Testing the biomineral archive: microstructural patterns of modern brachiopod shells

Biominerals, such as marine macroinvertebrate shells, serve as valuable archives for reconstructing environmental conditions in the recent and distant past. Brachiopod shells are among the most reliable high-resolution biomineral archives of climate and environmental change, as they resist diagenetic alteration due to their low-Mg calcite composition, are abundant and widespread in the fossil record, and precipitate shell material in near equilibrium with ambient seawater, with limited vital effects. Studying modern brachiopod shells is thus crucial to assess their potential as reliable archives of past environmental variability. Previous research has extensively examined the micro- and nanostructure of modern brachiopod shells, yet our understanding of their mesoscale structural patterns remains limited. This study investigates the organization, arrangement, and thickness of different shell fabrics (i.e., primary dendritic, secondary fibrous, and tertiary columnar) to identify systematic patterns of variation at interspecific, intraspecific, and intra-shell levels and how these relate to geochemical variation. A microstructural analysis was conducted on several two- and three-layered modern brachiopod shells using a JSM-IT500 (JEOL Ltd) scanning electron microscope (SEM). Specimens belong to eight terebratulid and rhynchonellid species from different settings and water depths: Liothyrella neozelanica (Thomson, 1918), Calloria inconspicua (Sowerby, 1846), Magasella sanguinea (Leach, 1814) and Notosaria nigricans (Sowerby, 1846) from New Zealand, Liothyrella uva (Broderip, 1833) from Antarctica, Gryphus vitreus (Born, 1778) from the Mediterranean Sea, and Terebratalia transversa (Sowerby, 1846) and Hemithiris psittacea (Gmelin, 1791) from NW USA. Results show differences between the three-layered species: G. vitreus displays a more regular and ordered microstructure, whereas L. neozelanica has frequent intercalations of fibrous and columnar layers. In both species, the tertiary layer is thicker in the central shell portion but thins toward the anterior margin until disappearing, while the umbonal and anterior regions present irregular patterns. Two-layered species exhibit interspecific variation while maintaining the typical shell architecture composed of an external thin primary layer and an inner fibrous fabric. No significant differences were observed between the ventral and dorsal valves in either the two- or three-layered species examined. Future studies should integrate these mesoscale structural patterns of extant brachiopod shells with high-resolution geochemical analyses to deepen our understanding of brachiopod biomineralization and further assess their reliability as environmental proxy archives.