It does look like an ammonoid. split right down the middle. The septa are showing and part of the shell wall, and the umbilicus would be where you show it, but it is not there. It would be very difficult to assign it to a species as the sutures are still buried and any ornamentation on the shell is not visible. The septa near the suture are more in the shape of the suture, but as you move toward the center of the shell the septa become more simple, almost to the point of looking nautiloid like. I think this is how your specimen is, you can see some of the curvature of the septa but not the sutures.
The dark lines in 1a and 1b are too thick, and along with 2, they don't have any sign of the shell wall. If I could see that they were not just squiggly line floating in space... then maybe. The piece between your fingers in 3 at least has equally spaced squiggly lines.
im starting to cool off on the ammonoid diagnosis for the squiggly lines. i was doing some reading on crustaceans from ohio and found that the tail of the decapod Palaeopalaemon looks rather similar to the pics in #5.