Most major features in cephalopod evolution may be interpreted as responses to the need for buoyancy control, which was a major problem in all ectocochlian shells. These early species were often too heavy. The shell of an ectocochlian shell needed to be in close approximation to hydrostatic equilibrium(neutral buoyancy), because if it were not, the animal could not have moved actively. The presence of hydrostatic appartus throughout the history of the group implies a mobile life-style. The chambered shell and the siphon were the features that ensured the organism's buoyancy, while retention of liquid in the camerae was one of the many devices used by cephalopods so they could sink to the bottom.

Figure 11: A portrayl of what a Nautiloid may have looked like on the ancient sea floor.