Structure of the organo-phosphatic shell of the brachiopod Discina

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Abstract
The secondary shell of Discina is composed of a succession of thin, impersistent, organo-phosphatic layers (laminae) pervaded by a vertical canal system and anastomosing organic strands. Four types of biomineral laminae are distinguishable, all composed of the same basic unit, an apatitic granule, between 4 and 8 nm in size, with a chitino-proteinaceous coat. These units are assembled and aggregated into spherules within the outer epithelium of the mantle. During exocytosis, spherules are further aggregated into mosaics, varying from flattened discs to spheroids, which are added incrementally to the shell succession more or less in their final polymerized and crystallized states. The distinctiveness of a lamina depends on the relative proportions of its organic and biomineral components and the aggregation of its apatitic mosaics. Compact laminae are composed almost exclusively of apatitic mosaics. In contrast, stratified, rubbly and baculate laminae respectively consist of plates, lenticles (or nodules) and rods of apatite in an increasing proportion of chitino-protein, culminating in wholly organic membranes. There is a discernible rhythmic sedimentation from a predominantly (or exclusively) organic to a mainly apatitic deposition, which reflects a recurrent cycle of secretion by the same group of cells. Spheroidal mosaics of apatite characterize the shell structure of extinct acrotretides and paterinides and probably represent traces of the original fabric. Exceptionally, swarms of inclusions, up to 1 µm or more in diameter and consisting of spheroidal mosaics grouped around central cavities, are embedded in laminar successions. The inclusions have thick, mainly proteinaceous coats and are encased in meshes of coarse particles representing nodes in the apatitic framework of the enclosing laminae. The inclusions appear to have crystallized immediately before or during exocytosis. Hemispherical hollows found in other shells are believed to be the casts of related structures. Arrays of pits and tubercles on the external surfaces of many extinct organophosphatic brachiopods may have originated in the same way. The sporadic development of discrete walls of spherular mosaics of apatite enclosing vertical canals also serve as a living model for the growth of the columnar laminae of Palaeozoic acrotretoids.

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