Ammonoids (Ammonoidea), commonly known as ammonites, are a subclass of extinct cephalopod molluscs that existed in seas from the Devonian (about 400 million years ago) to the late Cretaceous (65.5 million years ago). Their rapid evolution and worldwide distribution are excellent guide fossils for rock dating and have made possible the development of successions of biozones of great biostratigraphic precision.
