Ammonoid morphologies — Strange shapes?

Although most ammonoids are shaped like disks with an inward spiral (like the Maorites weddelliensis shown below), some ammonoids developed very unusual shells! These are known as "heteromorphic" ammonoids, meaning that they have many different forms- straight, U-shaped, and even curled into a cone like a snail shell! This great variety in shape indicates that ammonoids were quickly evolving and diversifying, which would have made them a very successful group of animals during the Cretaceous period.

The paperclip-shaped Diplomoceras maximum has such a strange shell form that it is hard to imagine how it would have moved around in the water. It was probably very unstable, but like all ammonoids, it would have been jet-propelled! Ammonoids drew water into their bodies and then squirted it out in front of them, pushing them backwards through the water.

The shell of Diplomoceras maximum is made up of straight sections connected by U-turns, causing it to be described as looking like a giant paper-clip or a saxophone!

Maorites weddelliensis has a more "typical" ammonoid shape.