AMMONOID CONCH ORNAMENTATION WAS NOT MERELY ORNAMENTAL: PHYSICAL TRADEOFFS BETWEEN ROCKING ATTENUATION AND DRAG REDUCTION

Ammonoid cephalopods displayed remarkable disparity in conch shapes – from first-order morphology (coiling) to second order-features (ornamentation). Conch ornamentation can be grouped into several categories, including ribs, tubercles, nodes, spines, carinae, and numerous others. These features became more ubiquitous throughout the Mesozoic, with coarser features eventually increasing in proportion relative to smooth conchs. This trend invites parallels to be drawn with other mollusks that experienced escalation of anti-predatory morphologies during the Mesozoic Marine Revolution. However, anti-predatory morphologies only explain a small subset of the observed ornamentation patterns in ammonoids. Alternatively, conch ornamentation would have had fundamental physical consequences for the swimming capabilities of these living animals. Coarse features (high amplitude and low frequency) would have been disruptive, suggesting increased hydrodynamic drag. However, this same attribute could have offered increased stabilization against rocking for these neutrally buoyant animals.

Rocking behavior was explored with five theoretical morphologies falling on a continuum of increasing ornamentation coarseness (smooth, fine, medium, coarse, and very coarse). Generalized ornamentation was constructed from amplitude and frequency measurements of 187 Mesozoic ammonoid species. Biomechanical models capable of submillimeter adjustments to their centers of mass were 3D-printed, allowing hydrostatic stability to be held constant. This property is responsible for generating a restoring torque when rotated from some equilibrium orientation. Each model was rotated underwater with a mechanical arm and released. Then, rocking behavior was monitored with 3D motion tracking. Coarser models progressively attenuate rocking amplitudes, allowing the models to come to rest more quickly. This higher hydrodynamic stability would have counteracted the rocking produced by jetting, improving directional motility. This property would have been even more beneficial for regions of the morphospace that lack hydrostatic stability; a trend supported by ornamentation patterns in the fossil record. These results highlight an evolutionary tradeoff involving conch ornament - between drag reduction and hydrodynamic stabilization.