Mechanisms of Shell Biomineralization in Recent Rhynchonelliform Brachiopods: Structure, Development, and Molecular Control

ABSTRACT

The phylum Brachiopoda, which originated in the Cambrian, includes the most diverse subphylum Rhynchonelliformea, comprising the three extant orders: Thecideida, Terebratulida, and Rhynchonellida. These animals possess a calcitic bivalve shell secreted by the mantle. Despite the long evolutionary history of brachiopods, the biomineralization processes in rhynchonelliforms remain poorly characterized. In this paper, we review published data on shell properties (morphology, microstructure, and chemistry) and mantle biology (ultrastructure, ontogeny, and molecular mechanisms) to propose a comprehensive hypothesis for their biomineralization process. We propose that shell secretion in rhynchonelliforms involves a conserved molecular mechanism shared with other carbonate‐skeleton‐producing animals. Furthermore, we suggest an ontogenetic model based on a cellular “conveyor belt” mechanism of shell secretion. The extremely narrow privileged space, together with the tight spatial association between secondary‐layer fibers and the cells of the outer mantle epithelium, indicates an exceptionally high degree of cellular control over shell growth. This precise control likely underpinned the evolution of the remarkably complex skeletal support of the lophophore in rhynchonelliform brachiopods. This apomorphic trait probably played a key role in the evolutionary success of these animals, enabling them to achieve the highest diversity and ecological dominance within the phylum.