Structure of shells from eggs of the Australian lizard Amphibolurus barbatus

Abstract

Eggshells of the Australian bearded dragon consist of a layer of calcite overlying a multilayered, fibrous shell membrane. Some layers of the shell membrane are comprised of tightly woven feltworks of fibers, whereas other layers consist of more open mats of fibers with numerous spaces among the fibers. The shell membrane is organized into a series of shallow crests and troughs with furrows delimiting the irregular plaques of crystal that cover the shell membrane. The morphology of the outer crystalline layer varies from compact calcareous rosettes to coarsely granular aggregations of crystals. Numerous porelike areas and fissures fenestrate the crystalline material. These openings presumably provide for exchange of gases and water between the egg and its environment. The calcareous layer is organized into columns of loosely aggregated crystals extending deep into the fibrous shell membrane and enclosing fibers of the membrane. The bearded dragon represents only the second lepidosaurian found to produce a flexible-shelled egg consisting of crystalline columns that extend into the shell membrane.