Ammonite

Ammonites are an extinct group of marine animals similar to the Nautilus. Like the nautilus, Ammonites had a soft body with tentacles that emerged from a hollow chambered shell filled with gas for buoyancy. Many Ammonites' shells echo the spiral shape of the nautilus, though some species of ammonites came in very bizarre shapes. Ammonite shells differ from Nautilus in other ways as well. For example, the walls that divided the chambers took on many complex ripples and folds adding extra internal structure to the shell and allowing it to be lighter than the shell of a Nautilus but no less strong. The curious leafy-looking patterns made where the folds of the chamber walls met the outside of the shell, known as sutures, can be used to distinguish species of ammonites. Ammonites are among the most common fossils found in collections, being plentiful in locations all over the world. They became extinct at the end of the Cretaceous period, the same time that the dinosaurs disappeared.

Individual shapes and fossils vary but your item will resemble the pictured example.