Is this a fossil

[Iand]

Found in a disused quarry in Oxfordshire. 

[CDiggs]

I think this is a fossil but not a bone or vertebra, looks more like part of an ammonite to me, but maybe someone more experienced with invertebrate material will be able to say more.

[Iand]

Thanks, I’m a complete amateur so starting from ground zero. 

[Avalonian]

Hi land. You appear to have found the internal mould of an ammonite chamber. The ammonite probably wasn't the same type or shape or age as the one shown in this blog post but the pictures will give you some idea of what you're looking at, I hope:

 

https://woostergeologists.scotblogs.wooster.edu/2011/11/13/wooster’s-fossil-of-the-week-a-baculitid-ammonite-cretaceous-of-wyoming/

[Ludwigia]

These chambers are known as "cat's paw" in layman's terms.