Didymoceras ammonite fossil found in the USA, dating 75 million years ago from the Campanian age of the Late Cretaceous epoch. This fossil measures 25 cm high. Ammonites are extinct marine animals which lived between 380 and 65 million years ago. They are distantly related to the modern-day nautilus. They lived in coastal waters and inland seas. Collection Jean-Philippe et Francois-Xavier Blouet, paleontologists.