Cuticular microstructure of some Silurian homalonotid trilobites from Sweden

Etched slices of the cuticle of the Silurian trilobite Homalonotus rhinotropis Angelin, 1854, from two localities in Skåne, southern Sweden, have been examined with the scanning electron microscope. The two major subdivisions of the cuticle are a “prismatic” (originally laminated) outer layer, about 20-30 μm thick, and a principal layer, 200-300 μm thick, which may be foliated or roughly laminated. The original finely-laminated aspect of the outer layer may sometimes be seen as a continuous layer, but is more frequently detected as “shaped inclusions” set in a prismatic background. These “shaped inclusions” are considered to have been produced by accumulation of minerals around the apices of 3-6 μm perpendicular canals early in calcification and preferentially preserved because of subtle differences from the products of later stages of the process. It is possible that calcification started before ecdysis. The difficulties involved in ascertaining the original structure of the principal layer are outlined, and possible reasons for differences in its preservation from the outer layer are proposed