ABSTRACT
Articulate brachiopods synthesise shells composed of both structural macromolecules (lipids, proteins, and polysaccharides) and low-magnesium calcite crystals (with intracrystalline proteins). The shell is the result of an evolutionary design for a hard, impact-resistant material to protect the soft tissues of the brachiopod. The composition and architecture of the primary and secondary layers of the articulate shell have been thoroughly documented using scanning electron microscopes (SEM), whereas transmission electron microscopy (TEM) of shell structure has largely been neglected. I have used TEM to investigate the ultrastructure of the calcitic fibres of the secondary shell layer in both the extant and Plio-Pleistocene brachiopods Calloria inconspicua (punctate shell) and Notosaria nigricans (impunctate shell), and in a Late Triassic Kutchithyris sp. (punctate shell) from New Zealand. I have identified a “tweed” texture in both the Recent and fossil, punctate and impunctate shells. This texture, known as modulated microstructure, has been well documented in dolomites, ankerite, and magnesian calcite. To my knowledge, this is the first record of modulated microstructure in extant skeletal carbonate, and specifically in brachiopods. Previous explanations for the modulated microstructures in sedimentary carbonates, such as diagenetic heating or ionic Mg/Ca exchange during diagenesis causing crystal lattice distortion, are rejected for the brachiopod shells examined. I suggest that incorporation of trace metals into the structure may strain the lattice, which could result in modulated microstructure. Synchrotron X-ray fluorescence microprobe measurements confirm the presence of Cu, Fe, Mn, and Zn in both Recent and fossil brachiopods in low concentrations (700 g/cm3). Modulated microstructures in brachiopod shells could also result from proteins trapped within the calcitic fibres of the secondary shell layer, which could distort the lattice.
