Seawater chemistry and biomineralization: did trepostome bryozoans become hypercalcified in the ‘calcite sea’ of the Ordovician?

Secular variations in the proportion of Mg and Ca ions in seawater during the Phanerozoic have driven alternations between calcite seas (Mg:Ca < 2) and aragonite seas (Mg:Ca > 2). There is mounting evidence that these changes in seawater chemistry have impacted the evolution of marine organisms constructing calcareous skeletons, favouring calcite as the CaCO3 biomineral during times of calcite seas but aragonite during times of aragonite seas. It has been suggested that some organisms became hypercalcified when the mineralogy of their skeletons matched seawater type. This paper tests the proposal that calcitic trepostome bryozoans (‘stony bryozoans’) became hypercalcified in the calcite sea of the Ordovician. Data on two independent hypercalcification proxies—the diameter of branches, and exozonal wall thickness—have been compiled from the literature for ramose trepostome species from the Ordovician (calcite sea), Devonian (calcite sea) and Permian (aragonite sea). No significant difference was found in branch diameter between the calcite and aragonite sea periods, whereas wall thickness was found to be greater in the Permian than in the Ordovician and Devonian, counter to expectations. Either these two parameters are inadequate as proxies for hypercalcification or, more likely, trepostomes did not become hypercalcified in the calcite sea of the Early Palaeozoic, probably because they exerted a higher degree of control over their biomineralization than some other groups such as corals.