Fig. 1.

A simplified phylogenetic tree of animals showing the distribution of CaCO₃ biomineralization. The major clades in which at least some members form CaCO₃ skeletons are highlighted in red. The phylogeny is after ref. 47. Dates in black, blue, or green are molecular clock estimates for early animal divergences (48, 49), and all nodes are positioned in time according to the molecular clock estimates from refs. 48, 49. Red dates in parentheses are the ages of the oldest fossils referable to the 3 principal phyla known to use ACC precursors in skeleton formation (mollusks, echinoderms, and cnidarians). The mollusk date comes from ref. 21, the cnidaria date is from ref. 22, and the echinoderm date is from ref. 50. Beyond any molecular clock uncertainties (48), the 3 phyla started biomineralizing long after diverging from one another.