Microstructural patterns of modern terebratulid brachiopod shells: Gryphus vitreus and Liothyrella neozelanica

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 focused extensively on the micro- and nanostructure of recent brachiopod shells, yet a gap remains in our understanding of mesoscale structural patterns. This study investigates the organization, arrangement, and thickness of different shell layers (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 three-layered modern brachiopod shells using scanning electron microscopy (SEM). Specimens belong to two terebratulid species from different settings and water depths: Gryphus vitreus (Born, 1778) from the Mediterranean Sea and Liothyrella neozelanica (Thomson, 1918) from New Zealand. Results show that 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 exhibit irregular patterns. Future studies should include other modern brachiopod species and integrate geochemical analyses to more robustly evaluate their potential as archives of environmental change.