Mechanical growth and morphogenesis of seashells

► We develop a model for seashell growth based on local aperture geometry. ► The framework can simulate realistic 3D shells with few parameters. ► We propose physical mechanisms for the morphogenesis of 3D shell ornamentation. ► The elastic response of the soft mantle tissue underlies shell morphogenesis. ► The geometrical/mechanical basis allows a range of shell ornamentation to be explored.