DID CALCITE-ARAGONITE SEAS DRIVE PHANEROZOIC ECHINODERM DIVERSITY? EXPERIMENTAL AND MACROEVOLUTIONARY INSIGHT

Throughout the Phanerozoic, fluctuating rates of seafloor spreading have shifted ratios of seawater magnesium (Mg) and calcium (Ca), thereby controlling whether the dominant calcium carbonate precipitant is calcite, resulting in “calcite sea” intervals, or aragonite+high-Mg calcite, resulting in “aragonite sea” intervals. These secular ocean chemistry transitions have long been implicated in controlling diversification and abundance of calcifying marine organisms. However, the effect of calcite-aragonite seas on marine calcifiers can only be fully understood by integrating historical data from the fossil record (e.g., biodiversity data in a time series) with experimental studies that establish physiological responses of calcifiers to varying Mg/Ca ratios, but to date, this approach has only rarely been taken.

Here, we use echinoderms as a model system to evaluate their short-term and long-term responses to changing Mg/Ca ratios. Our experimental investigations using living ophiuroids support prior findings that echinoderm regeneration rates decrease significantly in low Mg/Ca seawater when their high-Mg skeletal mineralogy is out of phase with seawater chemistry. By contrast, analyses of Phanerozoic echinoderm diversification dynamics reveal no significant correlations between calcite-aragonite seas and patterns of echinoderm extinction, standing diversity, or net diversification, regardless of whether or not mass extinction intervals are included. This decoupling between the short-term and long-term responses of echinoderms to calcite-aragonite sea fluctuations indicates that seawater Mg/Ca ratios have not played a major role in controlling Phanerozoic echinoderm diversification dynamics. This may be due to echinoderms adapting to slow changes in seawater chemistry and/or altering their skeletal Mg/Ca fractionation. Results highlight that a uniformitarian extrapolation of experimental results over deep time may not be appropriate when studying calcite-aragonite seas. To more fully evaluate the impact of Mg/Ca ratios on the Phanerozoic diversity of calcifying clades, future work should focus on quantitative approaches that merge both experimental data and fossil timeseries data.