Evolutionary and diagenetic changes in the chemico-structure of the shell of cranioid brachiopods

The laminar dorsal valve of living Neocrania consists of: a primary layer of rhombohedral tablets, composed of granular calcite and commonly forming slats orthogonal to the margin, associated with polysaccharides and a fibrous 60 kDa protein; and a secondary layer of spirally growing (10.4) rhombohedra, doped with the 60 kDa protein and interleaved with membranes of a fibrous 44 kDa protein. The ventral valve consists exclusively of a primary layer with the same composition and basic structure as that of the dorsal valve. Investigation of selected antecedents shows that the chemico-structure of the Neocrania shell has been virtually unchanged since the first appearance of the stock in the Early Ordovician (Arenig). The greatest phylogenetic change affected the ventral valve that varied, even in Ordovician genera, from a film of calcitic blades to a complete succession of primary and secondary laminae. The most profound diagenetic changes occurred before the Late Cretaceous with proteins degrading into peptides that were dispersed, with the loss of less stable amino acids, during laminar recrystallization. Palaeozoic shells suffered further recrystallization but, even after pressure solution, the original laminar fabric was replicated long after it had lost its constraining organic membranes.