The orientation of heteromorph ammonites depends upon on the positioning of the soft body parts of the animal within the shell. We propose that the body was a relatively small, compact mass only partially filling the living chamber. As the body moved from the aperture to the final septum and then back again, the centre of mass would be predicted to move in such a way as to cause the shell to rotate. The magnitude of this change is a function of shell shape, especially of coiling pattern. This effect may provide a functional explanation of heteromorph shell geometry. In addition, wider application of the model may lead to reinterpretations of the life modes of other ammonite groups.