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Ammonites are distant relatives of nautiluses, the only modern cephalopods that have retained an external shell. Their history begins in the early Paleozoic. The first cephalopods had compact, conical shells. Over time, the increase in size of these animals created a significant problem. The straight, rigid shell greatly reduced mobility, making them easy prey.

A slight curvature of the shell gave a maneuverability advantage, and over time it increased until a spiral was formed. Each coil of this spiral lay adjacent to the next, forming a solid structure. This shell design made it compact and controllable. It allowed for an active lifestyle—escaping predators and hunting agile prey. Thus, 410 million years ago, in the early Devonian period, a new subclass of cephalopods appeared—ammonites.