Any of a group of extinct shelled Devonian through Cretaceous periods (410–65 million years ago).
Most ammonoids were predators. The shells, either straight or coiled, served as protective and supportive structures and enabled the animals to compensate for varying water depths. Ammonoids are important index fossils because of their wide geographic distribution in shallow marine waters, rapid evolution, and easily recognizable features. See also {{link=belemnoid">belemnoid .