Stability-Maneuverability Tradeoffs Provided Diverse Functional Opportunities to Shelled Cephalopods.

Stability-maneuverability tradeoffs impose various constraints on aquatic locomotion. The fossil record houses a massive morphological dataset that documents how organisms have encountered these tradeoffs in an evolutionary framework. Externally shelled cephalopods (e.g., ammonoids and nautiloids) are excellent targets to study physical tradeoffs because they experimented with numerous conch morphologies during their long-lived evolutionary history (around 0.5 billion years). The tradeoff between hydrostatic stability and maneuverability was investigated with neutrally buoyant biomimetic models, engineered to have the same mass distributions computed for their once-living counterparts. Monitoring rocking behavior with 3D motion tracking reveals how stability influenced the life habits of these animals. Cephalopods with short body chambers and rapid whorl expansion (oxycones) more quickly attenuate rocking, while cephalopods with long body chambers (serpenticones and sphaerocones) had improved pitch maneuverability. Disparate conch morphologies presented broad functional opportunities to these animals, imposing several advantages and consequences across the morphospace. These animals navigated inescapable physical constraints enforced by conch geometry, illuminating key relationships between functional diversity and morphological disparity in aquatic ecosystems. Our modeling techniques correct for differences in material properties between physical models and those inferred for their living counterparts. This approach provides engineering solutions to the obstacles created by buoyancy, mass distributions, and moments of inertia, permitting more lifelike, free-swimming biomechanical models and aquatic robots.