Polished halves of fossilized ammonite, showing the interior makeup of the shell in separated chambers. Ammonites are extinct cephalopod mollusks that lived in the oceans during the Jurassic and Cretaceous periods. They are distantly related to the modern day nautilus and lived in coastal waters and inland seas. Certain species of ammonite existed at specific times within the geological timescale. Their remains can be used for the accurate dating of marine rocks