Evolving Mineralogy of Cheilostome Bryozoans in Calcite and Aragonite Seas

Bryozoans have evolved biomineralized skeletons twice from soft-bodied ancestors, once in the class Stenolaemata during the Ordovician, and a second time in the order Cheilostomata during the Jurassic. Whereas stenolaemate bryozoans have always employed calcite, cheilostome species may be calcitic, aragonitic or bimineralic. Published records of cheilostome mineralogy and our new analyses allow us to test the hypothesis that the mineralogy of this order responded to the switchover from calcite to aragonite seas in late Paleogene times. Among the 194 genera of recent cheilostomes for which analyses are available, about 61% are entirely calcitic, 8% entirely aragonitic and 9% entirely bimineralic, with 22% of genera containing species of varied mineralogy. Even in the absence of a phylogenetic tree for cheilostomes, the widely scattered taxonomic distribution of these different mineralogies implies multiple transitions between calcite, aragonite and bimineralic skeletons. Most primitive recent cheilostomes have calcite skeletons. Furthermore there is no evidence for aragonite in their early fossil relatives. The oldest evidence for aragonite biomineralization in fossil bryozoans is found in the latest Cretaceous, i.e. during a calcite sea. XRD analyses as well as preservational criteria (partial moulds) suggest that bryozoans increasingly employed aragonite, either alone or in combination with calcite, from the Eocene onwards. Some groups, such as free-living lunulites, provide clear evidence of switchovers from calcite to aragonite skeletons. Thus, although the first appearance of aragonitic skeletons in cheilostomes cannot be tied to seawater chemistry, the evolutionary trend towards an increasing number of aragonitic species may perhaps have been driven by the switchover to aragonite seas in the late Paleogene.