Fig. 1. Phylogenetic distribution of CaCO₃ biomineralization in animals.

Besides animals (shown), there are species (not shown) that make CaCO₃ skeletons in the foraminiferans, coccolithophorids, green algae, red algae, dinoflagellates, and even a few amoebozoans and brown algae. CaCO₃ skeleton–forming animals are shown in turquoise font; dark gray font indicates clades that form nonskeletal CaCO₃ biominerals, and light gray font indicates those that do not form CaCO₃ at all. Skeletons confirmed to be formed in part by particle attachment (PA) are underlined, and the age of the oldest unambiguous fossil found to date is in blue font. All fossil age estimates have an uncertainty of a few million years. For hemichordates, biomineralized fossils have not yet been identified. Data are from (15, 215, 242–246). All clades started biomineralizing after they diverged from one another.