Trilobites and the origin of arthropods.

While the question of whether the Arthropoda represent more than one phylum of animals is debatable, the jointed exoskeleton, a fundamental feature of arthropods, evolved independently in two groups that shared a worm-like common ancestor. The two major branches of Arthropoda, the primitively marine TCC and the primitively terrestrial (with one exception) Uniramia, independently arrived at arthropodization as the solution to the same problems of adaptation of the body mechanical system. New discoveries on trilobite anatomy show the unity of TCC as a group that shared a trilobite-like ancestor near the beginning of the Cambrian. With change in the constituency of Arthropoda through geologic time, the ways in which it would be categorized as a taxonomic group have also changed. The seeming isolation of the major modern arthropod groups is in large part an artifact of extinction of primitive intermediate forms such as trilobites which, in the Early Paleozoic, made the Arthropoda more diverse in basic modes of body organization than the group is at present. The appearance of fossilizable hard parts in arthropods resulted from shift in supporting function from the body cavity, primitively a hydrostatic skeleton, to the cuticle, which came to be strengthened in becoming an exoskeleton. Energetic efficiency, more than protection from predators or evolutionary size increase in itself, was probably the impetus behind the transition. On the scale provided by the general evolutionary trend toward progressive specialization of segments, TCC became arthropodized at earlier stages than did Uniramia. Among TCC, the shift may have been driven by the evolution of locomotory and feeding mechanisms that were exclusively geared to an aqueous medium.