THE IMPACT OF ASYMMETRIES IN THE ELEMENTS OF THE PHRAGMOCONE OF EARLY JURASSIC AMMONITES

ABSTRACT

In many Early Jurassic ammonites including Badouxia columbiae (Frebold 1967), the siphuncle is offset from the midline of the venter. In these cases, the septal suture line is expanded on the opposite side of the shell, suggesting an expansion of the septum on the non-siphuncle side. We use three computer models of B. columbiae to examine the hydrostatic implications of these asymmetries with respect to the presence of a possible weight-balancing mechanism. The first model is limited to the internal elements of a single chamber of the phragmocone including the siphuncle and septum. Cameral sheets are also considered mathematically as an optional component. The model indicates a counterbalance mechanism may have been present as the mass of the offset siphuncle (and optional cameral sheets) is at least partially corrected by the increased mass of the septum on the opposite side. In order to examine the affect of these asymmetries on the entire ammonite, two models representing complete ammonites at large and small shell diameters were constructed. In both cases, results show that the siphuncle and septal asymmetries within the phragmocone are very small with respect to the mass of the animal, and the effect of the offset components is insignificant. The asymmetric position of the siphuncle simply forced expansion of the septal suture line and septal folds on the non-siphuncle side of the shell but no significant counterbalance was in effect.