I'm not sure I know what you mean, I don't think it's a shell . I've got a bunch of shell fossils,this doesn't look like that. It looks like fossilized soft tissue,minus the shell. Maybe the shell was filled with mud and the shell broke away.I don't see any detail of a shell. The photos don't show that when I look at them. Anyway that's what I'm seeing. Thanks for your opinion.
That's why I said it most likely was just a rock. When a shell is buried, it is very common for mud or dirt to fill the hollow living spaces of the organism. They harden, as the shell sometimes breaks away. What is left is a rock that has the shape, size, and texture of the shell as it would be seen from the inside. I don't think it is a shell cast, not dung. But it has a very minute possibility of a snail cast.