Microstructure and Crystallographic Characteristics of Stenolaemate Bryozoans (Phylum Bryozoa and Class Stenolaemata)

The bryozoan Cass Stenolaemata is currently represented by the Order Cyclostomata. They produce skeletons made exclusively of calcite crystals assembled into different microstructures. Despite extensive previous research, no appropriate crystallographic techniques have been applied to decipher their crystallographic organization. We present an in-depth crystallographic study of the microstructures of three extant species (Fasciculipora ramosa, Hornera robusta, and Cinctipora elegans) using scanning electron microscopy and associated techniques (energy-dispersive spectroscopy and electron backscatter diffraction), together with atomic force microscopy and micro-computed tomography. We differentiate two microstructures, foliated and tabular calcite, and establish consistent models of their crystallography. Foliated calcite of F. ramosa and C. elegans consists of co-oriented laths arranged with their c-axes parallel to their elongation axis and to their main surfaces, and one a*-axis perpendicular to the latter (sheet texture). Tabular calcite of H. robusta consists of polygonal tablets with the c-axis as fiber axis (axial texture), perpendicular to the tablet surface. The foliated calcite of bryozoans is homeomorph to that of bivalves but has a significantly different crystallography. The tabular calcite of bryozoans lacks the spiral morphology of the tablet-shaped calcite of craniiformis brachiopods and has a different orientation of the c-axis with respect to the constituent tablets.