Calcite in the shells of Silurian brachiopods of the Pentamerida order

Fossil brachiopods are one of the least studied groups of organisms as objects of calcite biomineralisation processes. This paper considers the structure of the mineral substance in shells of (Silurian) Early Paleozoic marine invertebrates – brachiopods of two species of the Pentamerida order: Pseudoconchidium kozhimicum Nikiforov and Conchidium novosemelicum Nalivkin. The ventral valve walls of three shells, differing in the structural organisation of the prismatic zones: single-, double-, and triple-layered, have been studied. A clear hierarchy of structural elements composing the calcite shells at the micro- and macrolevels has been distinguished: subplates, prisms, blocks (columnar formations), spherulites; fibers (interprismatic) zones.