Brachiopod primary layer crystallography and nanostructure

Citation

Cusack M, Chung P, Dauphin Y & Perez-Huerta A (2010) Brachiopod primary layer crystallography and nanostructure. In: Alvarez F & Curry G (eds.) Evolution and Development of the Brachiopod Shell. Special Papers in Palaeontology, 84. Aberystwyth: Palaeontological Association, pp. 99-105.

Abstract
The primary layer of calcite-shelled brachiopods lacks structure and therefore differs markedly from the highly ordered secondary layer of calcite fibres in the Rhynchonelliformea and semi-nacre in the Craniiformea. Electron backscatter diffraction demonstrates that the crystallographic orientation of the primary layer is the same as that of the secondary layer calcite. In Terebratulina retusa and Terebratalia transversa, the c-axis of the primary layer calcite is perpendicular to the shell exterior as in the secondary layer fibres. In Novocrania anomala, the c-axis of the primary layer calcite is parallel to the shell exterior as in the secondary layer semi-nacre. Thus, while the primary layer may appear somewhat disordered, biological control is exerted on calcite crystallographic orientation from the first-formed primary layer. Atomic force microscopy of T. retusa primary and secondary layers indicates that the secondary layer fibres are composed of large triangular granules of fairly uniform dimensions (600 nm long), while the primary layer is composed of triangular units whose constituents are more spherical nano-granules of about 50 nm diameter. © The Palaeontological Association.

Keywords
brachiopod; calcite c-axis; primary layer; electron backscatter diffraction; atomic force microscopy