Article: Cuticular ultrastructure of the trilobite Ellipsocephalus polytomus from the Middle Cambrian of Öland, Sweden

Hand specimens and polished sections of the cuticle of the trilobite Ellipsocephalus polytomus Linnarsson from the Middle Cambrian of Oland, Sweden have been examined in incident light and, after etching, with the scanning electron microscope. A thin (25-50 micro m) outer layer comprises about twenty lamina units; the structure of these units is interpreted as representing the original inorganic material of the cuticle, and therefore also reflecting the structure of the original organic template. X-ray microanalysis strongly suggests that this outer layer is now composed of calcium phosphate. Cavities, 15 micro m in diameter, in the outer layer connect to 3 micro m diameter canals which extend across the principal layer of the cuticle: these resemble the gland ducts of a Recent millipede. Pore canal pathways may be represented by elongate openings on the undersurface of the outer layer, and structures resembling the interprismatic septa of Recent decapod crustaceans are seen in angled slices. Other primary microstructures identified are relict organic material and fibres which may have bound together the major layers of the cuticle. Horizontal tubules on the undersurface of the outer layer are possibly infilled borings of cyanobacteria. Major subdivisions of Ellipsocephalus cuticle in life are proposed as: a very thin outermost epicuticle, an outer laminated layer, and a principal layer, the original structure of which is represented only by disc-like extensions on the perpendicular canals which pass across it.