The unique fibrilar to platy nano- and microstructure of twinned rotaliid foraminiferal shell calcite Lastam, J. Sánchez Almazo, Isabel María Checa González, Antonio G. Diversification of biocrystal arrangements, incorporation of biopolymers at many scale levels and hierarchical architectures are keys for biomaterial optimization. The planktonic rotaliid foraminifer Pulleniatina obliquiloculata displays in its shell a new kind of mesocrystal architecture. Shell formation starts with crystallization of a rhizopodial network, the primary organic sheet (POS). On one side of the POS, crystals consist of blocky domains of 1 μm. On the other side of the POS crystals have dendritic-fractal morphologies, interdigitate and reach sizes of tens of micrometers. The dendriticfractal crystals are twinned. At the site of nucleation, twinned crystals consist of minute fibrils. With distance away from the nucleation-site, fibrils evolve to bundles of crystallographically well co-oriented nanofibrils and to, twinned, platy-blade-shaped crystals that seam outer shell surfaces. The morphological nanofibril axis is the crystallographic c-axis, both are perpendicular to shell vault. The nanofibrillar calcite is polysynthetically twinned according to the 60°/[100] (= m/{001}) twin law. We demonstrate for the twinned, fractal-dendritic, crystals formation at high supersaturation and growth through crystal competition. We show also that c-axis-alignment is already induced by biopolymers of the POS and is not simply a consequence of growth competition. We discuss determinants that lead to rotaliid calcite formation. 2023-03-14T13:28:27Z 2023-03-14T13:28:27Z 2023-02-07 journal article Lastam, J... [et al.]. The unique fibrilar to platy nano- and microstructure of twinned rotaliid foraminiferal shell calcite. Sci Rep 13, 2189 (2023). [https://doi.org/10.1038/s41598-022-25082-9] https://hdl.handle.net/10481/80597 10.1038/s41598-022-25082-9 eng http://creativecommons.org/licenses/by/4.0/ open access Atribución 4.0 Internacional Nature