Abstract Diversification of biocrystal arrangement patterns, incorporation of biopolymers at many scale levels and a hierarchical architecture are keys for biomaterial optimization. In contrast to molluscan microstructures that consist of ordered assemblies of regularly shaped biocrystals, shells of modern rotaliid foraminifera comprise variously sized and highly irregularly shaped crystals. Benthic and planktonic foraminiferal shell crystals are mesocrystals. Crystal morphologies range from polyhedral, fibrous, to bladed-platy, with most of them having fractal-dendritic morphologies. In this contribution we investigate the nanometer-scale structure and microstructural arrangement of the fractal-dendritic crystals and demonstrate for planktonic Pulleniatina obliquiloculata, Rotaliida, that these consist of twinned calcite. Each fractal-dendritic crystal-unit comprises twin individuals misoriented to each other by 60°. We report for the twinned crystals a unique microstructure, not yet observed for crystals of other carbonate-shelled organisms. The fractal-dendritic, twinned, crystals consist, at the site of nucleation in a rhizopodial-network, of minute fibrils. With distance away from the nucleation site, fibrils change shape and size and evolve to large, twinned, bladed/platy crystals. Arrays of the latter seam outer shell surfaces. We demonstrate that the twinned, fractal-dendritic, crystals form through growth competition at high supersaturation and pH and discuss the main determinants that lead to P. obliquiloculata calcite formation.