The Permian Period began about 299 million years ago and ended about 251 million years ago. Rocks referred to by geologists as the Dunkard Group crop out in the southeastern corner of the state and are contiguous with similar to rocks in adjacent portions of West Virginia and Pennsylvania. Dunkard rocks consist of thick sandstones deposited in river channels, red and gray shale and mudstone beds, non-marine limestones, and coal beds. They are entirely non-marine except for a thin bed in eastern Ohio that has yielded fossils of a brackish-water brachiopod, Lingula. The rocks indicate an upper delta plain environment similar to rocks of the upper part of the underlying Pennsylvanian. Indeed, there is no significant break between the Pennsylvanian and Dunkard rocks.
There has long been a controversy among geologists as to the age of Dunkard rocks. Some have considered these rocks to be entirely Early Permian in age; others have considered them to be partly Pennsylvanian and partly Permian; and others have considered them to be entirely Late Pennsylvanian in age. The lack of marine beds with diagnostic index fossils has made this a difficult problem to resolve. Plant fossils, which are abundant locally in some Dunkard rocks, do not provide a clear solution to the problem.
Nevertheless, Dunkard rocks mark the final filling of the Ohio basin with sediment eroded from the rising Appalachian Mountains to the south and east. The sea, which was so persistent in Ohio throughout the Paleozoic Era, was pushed out of the state, never to return again. As the Permian progressed, the Appalachian Mountains reached their final majestic heights as the southern continents of Gondwana merged with the northern continents of Euramerica forming the supercontinent of Pangea.
Fossils in Dunkard rocks are not common but persistent collecting has yielded an interesting record of amphibians and reptiles, freshwater sharks and lungfishes, and a diverse assemblage of plant fossils. A large amphibian, Eryops, reached a meter in length. Perhaps the most spectacular reptiles known from Dunkard rocks are Dimetrodon and Edaphosaurus. Both had large, sail-like structures on their backs that were probably used to help regulate body temperature. The carnivorous Dimetrodon reached lengths of up to 3 meters. A large freshwater shark, Orthacanthus compressus, characterized by spike-like two-pronged teeth and a sharp spine on the back of the skull, was a dominant predator in the lakes and streams of the delta plain.
Coal has been produced in modest amounts from Dunkard rocks, principally from the Waynesburg coal. Formerly, grindstones for various industrial uses were quarried from Dunkard sandstones and some of the sandstones were used for bridge abutments and foundations.