Bryozoans are active non-phototrophic biomineralizers that precipitate their calcareous skeletons in sea-water. Carbonate saturation states vary temporally and spatially in Paleozoic oceans, and we used the Bryozoan Skeletal Index (BSI) to investigate whether bryozoan calcification is controlled by seawater chemistry in Paleozoic trepostome and cryptostome bryozoans. Our results show that cryptostome bryozoan genera are influenced by ocean chemistry throughout the Paleozoic and precipitate the most calcite per autozooid at lower latitudes, where carbonate saturation states are generally higher, and less in mid-latitudes where carbonate will be relatively undersaturated. Trepostome bryozoan genera show a similar, but weaker, trend for the Ordovician to Devonian, suggesting that like the cryptostomes they are unable to metabolically overcome falling saturation states and simply precipitate less robust skeletons at higher-latitudes. Carboniferous to Triassic trepostomes differ however and show a trend toward increased calcification at higher latitudes, indicating they are able to overcome unfavorable carbonate saturation states. Analysis of Permian trepostomes, at the species level, indicates this is most pronounced in the southern hemisphere where calcification is matched by increased feeding capacity. It is proposed that this increased feeding capacity allowed them to metabolically overcome unfavorable carbonate saturation states. The differing responses of trepostome and cryptostome bryozoans to carbonate saturation states suggest that bryozoans should not be considered as a single group in climate driven marine extinctions. Likewise, it would suggest that modern stenolaemate and gymnolaemate bryozoans should be treated separately when considering their response to modern ocean chemistry changes.

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