Evolution of skeletal mineralogy in cheilostome bryozoans from calcite to aragonite seas

Under the calcite-aragonite seas hypothesis, the evolutionary history of calcifying marine organisms reflects changes in global seawater chemistry, alternately favoring precipitation of skeletons made of calcite or aragonite. Most calcifying groups including corals, coccolithophores, and stromatoporoids do not switch mineralogies in response to these changes; instead, they expand and dwindle as their favored seawater conditions come and go. Cheilostome bryozoans have been suggested as an exception to this rule, apparently transitioning between aragonite and calcite many times. These changes have never been surveyed in detail, but have important consequences for material properties, fossil preservation, and the capacity of marine organisms to adapt to environmental change. We used new large-scale phylogenetic, paleontological, and mineralogical data sets to analyze the evolution of skeletal mineralogy in cheilostomes as they diversified across the early Cenozoic calcite-aragonite seas transition. Ancestral state reconstructions and stochastic character maps indicate at least 50 independent acquisitions of partly or fully aragonitic skeletons from calcitic ancestors, with many more transitions toward the aragonitic state than away from it. Fossil faunas are dominated almost entirely by calcitic species in the Cretaceous, but bimineralic species become common by the Oligocene, and aragonitic species by the Pliocene-Pleistocene. Phylogenetic and fossil analyses reveal a coherent timeline consistent with the shift to aragonite seas. Cheilostome skeletal development may be predisposed to mineralogical flexibility, with adaptive consequences for colony construction and modularity.