AMMONITES AS CARTESIAN DIVERS

Many constructional and taphonomic incongruencies dissolve if ammonite s are modeled as active Cartesian divers, whose last septum remained a rubber-like, ulcalcified 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 b ehind could be operated like a swim bladder by sutural muscles spannin g across the septal mantle. Other septal functions probably served as evolutionary stepping stones on the way to this unique design.