Texture and Microstructure of Modern Rhynchonellide Brachiopod Shells—An Ontogenetic Study

Abstract

We have investigated the texture and shell microstructure of juvenile, adolescent and adult forms of the modern brachiopod Notosaria nigricans with electron backscatter diffraction (EBSD). We examine textural and microstructural features of calcite biomineralization in the course of growth development. Based on the size and morphology of calcite crystals the shell of Nolosaria nigricans is structured into two main layers which each consist of sublayers. The primary layer on the outside of the shell shows an outer sublayer composed of nanosized crystallites and an inner sublayer with micrometre sized crystallites. The primary layer is followed inward by a secondary shell layer composed of fibrous calcite crystals. Sublayers of the secondary layer are distinguished by an alternation of domains where the morphological axis of the calcite fibers run in different directions in the plane parallel (or up to 10° inclined) to the shell surface. While in the juvenile brachiopod we have only three sublayers with distinctly oriented fibers in the secondary layer. the adult brachiopod shows several alternating domains. The crystallographic texture of both shell layers is a strong fiber texture. Calcite c-axes are perpendicular (to 22° subperpendicular) to the shell vault and rotates with the curvature of the shell. Accordingly, the calcite c-axis is perpendicular (to slightly inclined) to the morphological fiber axis. In juvenile as well as in adult Nolosaria nigricans there is a slight inclination in c-axis orientation between the shell sublayers.