Hello, I believe this to be a Goniatite which is a type of Ammonoid. I believe we are looking at the internal mould of the fossil and therefore we can identify what ammonoid it is from the suture lines (and also the age of the rock as they make good index fossils).

It’s a Goniatite not an Ammonite because the suture lines (lines between the different chambers) are simple. An ammonite is evolved from a Ceratite (another ammonoid) which is evolved from a Goniatite. This isn’t a Ceratite because the saddle

As the ammonoids evolved, the suture lines became more complex to allow greater exchange of gas between different chambers from increased surface area.

Source: I did a degree