Aragonite and Calcite Seas and the Evolution of Carbonate Skeletons

A longstanding question in paleontology has been the role of calcite and aragonite seas in influencing the evolution of carbonate skeletons. I focused on the first appearances of carbonate biomineralization during Ediacaran through mid-Ordovician time when at least two dozen animal clades are thought to have evolved skeletons independently. Of the 19 clades whose original mineralogy could be inferred with some confidence, all those with aragonitic skeletons (N=8) appeared before those with calcitic skeletons (N=11). The sequence of skeletal mineralogies corresponds with changes in the Mg/Ca ratio of seawater (thought to be the primary driver of aragonite-calcite seas). This suggests that the principal factor determining carbonate skeletal mineralogy is the Mg/Ca ratio of seawater at the time of initial skeletal acquisition in a clade.

After skeletons evolve, however, the influence of changing aragonite-calcite seas on skeletal mineralogy appears to drop. Oscillations in seawater chemistry may affect the relative success of aragonitic vs. calcitic taxa and may cause fluctuations between low-Mg- and high-Mg-calcite mineralogy within a clade, but there are relatively few examples of an evolutionary switch from aragonite to calcite biomineralization or vice versa. Many reports of mineralogical switching instead likely reflect de novo acquisition of skeletons in a non-mineralizing clade or environmental conditions overriding biological control of mineralization.

A possible explanation for these observations is that natural selection for reduced energetic costs dictates the initial choice of mineralogy. However, because shell mineralization involves a diverse suite of proteins, many polymorph-specific, switching skeletal mineralogies may require the evolution of a new and extensive array of proteins. As a result, once mineralization has evolved, the cost of switching mineralogies may be greater than that of producing a mineralized skeleton not favored by seawater. How and why a few taxa, including molluscs and bryozoans, apparently did switch mineralogies is not known.