Carbonate-producing biota on Cretaceous carbonate platforms changed from early Cretaceous aragonitic coral-algal and coral-rudist associations to Turonian-Maastrichtian calcite-dominated rudist associations. Abundance patterns of rudist bivalves examined from a large database show a shift from Barremian- Cenomanian aragonite-dominated to post-Cenomanian calcite- dominated shells. These skeletal-mineral changes agree with a decrease in the Mg/Ca ratio of seawater owing to high rates of production of oceanic crust during the Barremian−early Turonian and its effect on seawater saturation with respect to aragonite and calcite. Increasing Sr concentrations in biotic calcite from the Barremian to the Maastrichtian reflect the shift from aragonite-dominated to calcite-dominated platform sedimentation, because aragonite precipitation is a major sink for seawater Sr. It is speculated that a particularly low Mg/Ca ratio and the corresponding low supersaturation of seawater with respect to aragonite during the middle Cretaceous was a major cue for the radiation of calcite- dominated rudist bivalves, which became important carbonate producers on Turonian-Maastrichtian carbonate platforms. Changing Mg/Ca ratios of seawater are not considered to have caused middle Cretaceous crises in the evolution of carbonate platforms, but seawater composition could have selected for calcite-dominated biota when carbonate production resumed.