in sheer numbers of different kinds of body plans (described as disparity), the arthropods were the most successful of Cambrian animals. How much of this success was due to their principal body plan characteristic—segmentation?

Segmented animals are the most diverse of all animals on the planet today, and most are arthropods. All arthropods, including the highly diverse insects, show repeated body units and body regions based on groupings of individual segments that have specific functions for the animal. The feature uniting the group is the presence of a jointed exoskeleton that encloses the entire body. This exoskeleton even extends into the gut. The exoskeleton cannot grow, so it must be periodically molted and another, slightly larger, one produced. The body has a well-differentiated head, trunk, and posterior region in varying proportions. Appendages are commonly specialized. On terrestrial arthropods the appendages are usually single and enormous, but the marine forms generally have two branches or parts per appendage, an inner leg branch and an outer gill branch, and are thus termed “biramous.”

Arthropods are not alone in being segmented. All annelids are segmented, and some members of generally nonsegmented groups, such as the monoplacophoran mollusk, show segmentation. It appeared early in the history of animals and indeed the Cambrian trilobite fossils, the most common of the earliest preserved animal, show segmentation.

Reconstruction of Marella, a Cambrian arthropod from the Burgess Shale. As can be seen here, each segment behind the head bears a pair of large gills. The area of gills to overall body volume is thus very large in this kind of body plan.



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