This fossil(?) was found on the roof of a cave in the Renault or Ste. Genevieve Limestones in Kentucky. Mississipian period. I apologize for no scale. It is about 6 inches long
I talked to some usgs fossil guys but they weren't sure. They thought it could be from an armored fish.
It is unlike any fossil I've ever seen. I originally thought it was just chert, but on closer look, it appeared to have bilateral symmetry. It seems like whatever was in there was replaced by the chert. Although, I'm not really familiar with how fossils form.
EDIT: the fossil was very much 3D. The part closest to my finger, facing perpendicular to the wall appears to be be concave.