Article: Buoyancy control and siphuncle function in ammonoids

The question of buoyancy control of ammonoids in relation to the function of the siphuncle is analysed in the light of flotational experiments on exact models of moderately evolute and highly evofute shell types, and the structure of the siphuncle. It is demonstrated that, if the mode of life of the ammonoid animal were analogous to that of living Nautilus, the relatively more buoyant shell of most ammonoids would have needed considerably more liquid in its chambers than Nautilus, with many of the chambers completely filled. The structure of the siphuncle, and its location in the last chambers of the majority of coiled ammonoids, is such that it may have been non-functional in these chambers so that the animal did not vary the quantity of liquid in them. We believe that most ammonoids were fairly efficient at moving themselves vertically but less efficient as swimmers.