Trepostome bryozoans buck the trend and ignore calcite-aragonite seas

Abstract

Trepostome bryozoan skeletalisation did not passively respond to changes in seawater chemistry associated with calcite-aragonite seas. According to Stanley and others, trepostome bryozoans were passive hypercalcifiers. However, if this was the case, we would expect their degree of calcitic colony calcification to have decreased across the Calcite I Sea to the Aragonite II Sea at its transition in the Middle Mississippian. Data from the type species of all 184 trepostome genera from the Early Ordovician to the Late Triassic were utilised to calculate the Bryozoan Skeletal Index (BSI) as a proxy for the degree of calcification. BSI values and genus-level diversity did not decrease across the transition from the Calcite I Sea to the Aragonite II Sea. Nor were there any changes in the number of genus originations and extinctions. This suggests that trepostome bryozoans were not passive hypercalcifiers but active biomineralisers that controlled the mineralogy and robustness of their skeletons regardless of changes in seawater chemistry.

Latitudinal influences on bryozoan calcification through the Paleozoic

“…2). In other words, the BSI of trepostome taxa does not appear to be uniformly controlled by carbonate saturation state through the Paleozoic, just as trepostome taxa do not respond to calcite-aragonite sea transitions (Key et al 2022). When examined in more detail, however, earlier Paleozoic forms of the Ordovician to Devonian show, like the cryptostomes, a weak trend of higher BSI at lower latitudes (Figs.…”

“…Therefore, it might be expected that Paleozoic bryozoans should have larger or more robust skeletons in warmer lower-latitude waters and would be smaller or less robust in colder water at higher latitudes in response to the decreased saturation state with increasing latitude. Following on from Taylor and Kuklinski's (2011) and Key et al's (2022) finding that bryozoans are active and not passive biomineralizers, this paper investigates the influence of paleolatitude on the calcification of selected Paleozoic stenolaemates (Trepostomata and Cryptostomata) and the potential difficulty of calcium carbonate precipitation in these bryozoans driven by oceanographic variation in carbonate saturation state. As carbonate saturation state cannot be directly measured from the fossil record, we use paleolatitude as a proxy.…”

“…Many phylogenetically more primitive calcareous organisms, particularly phototrophic ones, are passive calcifiers, and they were influenced by changes in ocean chemistry through the Phanerozoic (Stanley and Hardie 1998, 1999), and bryozoans were initially proposed to be among this passive calcifying group. Paleozoic bryozoans are now known to be active biomineralizers (Taylor and Kuklinski 2011; Key et al 2022) rather than passive calcifiers and show no response to secular changes in ocean calcite-aragonite chemistry. Bryozoans, however, must still accommodate biomineralization issues associated with p CO 2 fluctuations and temperature-controlled changes in marine carbonate saturation state and the availability of skeleton-building ions (James and Jones 2015).…”