STABLE HOOKS: EFFECTS OF THE U-SHAPED BODY CHAMBER OF ADULT HETEROMORPHIC AMMONOIDS ON HYDROSTATIC STABILITY AND HYDRODYNAMIC RESTORATION

A common adult modification in heteromorphic ammonoids is the uncoiling of their shells, resulting in a coiled juvenile portion followed by a shaft that terminates into a U-shaped hook. This morphotype is characteristic for several diverse ammonoid families, including Scaphitidae and Ancyloceratidae. Uncoiling of the shell influences the organismal mass distribution and therefore hydrostatic stability. Hydrostatic stability is proportionate to the distance between the centers of buoyancy and mass, and is a metric for the degree of resistance to displacement from the equilibrium position of the living ammonoid.

Virtual models were constructed from specimens of three scaphitids, a Hoploscaphites crassus macroconch, H. crassus microconch, H. nicolletii, and one ancyloceratid, Audouliceras renauxianum. The specimens in the aforementioned order exhibit increasingly uncoiled apertures at adulthood along with increasing hydrostatic stability. These physical properties were tested in a hydrodynamic setting with neutrally-buoyant, 3D printed models that have theoretically equal masses and mass distributions to their virtual counterparts. Each 3D printed model was displaced from its equilibrium orientation to measure the magnitude of the restoring moment that corresponds to its stability.

The Hoploscaphites models and Audouliceras model both exhibit syn vivo orientations of around +90 degrees (upturned apertures at adulthood). After displacing from these orientations at equilibrium, the Hoploscaphites crassus macroconch exhibits an oscillating, nautilus-like restoration, while the H. crassus microconch, H. nicolletii, and Audouliceras renauxianum restore themselves much more quickly. This behavior is due to both increasing stability and hydrodynamic drag between each of the modeled species. These experiments suggest that more uncoiled U-shaped body chambers increase hydrostatic stability, hydrodynamic drag, and resistance to rocking. Although increasing stability may not have been the primary function of such a modification, it is nonetheless a consequence of redistributing the total organismal mass.