  Ammonoids are a group of extinct mollusks belonging to the same class of the
living genus Nautilus (Cephalopoda). In both Nautili and ammonoids, the
(usually planospiral) shell is divided into chambers separated by septa that
during the lifetime were filled with gas at atmospheric pressure. The
intersection of septa with the external shell generates a curve called the
suture line, which in living and most fossil Nautili is fairly uncomplicated.
In contrast, suture lines of ancient ammonoid were gently curved and during the
evolution of the group became highly complex, in some cases so extensively
frilled to be considerable as fractal curves. Numerous theories have been put
forward to explain the complexity of suture ammonoid lines. Calculations
presented here lend support to the hypothesis that complex suture lines aided
in counteracting the effect of the external water pressure. Additionally, it is
found that complex suture lines diminished shell shrinkage caused by water
pressure, and thus aided improve buoyancy. Understanding the reason for complex
sutures in ammonoids does not only represent an important issue in
paleobiology, but is also a challenging problem in the resistance of complex
mechanical structures subjected to high pressure.
