I have a limestone replacement/cast fossil that looks very much like that. It is about 1 & 1/2 inches in diameter. While I found it in railroad bed gravel, its likely source is a large limestone quarry nearby that has a rich field of Devonian coral reef fossils. I judged it to be a gastropod (snail) as that is just how it appeared. I have a few ammonites and they tend to be very much larger and generally more elaborately shaped/textured than the gastropodae of that era.
However, an ammonite is also a very good possibility, especially if it is a large specimen. How big is it?
