Ammonoids are related to the modern nautilus. Ammonoids lived in a chambered, coiled shell, the many chambers connected by a thin tube. They were able to change the contents of the chambers to alter their buoyancy. They were hunters, able to move quickly in search of prey.

Ammonoids developed a pattern of ribbed shells. The junction between the chamber walls and the outer shell is called the suture.

The earliest ammonoids are called goniatites (Devonian to Permian periods) and have a very simple suture pattern. They were followed by the ammonites during the Jurassic and Cretaceous periods. Ammonites developed a very complicated suture pattern that makes each species easy to identify.