I found this fossil a few weeks ago. It was in a creek bed that flows during the wet season but has pools in the dry. I know the creek has sandstone, but it also looks like it has limestone and possibly slate. There is also a lot of rocks containing rust. This fossil appears to be stained with it. I am located in Western Kentucky near Hopkinsville.
One side looks like it is ribs. The other side is smooth and one part is unusually round. There are pockets that appear to be filled with sediment that has solidified into rock.
I've added pictures from all sides and others with measurements. I will add them as replies since the files are too large. I wish I had more, but this is all I have.