Respiration in trilobites: A reevaluation

It has often been taken for granted that the outer branch of trilobite limbs had a gill function. Since there are no thorough analyses of the case, we decided to make an attempt. Morphological characteristics of arthropod gills are defined. There is a need for a large area/volume ratio with a cuticle thin enough to perform gaseous exchange and an anastomosing hemocoel for blood circulation. Protection and stability are often provided in carapace folds, where the upper/dorsal sheet is sclerotized and the lower one houses the gill area. Where the bivalved ostracode carapace is not attached to the body, it is such a fold. More often segmental folds are in the shape of lateral tergopleurae without ventral gill areas. Vascular channels and pillars connecting the sclerotized and non-sclerotized cuticle of the two sides are typical features, and are traced in trilobites. The trace is seen only as a pattern in the pleural exoskeleton between the axis and the doublure, exactly the area where there was no ventral exoskeleton. This therefore seems to be the place for the gills. The gill lamellae in limulids are folds from the basal part of the appendage, whereas the lamellae in trilobites are setae on a distal limb branch, the exopod; the two types of lamellar structures thus are not homologous. It was concluded in a previous study of ours that the exopod setae of trilobites had mechanical rather than respiratory functions. The conservation of the trilobite type of exopod throughout the lamellipedians indicates a fundamental role as ventilator of the gills.