Paleontology and geology
Shallow tropical seas continued to cover Ohio during the Early Devonian, producing thick deposits of limestones on the sea floor. By the end of the Devonian, muds from the rising Appalachian Mountains to the east were being deposited into an oxygen-poor sea. Due to the low oxygen level, very few organisms could survive on the sea floor. Fossils of brachiopods, crinoids, trilobites, and placoderms (armored fish) are found in the shales formed during this time. This map indicates the presence of Devonian rocks in northern Ohio, extending to the south in the central part of the state.