Orthoceras and ammonite fossils in a slab of limestone. Both Orthoceras and ammonites are extinct marine cephalopods related to the squid and octopus. Orthoceras lived between about 470 and 360 million years ago, and have been found in Europe, North Africa, and North America. Ammonites (spiral fossils), of the subclass Ammonoidea, lived between 400 and 65 million years ago. This sample measures 550 millimetres across.