From Ohio History Central
The Pliocene Epoch began about 5 million years ago and ended with the beginning of the Pleistocene Ice Age about 2 million years ago. During this time, the climate was warm and a diverse fauna of mammals roamed the land. No deposits of Pliocene age have been found in Ohio, because either they were not deposited or, more likely, they were removed by the scouring action of the Pleistocene glaciers or stream erosion. A fauna of Pliocene animals, including camel and rhinoceros, was found preserved in a sinkhole in Indiana. The sinkhole formed in limestone during the Pliocene and animals periodically fell into the hole and were buried in sediment. Later glaciers of the Ice Age overrode the sinkhole. Quarrying of limestone eventually exposed the ancient sinkhole and its fossil treasures. This find almost certainly confirms that these animals lived in neighboring Ohio.