From Ohio History Central
The Paleozoic (early life) Era began about 542 million years ago and ended about 251 million years ago. It is divided into seven periods: Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian. Paleozoic rocks form the bedrock of Ohio. Ordovician through Pennsylvanian, and possibly Permian, are present at the surface. Cambrian rocks are present in the subsurface but none crop out in Ohio. Rocks known as the Dunkard Group, in the far southeastern part of the state, may be Late Pennsylvanian or Early Permian in age. Complex, multicellular animals first became abundant and widespread during the early part of the Paleozoic. During the Era, vertebrates such as fishes, amphibians, and reptiles appeared and land plants appeared and had diversified such that forests were present by the end of the Devonian.