From Ohio History Central
Sharks are fishes with cartilaginous skeletons and are only rarely are they completely preserved. Most often, their teeth, fin spines, and microscopic scales are found. Shark remains are present in Devonian, Mississippian, and Pennsylvanian marine rocks, and remains of freshwater sharks are found in Pennsylvanian and Permian rocks. They were a diverse group in the Late Paleozoic and adapted not only for feeding on other fishes but many species had crushing teeth adapted to feeding on shelled invertebrates. These teeth are moderately abundant in some Pennsylvanian marine rocks in eastern Ohio.
Teeth and scales of sharks are found in the Columbus Limestone. The Upper Devonian Cleveland Shale Member of the Ohio Shale, in the Cleveland area, has produced concretions that yield exquisitely preserved remains of one of the earliest sharks, Cladoselache. Soft parts of the sharks are preserved, including muscle tissues.