Did organisms with calcitic skeletons become hypercalcified during times of calcite seas? A test using Paleozoic trepostome bryozoans

Secular changes in seawater chemistry that have driven oscillations between calcite and aragonite seas may have impacted the biomineralization of marine organisms with calcium carbonate skeletons in various ways. There are good reasons for believing that calcite biomineralizers will be favoured in calcite seas, and aragonite biomineralizers in aragonite seas. One hypothesized result of this preference is the hypercalcification of clades employing the CaCO3 mineral matching seawater type, to the extent that such clades have the potential to produce skeletons sufficiently robust to contribute to reef-building. For example, it has been suggested that Paleozoic stenolaemate bryozoans, which were exclusively calcitic, became hypercalcified during the Ordovician to Early Carboniferous. This hypothesis is here tested for trepostomes using data from the literature. Branch and exozonal wall thicknesses were compiled for dendroid trepostomes in three time intervals: Ordovician (calcite sea), Devonian (calcite sea) and Permian (aragonite sea). Hypercalcification predicts that Ordovician and Devonian trepostomes should have thicker branches and walls than Permian trepostomes. However, neither of these parameters differs significantly between calcite and aragonite seas. While it is possible that branch and wall thickness are inadequate proxies for hypercalcification, which ideally should be quantified as the mass of CaCO3 deposited per unit time, the favoured explanation for this result is that trepostome bryozoans resided closer to the biologically-controlled than biologically-induced spectrum of biomineralization than has been previously supposed.