Polished cross section of a Middle Jurassic ammonite from southern France; in the Eichstätt —Lilly Stunzi, Zurich
Any of a group of extinct shelled cephalopods, related to the modern pearly nautilus, that are commonly found as fossils in marine rocks of the 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 belemnoid.