Hydromechanics of ammonoid conch ornamentation: trade-offs between rocking attenuation and drag reduction

Ammonoid cephalopods are excellent model systems for evolutionary biomechanics due to their volatile evolutionary dynamics and remarkable fossil record. During the Mesozoic marine revolution, natural selection increasingly favored ammonoid shells with specific ranges of ornamentation patterns (projections that influence surface roughness). While this evolutionary pattern has been attributed to enemy-driven evolution (i.e., escalation), many morphologies lack clear defensive roles. Using a combination of 3D modeling, physical experiments, and computer simulations, we investigate these patterns from a hydromechanical perspective. We model theoretical morphologies along a continuum of increasing ornamentation coarseness. Neutrally buoyant, 3D-printed models, weighted to match the mass distribution of their virtual counterparts, demonstrate that coarser patterns progressively attenuate rocking motions. Flow visualization experiments reveal these coarser patterns produce higher energy dissipation rates in the disturbed fluid. Computational fluid dynamics simulations were performed to characterize the hydrodynamic costs of ornamentation patterns over the majority of biologically relevant swimming speeds and shell sizes for planispiral ammonoids. Only the coarsest categories incur substantial increases in hydrodynamic drag. However, ornamentation patterns with intermediate coarseness effectively avoid this physical trade-off, experiencing dynamic stabilization without considerably reducing swimming efficiency. These trade-off-defying morphologies were progressively favored during the Mesozoic, becoming more abundant than others by the end of this era. Ultimately, these experiments highlight important hydromechanical selective pressures involved in ammonoid evolutionary trends and some fundamental constraints on aquatic locomotion more broadly.