The evolution of gigantism on temperate seashores

The extent to which animal lineages achieve large body size, a trait with broad advantages in competition and defence, varies in space and time according to the supply of (and demand for) resources, as well as the magnitude and effects of extinction. Using the maximum sizes of shallow‐water marine shell‐bearing molluscs belonging to nineteen guilds (groups of species with similar habits and food sources) in seven temperate regions from the Early Miocene to the Recent, the present study examined the controls on productivity and predation that enable and compel large size to evolve. The North Pacific (especially its eastern sector) has been most favourable to large‐bodied species from the Pliocene onward. Large productive kelps (Laminariales) evolved there in conjunction with herbivorous mammals, setting the stage through positive feedbacks between production and consumption for the evolution of large molluscan herbivores and suspension‐feeders. The evolution of bottom‐feeding predatory mammals together with other large predators created intense selection for large molluscan sizes. Very large molluscs in the Early Miocene were concentrated in the southern hemisphere, especially among metabolically passive species. Extinctions, which preferentially targeted the largest members of guilds in most regions, were more numerous in the southern hemisphere and the North Atlantic than in the North Pacific. Minimal disruption, together with the early evolution of metabolically‐active consumers and the positive feedbacks they engendered, accounts for the evolution of molluscan gigantism in the North Pacific.