surface find----gastropod fossil with decorative carving

[Charl]

It's not clear from the pictures whether it is actually a continuous spiral as you would find in a 'snail-like' gastropod shell or a series of cushion-like structures butted together in the manner of a tiered wedding cake. The answer is important in determining what it might be. The close-up structure most resembles a coralline sponge and that could certainly grow in 'tiered wedding cake' fashion.

For sure it isn't an artefact and I don't think there is any man-made 'carving' on it. There are a number of organisms which can leave feeding traces that look like carved lines in limestone/carbonate-type reef fossils and I would suspect that's what the marks on the 'bottom' are.

I would pay attention to what Red-Coat is saying here. I believe he knows what he is talking about, and more photos might really help. I also agree that the lines being feeding traces is likely for marine reef fossils. Worm trails, for instance are commonly seen on shellfish remains.