Chemico-structure of the organophosphatic shells of siphonotretide brachiopods

The organophosphatic shell of siphonotretide brachiopods is stratiform with orthodoxly secreted primary and secondary layers. The dominant apatitic constituents of the secondary layer are prismatic laths and rods arranged in monolayers (occasionally in cross-bladed successions), normally recrystallized as platy laminae. Sporadically distributed, interlaminar, lenticular chambers, containing apatitic meshes of laths and aggregates of plates and spherulites, probably represent degraded, localized exudations of glycosaminoglycans (GAGs) with dispersed apatite. The shells of Helmersenia and Gorchakovia are perforated by canals with external depressions (antechambers) that possibly contained chitinous tubercles in vivo. The immature shell of Siphonotreta and most other siphonotretids is similarly perforated and pitted; but the mature part bears recumbent, rheomorphic, hollow spines that grew forward out of pits. Internally, spines pierce the shell as independent structures to terminate as pillars in GAGs chambers. Spines and pillars were probably secreted by collectives of specialized cells (acanthoblasts) within the mantle. The shell of the oldest siphonotretide, Schizambon, is imperforate but the ventral valve has a pedicle foramen that lies forward of the posterior margin of the juvenile valve. This relationship characterizes all siphonotretides, suggesting that the pedicle, in vivo, originated within the ventral outer epithelium and not from the posterior body wall as in lingulides.