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 o f rhombohedral tablets, composed of granular calcite and commonly forming s lats orthogonal to the margin, associated with polysaccharides and a fibrou s 60 kDa protein; and a secondary layer of spirally growing (10.4) rhombohe dra, doped with the 60 kDa protein and interleaved with membranes of a fibr ous 44 kDa protein. The ventral valve consists exclusively of a primary lay er with the same composition and basic structure as that of the dorsal valv e. Investigation of selected antecedents shows that the chemico-structure o f the Neocrania shell has been virtually unchanged since the first appearan ce 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 seco ndary laminae. The most profound diagenetic changes occurred before the Lat e Cretaceous with proteins degrading into peptides that were dispersed, wit h the loss of less stable amino acids, during laminar recrystallization. Pa laeozoic shells suffered further recrystallization but, even after pressure solution, the original laminar fabric was replicated long after it had los t its constraining organic membranes.