Ammonites are extinct members of the Cephalopod class. Modern members include nautilus, squid and octopus. They first appeared during the Silurian Period (435 million to 410 million years ago) and were abundant and widespread in the seas of the Jurassic and Cretaceous periods (175 million to 65 million years ago). Ammonites are important index fossils—that is, they often link the rock layer in which they are found to specific geological time periods.
Ammonites varied greatly in size. The largest known as small as 2 cm (0.75 in) in diameter. During the Jurassic and Cretaceous periods, ammonites evolved more streamlined shells for swimming and the structure of the shell became stronger. Different shell shapes emerged as well, such as snail-like or uncoiled.