Seawater chemistry and early carbonate biomineralization.

The first appearances of aragonite and calcite skeletons in 18 animal clades that independently evolved mineralization during the late Ediacaran through the Ordovician (approximately 550 to 444 million years ago) correspond to intervals when seawater chemistry favored aragonite and calcite precipitation, respectively. Skeletal mineralogies rarely changed once skeletons evolved, despite subsequent changes in seawater chemistry. Thus, the selection of carbonate skeletal minerals appears to have been dictated by seawater chemistry at the time a clade first acquired its mineralized skeleton.