According to the plaque on the mantlepiece: This is a really cool heteromorph ammonite (Nostoceras sp.) fossil from the Tulear Province of Madagascar. Unlike most ammonites with a tightly-coiled spiral shell that retains its shape throughout its life, heteromorph ammonites have irregularly-coiled or uncoiled shells. They are Late Cretaceous in age (Campanian Stage) or approximately 70 million years old.