PHYLOGENETIC AND PALEONTOLOGICAL PERSPECTIVES ON THE EVOLUTION OF SKELETAL MINERALOGY IN CHEILOSTOME BRYOZOANS

Under the calcite-aragonite seas hypothesis, the evolutionary history of calcifying marine organisms reflects five phases in the Phanerozoic history of global seawater chemistry, alternately favoring precipitation of skeletons made of calcite or aragonite. The hypothesis proposes an exceptionally straightforward and testable link between earth and life history, but documentation of the biotic side of this story remains preliminary, especially in the context of phylogeny. Cheilostome bryozoans diversified and rose to ecological prominence across the most recent transition from calcite to aragonite seas; they make an ideal case study in the deep-time evolution of skeletal mineralogy because of their rich fossil record, recent work on their phylogenic relationships, and growing databases on skeletal chemistry. Here we explore the imprint of global seawater changes on cheilostome evolutionary history using new phylogenetic comparative analyses and study of the taxonomic composition of fossil faunas. Ancestral state reconstructions and stochastic character maps indicate dozens of independent acquisitions of partly or fully aragonitic skeletons in cheilostome lineages, with many more transitions toward the aragonitic state than away from it. Complementary findings emerge from studying the taxonomic composition of fossil cheilostome faunas: the proportion of species indirectly inferred to be non-calcitic in cheilostome faunas has doubled since the Eocene. Skeletal mineralogy has only been directly measured in fossils in a few previous studies, but their findings generally align with ours. Despite distinct challenges in interpreting phylogenetic comparative results (uncertainties in tree dating, modeling trait evolution) and the fossil record (inferring mineralogy from taxonomy, taphonomic bias), these approaches agree in pointing to a gradual, continuing, and phylogenetically widespread trend in skeletal mineralogy paralleling changes in seawater chemistry. Remaining uncertainties include the timing of this change in cheilostomes, the consequences of different mineralogies for functional ecology, and the interaction of latitudinal factors and secular changes in seawater chemistry in shaping skeletal evolution.