Luckily, the curator was there and she told us it was a Devonian Period arthrodire (armored fish). They make great displays once there in 3-D form (http://www.palaeocritti.com/dunkleosteus-terrelli). I believe there is one in the lobby at the Falls of the Ohio State Park visitor center.
If I had to guess, this fossil probably came from the Cleveland Shale (member of Ohio Shale). It is probably a Dunkleosteus.
