozoans, brachiopods, and mollusks thrived in the warm sea that covered most of the Inland Basin. Fish and sharks were also common, but their fossils are much rarer. At the time, what is now the Midwest was just south of the equator. Sediment was washed from the rising Acadian mountain range far to the east, carried down rivers, and deposited into the sea where it settled to the bottom and occasionally buried the organisms living there. Driftwood from the world’s first terrestrial forests, made up mostly of lycopod trees, sometimes found its way to the seafloor to be preserved alongside the shells of the animals living there.

The Carboniferous and a Transition to Terrestrial Environments

At the end of the Devonian, fluctuations in sea level caused the water to retreat from portions of the Inland Basin. By the beginning of the Carboniferous, the landscape was dominated by deltas and swampy forests, similar to what occurred in the Central Lowland region at this time. In addition to plants, the fossils of freshwater fish, sharks, early amphibians, and arthropods can be found in coal beds from the late Carboniferous. At the beginning of the Permian, Ohio was a terrestrial environment where lake and river deposits preserved horsetail and fern fossils. These are the youngest fossils found in the Inland Basin’s bedrock.