Chemico-structural differentiation of the organocalcitic shells of rhynchonellate brachiopods

Citation

Cusack M & Williams A (2001) Chemico-structural differentiation of the organocalcitic shells of rhynchonellate brachiopods. In: Long SL, Howard C, Brunton C, Robin L & Cocks M (eds.) Brachiopods: past and present. London: Taylor & Francis, pp. 17-27.

Abstract
The standard shell structure of rhynchonellate brachiopods, as typified by the living rhynchonellate Notosaria consists of a primary layer of growth-banded calcite succeeded by a secondary layer of fibres sheathed in interconnected glycoproteinaceous membranes. The extreme variations include the atrophy of the secondary layer in living thecideidines, like Thecidellina, and the development of a tertiary layer of 'prismatic' calcite in the terebratulide, Liothyrella. An ultrastructural comparison of the three kinds of shells shows that they are virtually identical in the correlated layers. The basic calcitic unit is a granule, ca. 15nm in diameter, aggregated as rhombohedral tablets and spherules that are probably 'glued' together by a polysaccharide to form laminae. In the primary and tertiary layers, laminae are nearly horizontal. In secondary layers, laminae are variably inclined but normally retain the same crystallographic alignment not only in the contiguous fibres but also with those of the primary and tertiary layers. Laminae of fibres commonly aggregate as rhombohedral laths. Such laths cleave parallel with rhombohedral angles subtended by the proximal boundaries of terminal faces. Up to twenty-one soluble intracrystalline proteins of different molecular weights, varying from 16 to 109kDa, have been extracted but none could be identified as specific to, and therefore involved exclusively in the calcification of, one or other of the principal layers of the shell. Some proteins are actin-like strands ramifying the shell; others, occupying grooves and pits, could be inclusions of intercrystalline substrates. Many represent molecular transformations that occurred after the divergence of the thecideidines and the terebratulides from their sister rhynchonellides.