Biogenic calcite granules - are brachiopods different ?
Résumé
Brachiopods are still one of the least studied groups of organisms in terms of biomineralization despite recent studies indicating the presence of highly complex biomineral structures, particularly in taxa with calcitic shells. Here, we analyze the nanostructure of calcite biominerals, fibres and semi-nacre tablets, in brachiopod shells by high-resolution scanning electron microscopy (SEM) and atomic force microscopy (AFM). We demonstrate that basic mechanisms of carbonate biomineralization are not uniform within the phylum, with semi-nacre tablets composed of spherical aggregates with sub-rounded granules and fibres composed of large, triangular or rod-like particles composed of small sub-rounded granules (40-60 nm). Additionally, proteinaceous envelopes surrounding calcite fibres have been shown for the first time to have a dual function: providing a micro-environment in which granules are produced and acting as the organic template for particle orientation as fibre components. In summary, these new findings in brachiopod shells reveal a complex and distinctive style of biomineralization among carbonate-producing organisms.