ment of the body—as repeated. Valentine proposes that this striking body plan arose in response to locomotory needs, stating:
Clearly, the segmented nature of the arthropod body is related to the mechanics of body movement, particularly to locomotion, with nerve and blood supplies in support.
There is no doubt that this type of body plan is an adaptation that aids locomotion. But here we can depart from Valentine and suggest that the main function of this kind of body plan is to allow repeated gill segments, each small enough to be held in optimal orientation beneath the segments. The flow of water across these gills, while at first glance appearing to be passive, may actually represent a pumping gill.
Look at trilobite morphology. While the upper surfaces of the trilobite carapace is commonly preserved, the underside, bearing appendages and gills, is rarely preserved.
Trilobites have long been known to have a curious food acquisition system. The same appendage terminations mark the fusion of the walking legs and gills and end in a blunt paddle structure known as a gnathobase. Paleontologists have long surmised that a forward-moving current created by limb movement would move food material to the mouth underneath the animal. But such a current would also serve to bring new water across the gills, and with the carapace of the animal sitting overhead like a roof or tent, the trilobite or other arthropod could build a directed water current defined by the body above and the sediment or sea bottom below. I propose that oxygen acquisition purposes primarily led to this body plan and that it was only secondarily co-opted for food acquisition. The environmental conditions of oxygen levels far lower than those of today would have provided the stimulus to begin this evolutionary pathway leading to the arthropod body plan.
Hypothesis 3.1: The repeated-segment body plan came about to allow the formation of a large gill surface area, with the gnathobase water current system evolving as part of this respiratory structure. The overall shape of the arthropod creates a defacto pump gill system. Segmentation evolved as a way of increasing gill surface area during the latest Neoproterozoic (from about 600 million years ago to 538 million years ago).