Structure and development of upper Permian brachiopod shells from North Iran

The calcareous shell of several Rhynchonelliform brachiopods from the Nesen Formation (Late Permian) of North Iran was studied under scanning electron microscopy and cathodoluminescence to define their ultrastructure and their state of preservation versus diagenetic alteration. The Strophomenata shell succession is composed of a cross-bladed laminar secondary layer crossed by pseudopunctae; a prismatic tertiary layer has been observed in a few species only. The Rhynchonellata shell succession is composed of a fibrous secondary layer and a prismatic tertiary layer in all the studied species. Specimens were classified into six morphological types based on layer fabric and the morphology of its structural units (laminae, fibres and prisms). Subsequently these morphological types were analyzed through cathodoluminescence to assess their eventual diagenetic alteration. Relative frequencies were calculated for each morphological type and for the degree of luminescence based on the different lithology of the host rocks to evaluate the main factors that affect shell preservation. Results show that diagenetic alteration depends on shell fabric, taxonomy and lithology of the host rocks, but it appears that the first and the second features are the most important factors in determining the fate of the shell and its preservation.