It isn't known why cephalopods developed such extremely complex partitions,
and sutures, although they probably strengthened the shell as corrugations
strengthen cardboard. Another possibility is that the increasing complexity
of sutures led to decrease the bulk of total septal substance and may have
thus contributed to the solution of the buoyancy problem. This would have
served to strengthen the shells to withstand greater hydrostatic pressure,
while maintaining or even decreasing the shell's mass.
In any case, except for the Nautilus, which has simple, smooth partitions, the shelled cephalopods died out at the end of the Cretaceous. Perhaps the more specialized forms could not adjust to the new conditions at the end of the Cretaceous, but this seems unlikely for such a successful group.
Figure 13: Cephalopod Sutures, Arrow points toward the aperture