Many constructional and taphonomic incongruencies dissolve if ammonites are modeled as active Cartesian divers, whose last septum remained a rubber-like, uncalcified membrane until the next chamber cycle began. If the prochoanitic invagination of the septal mantle served as a gas gland able to produce gas at or above ambient pressure, the chamber behind could be operated like a swim bladder by sutural muscles spanning across the septal mantle. Other septal functions probably served as evolutionary stepping stones on the way to this unique design.