Chemico-structural evolution of linguloid brachiopod shells

Chemico-structures of shells representing all families presently assigned to the Linguloidea have undergone significant transformations since the Early Cambrian. Superficial hemispherical to hemi-ellipsoidal pits on the larval and/or mature shells are interpreted as casts of deformable, membrane-bound vesicles of mucus or rigid vesicles of glycoproteins or GAGs with thickened coats. Flat-bottomed, sub-circular imprints characterize acrotheloids and many acrotretides, and could be impressions of biconvex tablets of apatite like those exocytosed within the primary layer of the obolid ‘Lingulella’? antiquissima, whilst the rhomboidal imprints of the Paterula shell could have held tablets of proteinaceous silica like those of living discinid larvae. The ancestral fabric of the linguloid secondary layer was probably composed of rubbly and virgose sets, but trellised rods of apatite (baculation) are characteristic of most linguloids and also acrotheloids. This condition was suppressed in shells identified as ‘Lingula’ from at least the Early Carboniferous to the present day. In early Palaeozoic acrotretides and lingulellotretids, columnar and camerate fabrics evolved in place of baculation. Baculation in Discinisca tenuis and Glottidia pyramidata is associated with the amino acids glutamic acid, glycine, alanine, arginine and proline which may be components of an organic polymer axial to baculate accretion.