Nature Geosci. doi:10.1038/ngeo251 (2008)

Corals, sponges, bivalves and other calcifying marine creatures secrete their calcium carbonate shells and skeletons in the form of aragonite, calcite or both. The evolutionary successes of aragonitic and calcitic genera over the past 500 million years were influenced much more strongly by mass extinctions than by the chemical composition of the seas, researchers have found.

This is surprising because the oceanic ratio of magnesium to calcium has fluctuated throughout this period; a high ratio facilitates aragonite precipitation, a low one promotes calcite precipitation. But Wolfgang Kiessling of Humboldt University in Berlin, Germany, and his colleagues show that this cycling does not match the relative abundances of aragonite- and calcite-secretors in the fossil record. The mineralization changes correlate with mass extinctions, which seem to alter the fortunes of the two categories at random.