The exoskeleton encloses the soft parts like a suit of armor and that may be its major function: protection. But the consequences of this kind of skeleton are huge: there can be no passive diffusion of oxygen across any part of the body. To obtain oxygen the first arthropods, all marine, had to evolve specialized respiratory structures or gills. The combination of segmentation and an exoskeleton used by the arthropods was clearly a design winner: the arthropods, all with this design, have more species today on Earth than any other phylum. Some (perhaps most) of that success must be due to their characteristic segmented body. In his 2004 book, The Origin of Phyla, James Valentine reflected on what is a major evolutionary puzzle: why were there so many kinds of arthropods in the Cambrian and such large populations of individuals belonging to the many kinds of this group then present? It is worthwhile to look at what he has written on this subject:
A marvelous diversity of early arthropod body types has come to light, so many and so distinctive as to pose important problems in applying the principles of systematics. The most diverse of the extinct arthropod groups is the Trilobita…. However, a large number of non-trilobite fossils with jointed bodies and appendages display great disparity in just those features that form the defining characteristics of the living higher arthropod taxa—tagmosis, including segment numbers and the number, type and placement of appendages. Most Early and Middle Cambrian forms have such unique assemblages of these characters [body parts] that they cannot be included in any of the living higher taxa as they are defined within crown groups and many of the fossil taxa are quite distinct from each other as well. These disparate arthropod types are phylogenetically puzzling…. This evidently sudden burst of evolution of arthropod-like body types is outstanding even among the Cambrian Explosion taxa.
Hence we have an interesting puzzle. What we call arthropods are composed of what appear to be many separately evolving groups that have, through convergent evolution, produced body plans of great diversity save for one aspect: all have limbs on each segment that are biramous—each appendage carries a leg of some sort and a second appendage, a long gill.
Why would early primitive (or basal) animal groups opt for segmentation? Perhaps this is the wrong word, for Valentine and others have noted that the arthropods are not so much segmented—which at least in annelids consists of largely separated chambers for each seg-