Organization, life and systematies of trilobites

The alimentary and vascular systems are discussed. The "alimentary prosopon" of polymerid trilobites is thought to show the course of the superficial dorsal vascular system. The arrangement of segments in the cephalon and thorax is investigated. Evidence of six cephalic segments is found. The articulation and enrollment mechanisms are scrutinized. Spiral enrollment evolved in Early Cambrian times and characterized a ptychopariid group which forms an evolutionary end line. Sphaeroidal enrollment is found in most other trilobites. An attempt at a new classification is made. The concepts of the Redlichiida, Phacopida, Odontopleurida, and Ptychopariida are profoundly changed, and the Illaenida are recognized as an order for the first time. Except for the Olenellida, all orders may have descended from the Redlichiina. The mode of life and feeding is diseussed, arguments being drawn from features of dorsal and ventrai morphology and from trace fossil evidence. The agnostids may have been parasitic, while the re is evidence for carnivorous habits in other instances. The exites commonly assisted in the food search in burrowing trilobites. Most trilobites were benthic but a few may have been pelagic.