Rhaeboceras halli fossil from South Dakota in the United States. The fossil dates to the late Campanian Stage in the Cretaceous Period, around 73 Ma (mega-annum, or million years). This ammonite is a shelled cephalopod related to extant squids.
Some chambers of this ammonite are hollow, allowing the internal structure to be observed. Present on this ammonite is the original nacre (aragonitic mother of pearl).