An extinct aquatic mollusc of the class Cephalopoda. Ammonites were abundant in the Mesozoic era (225–65 mya) and are commonly found as fossils in rock strata of that time, being used as index fossils for the Jurassic period. They were characterized by a coiled shell divided into many chambers, which acted as a buoyancy aid. The external suture lines on these shells increased in complexity with the advance of the group.