HOW STABLE WERE ORTHOCONIC CEPHALOPODS? HYDRODYNAMIC ANALYSES OF RESTORING MOMENTS FROM NEUTRALLY BUOYANT, 3D PRINTED MODELS OF ECTOCOCHLEATE CEPHALOPODS

Orthoconic (straight-shelled) cephalopods were generally more hydrostatically stable than their planispiral counterparts. This is especially true for orthocones without cameral/endosiphuncular deposits that could have acted as stability-reducing counterweights. The orthocone shell ideally separates the center of mass from the center of buoyancy which results in a stable, vertical orientation during life. Hydrostatic stability is quantified by the stability index, which is proportionate to the separation between the centers of buoyancy and mass. Larger stability indices yield larger restoring moments acting to vertically align these two centers. However, in a hydrodynamic setting (after rotating the neutrally buoyant shell) there are many chaotic factors that influence the motion of restoration. This stresses the necessity of exploring how the stability indices of ectocochleate cephalopods relate to hydrodynamic restoration. These principles are essential to better understand the functional morphology of ectocochleate cephalopods.

Virtual 3D models of the extant Nautilus pompilius and the orthoconic ammonite, Baculites compressus, were constructed to compute the hydrostatic properties of these ectocochleate cephalopods during life. These hydrostatic properties include the conditions for neutral buoyancy, hydrostatic stability, and static syn vivo orientation. These properties were tested in a physical hydrodynamic setting by constructing models with theoretically identical external features and mass distributions to the virtual models. These models were then 3D printed, made neutrally buoyant, then displaced from a stable condition in order to compute their restoring moments.

The low stability of the Nautilus pompilius model behaved as an underdamped harmonic oscillator during restoration (similar to a pendulum interacting with air resistance and gravity). However, the Baculites compressus model had a more chaotic motion that followed overdamped harmonic oscillation (quick decay to equilibrium). The high hydrostatic stability of orthocones requires that a large amount of thrust be generated via active locomotion to assume horizontal orientations. This suggests that vertical migration (and rapid escape) may have been the primary form of movement for orthoconic cephalopods.