Crystallography of Echinoid Calcite

Each element of an echinoderm skeleton is a single crystal of calcite. The universal stage method is used to determine orientations of crystals in coronal plates of fossil and Recent regular echinoids. Stereographic diagrams are presented which relate crystallographic orientation to the symmetry of the echinoid skeleton. Most species exhibit one of two preferred orientations and show little variation within a single individual. In one of the preferred orientations, c-axes are perpendicular to the test surface; in the other, they are tangent to the test surface and are aligned longitudinally. In the present sample, crystallographic orientation shows no variation within single genera, and most echinoid families studied have but one orientation. The two preferred orientations appear to have evolved independently several times during echinoid history. Preferred orientation, as an adaptation to the environment, may be related to the organisms' sensitivity to light.