Biomineralization and Geochemistry

All stenolaemate bryozoans possess biomineralized skeletons, as do the cheilostome gymnolaemates, which are the overwhelmingly dominant order of bryozoans living today. The fine‐scale structure of these skeletons are the smallest morphological features pertinent to our understanding of bryozoan palaeobiology. Biomineralized skeletons are composite materials combining inorganic and organic phases (Dove, De Yoreo, and Weiner 2003). In the case of bryozoans, the inorganic phase consists of one or both of the calcium carbonate minerals calcite and aragonite. These minerals take the form of numerous tiny, micron‐ sized crystallites arranged in distinctive patterns. The organic phase comprises an outer covering called the cuticle, which is the equivalent of the periostracum of brachiopod and mollusc shells, as well as thin organic sheets between the crystallites. Both the cuticle and intercrystalline organic sheets in bryozoans have a predominantly proteinaceous composition but also contain chitin. Research on other biomineralizing groups has shown that biogenic crystallites are composed of numerous nanogranules, each submicron‐sized (see De Yoreo et al. 2015). The nanogranules are arranged with their crystallographic axes in the same orientation. Hence the component crystallites of the skeleton resemble single inorganic crystals. A substructure evident in some SEM images suggests that nanogranules may also be the building blocks of bryozoan crystallites. Specialized epithelial cells are responsible for the secretion of the bryozoan skeleton. At the growing edges of the cheilostome Membranipora, the secretory epithelium initially consists of columnar palisade cells responsible for formation of the cuticle (Tavener‐Smith and Williams 1972). New cells are formed at the growing edge in a conveyor belt‐like manner, pushing back the older palisade cells, which become flattened and commence secreting the mineralized part of the skeleton. Thus, mature skeletal walls have an outer cuticle underlain successively by mineralized skeleton and an epithelial layer.