Article: Sculpture and microstructure of the exoskeleton in chasmopinid and phacopid trilobites

This paper describes the detailed skeletal anatomy of chasmopinid trilobites from the Ordovician of the Oslo region, and Phacops from the Polish Devonian. Lamination of the cuticle is similar to that previously reported, and the outermost prismatic layer has been conclusively proved. Several types of tubercles or pustules on the surface of the exoskeleton have been identified, for which a classification is proposed. Canals through the exoskeleton range from about 1-40 micro-m in diameter. In Chasmops spp. apparently smooth tubercles have a thin central canal through the outer zone changing downwards into a wide canal. This smooth tubercle organ shows some resemblance with the sensory campanular organ (mechanoreceptor) in Recent arthropods. Sections of the big composite tubercles or pustules in Phacops have revealed two sets of ducts, partly with different directions: (1) numerous close-set fine (4 micro-m) canals in a fan-like arrangement, and (2) diffuse ramifying branches or tubes with distal bundles of tubules, the blunt terminals of which are inserted into the lower surface of the outer zone of the exoskeleton. This internal tubules organ might possibly, in general features, be analogous with the much smaller campanular organ in the halteres in Diptera. Just below the prismatic layer there are numerous small (10-20 micro-m) subsphaeric cavities apparently connected with the epidermis by a single thin (1.5-2.0 micro-m) perpendicular canal, which are termed Osmolska cavity organs.There is a considerable variation in muscle-scar pattern. Some species have calluses on the inner surface corresponding to shallow pits on the outer surface. Similar pits and dark patches, and knobs or calluses on the inner surface of the exoskeleton, are scars or small apodemes of muscles leading from the dorsal exoskeleton to the presumed thin membrane representing the ventral body wall inside the marginal doublure.