ABSTRACT
As we have seen in Chapters 17 through 19, a chambered shell for buoyancy with a mantle siphon for jet propulsion has been a very successful construction in Late Paleozoic and Mesozoic ammonoids. The present day “living fossil” Nautilus, equipped with a similarly coiled shell, has long served as a model for ammonoid lifestyle. But it also became clear that this model has it limits. In spiral shells, the center of buoyancy is situated above the center of gravity; therefore they are unsuited for fast jet swimming. A stronger jet would just make the animal rotate like a pinwheel. Consequently, no coiled cephalopod has an ink sack, as used by modern teuthoids and octopuses to irritate an aggressor and to quickly escape in the opposite direction. For the same reason, cephalopods with spiral shells are unlikely to have been predatory hunters, even if they could bend their siphon for slow forward swimming (Pl. 17.4). Nautilus is best characterized as a scavenger, while derived ammonoids, as Cartesian filtrators, made the best out of the phragmocone buoyancy principle but at the cost of suboptimal streamlining. Thus, they could afford to transform their beaks into pumping organs and protective opercula (Pl. 17.5).
