In contrast to modern shallow-marine ooids and carbonate muds, which are composed for the most part of aragonite, Paleozoic counterparts probably were deposited with a primary calcite mineralogy, owing to significantly lower Mg/Ca ratios in Paleozoic seas. Although evidence supporting a postulated calcite mineralogy for ancient micrites is equivocal, the fabric of coeval oolites supports such an interpretation. The fabric of carbonate invertebrate remains from Phanerozoic sequences also demonstrates a concurrent, albeit gradual, shift from calcite to aragonite in calcified marine metazoans. This suggests that changing ocean chemistry has been an important factor not only in controlling the primary mineralogy of ooids, but also in influencing the evolution of major carbonate-mineralizing groups. Organisms that secreted calcite were dominant during most of Paleozoic time, whereas groups that progressively evolved the ability to precipitate aragonite were better adapted to changing ocean chemistry, have largely replaced calcite forms, and now dominate in modern shallow-water marine communities.