SUMMARY AND CONCLUSIONS

The centroids of the internal components of the phragmocone including septa, siphuncle and possibly cameral sheets were offset from the centre in the Sinemurian ammonite, B. columbiae. When only these asymmetric components are considered in isolation, a counterbalance mechanism seems to be present. The mass of the offset siphuncle (with optional cameral sheets) is at least partially corrected by the mass of the septa, which is displaced in the opposite direction in the Z-plane. However, when the complete animal is considered at both large and small shell diameters, the individual components within the phragmocone are very small with respect to the mass of the entire animal and the effect of the offset components is virtually negligible. Thus, the existence of a weight-balancing mechanism seems unlikely. More probably, the asymmetry in the position of the siphuncle simply forced expansion of the suture line and associated septal folding on the opposite side. Although the displaced masses on either side of the plane of bilateral symmetry did counterbalance each other to some extent, their magnitude relative to the mass of the entire animal was inconsequential.