Evolution of a Rhythmic Lamination in the Organophosphatic Shells of Brachiopods

The secondary layer of organophosphatic-shelled brachiopods consists of stratiform laminae in glycosaminoglycans (GAGs), pervaded by canals with chitinous walls. The laminae are composed of various aggregates of protein-coated granules of apatite, 4–8 nm in size, 10 or so soluble proteins, β-chitinous meshes and sheets, and a possibly proteinaceous actin-like network. The succession is disposed in rhythmic sets grading between predominantly apatitic and wholly membranous laminae. The most conspicuous set consists of apatitic rods (baculi) in trellised arrays associated with laminae of compacted mosaics and spherules of apatite. The baculi are composed of mosaics accreting around axial fibrous proteins; apatitic aggregates are also scattered throughout the GAGs and accrete on proteinaceous networks and chitinous meshes. Baculi, subtended between two compact laminae, first appeared in Early Cambrian times and are a synapomorphy of lingulides. Subsequently, the secretion of one or another of the compact laminae was suppressed in the two surviving clades, with sets of the lingulid Glottidia and the discinids, respectively, grading inwardly from, and to, compact laminae. Suppression of baculate secretion also occurred in the dorsal valve of living Discinisca while, in Lingula, baculi have been replaced by botryoidal aggregates of mosaics at least since Carboniferous times.