Calcite and aragonite seas and the de novo acquisition of carbonate skeletons.

A longstanding question in paleontology has been the influence of calcite and aragonite seas on the evolution of carbonate skeletons. An earlier study based on 21 taxa that evolved skeletons during the Ediacaran through Ordovician suggested that carbonate skeletal mineralogy is determined by seawater chemistry at the time skeletons first evolve in a clade. Here I test this hypothesis using an expanded dataset comprising 40 well-defined animal taxa that evolved skeletons de novo in the last 600 Myr. Of the 37 taxa whose mineralogy is known with some confidence, 25 acquired mineralogies that matched seawater chemistry of the time, whereas only two taxa acquired non-matching mineralogies. (Ten appeared during times when seawater chemistry is not well constrained.) The results suggest that calcite and aragonite seas do have a strong influence on carbonate skeletal mineralogy, however, this appears to be true only at the time mineralized skeletons first evolve. Few taxa switch mineralogies (from calcite to aragonite or vice versa) despite subsequent changes in seawater chemistry, and those that do switch do not appear to do so in response to changing aragonite-calcite seas. This suggests that there may be evolutionary constraints on skeletal mineralogy, and that although there may be increased costs associated with producing a mineralogy not favored by seawater, the costs of switching mineralogies are even greater.