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Oscillations between the dominance of aragonite and calcite in abiotic marine CaCO3 precipitates throughout Earth history are closely coupled with the evolution of Earth’s seawater composition and represent the environmental context in which organisms evolved their ability to biomineralize. The most important factor controlling these Phanerozoic oscillations in CaCO3 polymorph composition is the ratio of Mg:Ca in seawater, which is thought to separate aragonite and calcite precipitation along a distinct temperature-controlled threshold. A sharp threshold, however, is contradicted by overlapping aragonite and calcite precipitation fields at a range of experimental conditions. Here we present experimental data that enable us to quantify the proportions of CaCO3 polymorphs as a function of Mg:Ca ratio and temperature. This allows us to convert published Mg:Ca ratio proxy data and models of the Phanerozoic Mg:Ca ratio into proportions of abiotic CaCO3 polymorphs at a given temperature, and thus provides a temperature-corrected view of aragonite-calcite sea conditions. In this revised view, abiotic calcite precipitation was inhibited during aragonite sea intervals at temperatures above 20 °C, whereas calcite sea intervals were characterized by the co-precipitation of aragonite and calcite in environments above 20 °C. This continuous prominence of aragonite precipitation in Phanerozoic warm-water environments explains the Phanerozoic increase of aragonite over calcite skeletal composition in calcifying organisms.

Understanding the influence of aragonite/calcite sea conditions on the evolution of biocalcification relies strongly on the correct interpretation of the original composition of calcareous taxa. Aragonite dissolves or inverts into calcite over geological time, and its preservation is currently unknown to predate the Pennsylvanian. Here we present direct evidence for the common occurrence of relic aragonite in Ordovician and Silurian trimerellid brachiopods, thereby extending the known range of aragonite preservation by more than 130 million years. Together with associated hypercalcifying taxa of putatively original aragonite or high-magnesium calcite composition and considerations of the temperature dependence of aragonite and calcite precipitation, our results suggest that the evolution of aragonite biomineralization might have presented an adaptive advantage in shallow marine tropical waters of calcite seas. A targeted search for Paleozoic aragonite should both resolve the original composition of consistently recrystallized taxa and enable the reassessment of the aragonite/calcite sea paradigm in a paleoenvironmental context.

Tommotiids are distinctive components of the early Cambrian small shelly fauna, almost invariably represented by isolated phosphatic sclerites derived from a multielement protective cover (scleritome). The unusual range of tommotiid sclerite morphologies and unknown construction of the scleritome have severely hampered our understanding of their phylogenetic affinities. However, recent description of rare, articulated scleritome material belonging to the tommotiid genera Eccentrotheca and Paterimitra support the hypothesis that some tommotiids fall within the stem group of the lophophorate phyla Phoronida and Brachiopoda and that at least some tommotiid sclerites are homologous precursors of the shells of organophosphatic brachiopods. Here we show that the shell microstructure of Eccentrotheca and Paterimitra share substantial similarities with paterinid brachiopods. While paterinids possess an overall brachiopod morphology, their microstructure appears more similar to Eccentrotheca and Paterimitra than to nonpaterinate lingulids. These findings strongly support the existence of a brachiopod total group that is solidly rooted within tommotiids, and identify the organophosphatic skeletal composition as plesiomorphic with calcareous shells as derived. The microstructural changes of the proposed tommotiid-brachiopod transition probably reflect an adaptation to fluctuating food and phosphorous intake that came with the switch to a sessile life style at the base of the tommotiid clade.