Ammonite

Ammonite, common name for an extinct group of cephalopods that usually had a tightly coiled, spiral-shaped shell. These squid-like animals appeared during the Devonian period approximately 400 million years ago and died out along with the dinosaur lineages at the end of the Cretaceous period 65 million years ago. The shell of the ammonite was chambered, like that of its still-living relative, the nautilus. By pumping water into or out of the chambers, the animal could move down and up in the water. The animal lived in the last and largest chamber. Some kinds of ammonites, however, did not have a tightly coiled shell; Baculites, for example, had a straight shell.

Ammonites abounded in shallow marine environments on the edges of continents, although they were rather slow swimmers and avoided extremely shallow wave-dominated waters close to shore. Ammonites also thrived in continental seas such as those that covered the interior of North America during the Late Cretaceous period, and in the ancient Tethys Sea that once covered southern Europe, northern Africa, and parts of Asia.

Ammonites went through distinct, easily recognizable morphological changes through time, and are therefore very useful indicators of age for the marine environment. Comparing the types of ammonite fossils in rock layers indicates the relative ages of the rock.

Scientific classification: Ammonites belong to the phylum Mollusca, the class Cephalopoda, and the subclass Ammonoidea.