Evolution of a rhythmic lamination in the organophosphatic shells of brachiopods

The secondary layer of organophosphatic-shelled brachiopods consists of str atiform laminae in glycosaminoglycans (GAG;s), pervaded by canals with chit inous walls. The laminae are composed of various aggregates of protein-coat ed granules of apatite, 4-8 nm in size, 10 or so soluble proteins, P-chitin ous meshes and sheets, and a possibly proteinaceous actin-like network. The succession is disposed in rhythmic sets grading between predominantly apat itic and wholly membranous laminae. The most conspicuous set consists of ap atitic rods (baculi) in trellised arrays associated with laminae of compact ed mosaics and spherules of apatite. The baculi are composed of mosaics acc reting around axial fibrous proteins; apatitic aggregates are also scattere d throughout the GAGs and accrete on proteinaceous networks and chitinous m eshes. Baculi, subtended between two compact laminae, first appeared in Ear ly Cambrian times and are a synapomorphy of lingulides. Subsequently, the s ecretion of one or another of the compact laminae was suppressed in the two surviving clades, with sets of the lingulid Glottidia and the discinids, r espectively, 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Suppression of baculate secretion also occurred in the dorsal valve of living Discinisca while, in Lingula, baculi have been replaced by botryoidal aggregates of mo saics at least since Carboniferous times.