Sr/Ca and Mg/Ca ratios in biomineral CaCO3 have recently been regarded as more reliable than δ18O values as proxy for paleotemperature because they are less affected by salinity or polar ice volume. We argue, however, that vital effects can exert a greater control than paleotemperature over fossil Sr/Ca. Seasonal perturbations in isotopic data from the Eocene bivalve Venericardia planicosta reveal a gradually decreasing annual growth rate through ontogeny. High-resolution Sr/Ca ratios, analyzed with a new proton-microbe technique, increase markedly through ontogeny, however, suggesting that more Sr was incorporated as growth rate slowed rather than as a result of changing paleotemperature. Comparative δ18O and Sr/Ca data from the broadly coeval marine gastropod Clavilithes macrospira, which exhibits a linear growth rate through ontogeny, also shows a significant increase in Sr concentration with age as well as seasonal, possibly temperature-related variations. Our observations show that neither growth, calcification rate, nor temperature can be the sole factor controlling Sr incorporation into molluscan aragonite. Metabolic activity, related to factors such as temperature, salinity, age, and growth rate, is likely to exert the primary control over Sr/Ca ratios in aragonitic mollusks.