Mineralogy of cheilostome bryozoans across the Eocene–Oligocene boundary in Mississippi, USA

Abstract

Different cheilostome bryozoan species construct their skeletons from calcite, aragonite, or a combination of these two minerals. Calcite is the primitive biomineral in cheilostomes, but an increasing number of clades since the Late Cretaceous have evolved the ability to biomineralize aragonite. This change in bryozoan mineralogy paralleled the switchover from calcite to aragonite seas. Here, we provide a first test of whether changing bryozoan biomineralogy can be correlated with a particular calcite to aragonite sea transition by comparing the mineralogical composition of bryozoan assemblages from the Eocene with those from the Early Oligocene, a time of major climatic and oceanographical changes that many believe to be a time of transition. Bryozoan assemblages from deposits in Mississippi and western Alabama preserving skeletal aragonite (as indicated by the occurrence of aragonitic molluscs) were sampled, cleaned, and sorted by species for XRD analysis to determine their mineralogy. The hypothesis that the proportion of species employing aragonite increases from the Eocene into the Oligocene through the transition to aragonite seas is not supported by our data. Among 23 Eocene species, 14 (61 %) were found to be calcitic, 7 (30 %) aragonitic and 2 (9 %) bimineralic, whereas among 28 Oligocene species 19 (68 %) were calcitic, 5 (18 %) aragonitic and 4 (14 %) bimineralic. Possible reasons why the expected mineralogical change is lacking include incorrect dating of the calcite to aragonite sea switchover at the Eocene–Oligocene boundary, the compensating effects of cooling at this time favouring calcite over aragonite biomineralization, or high levels of biological control over their mineralogy by the bryozoans.