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

Abstract

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.

Organization and Formation of the Crossed-Foliated Biomineral Microstructure of Limpet Shells

“…Then, microvilli extrude their precursor particles (amorphous calcium carbonate + organics) toward the advancing laths, thus, producing a flow across the thin extrapallial space, with the consequence that crystals would tend to orient in the directions of flow. It is known that, under hydrodynamic conditions, crystals are able to elongate in the direction of the ion flow due to differences in the growth rates of their faces. − The ability to change the growth direction at constant lattice orientation has been reported in the laths of the foliated calcite of bivalves and stenolaemate bryozoans . In the CCF material, we are dealing with a polycrystalline material such that either the individual crystals can change their crystalline orientations or the more conveniently oriented laths (in parallel to the elongation of the zebra stripes) outcompete others.…”

“…In both groups, the c -axis is approximately parallel to the growth axis of the laths; the orientations of the a -axes with respect to the main surfaces of laths are inconsistent in inarticulate brachiopods 38 , and are approximately parallel to the main surfaces in stenolaematans. 39 Accordingly, the crystallography of the laths of the Patella crossed-foliated microstructure is unique.…”

“… 46 − 48 The ability to change the growth direction at constant lattice orientation has been reported in the laths of the foliated calcite of bivalves 37 and stenolaemate bryozoans. 39 In the CCF material, we are dealing with a polycrystalline material such that either the individual crystals can change their crystalline orientations or the more conveniently oriented laths (in parallel to the elongation of the zebra stripes) outcompete others. All things considered, we suggest that, given enough time, the components of the crossed-foliated microstructure can change their crystallographic directions progressively toward the flow of the precursor particles ( Figure 8 b) induced by the mantle cells.…”

Late Cambrian Pywackia is a cnidarian, not a bryozoan: Insights from skeletal microstructure

“…1–4; Ma et al, 2014). Stenolaemate bryozoan skeletal microstructures are well documented (see Boardman, 1983; Hickey, 1987) and are therefore not illustrated here; for a review using contemporary imaging techniques, see Grenier et al (2023).…”