Wonderful Life: The Burgess Shale and the Nature of History by Stephen Jay Gould Page B

compression and fracturing of specimens is extensive. But Bruton found that he had to go digging in order to reveal an anatomical totality. The appendages of many marine arthropods contain two branches (see pages 104–5, in the inset on arthropod anatomy)—an outer branch bearing gills, used for respiration and swimming, and an inner branch, or walking leg, often used in feeding as well. Hence, as you cut through the outer covering over the center of the body, you first encounter the gill branches, then the leg branches. Bruton found that he could begin with a complete outer covering (figure 3.4), and then dissect through to reveal a layer of gills (figure 3.5), followed by a set of walking legs (figure 3.6). (These drawings are all done directly from the fossils themselves, using a camera lucida attached to a binocular microscope.) Bruton described his method in the conventional passive voice of technical monographs:
    Preparation of specimens shows features … to occur at successive levels within the rock and these can be revealed by carefully removing one from above the other, or by removing the thin layer of sediment that separates them.… The method of approach has been to remove successively first the dorsal exoskeleton … to reveal the filaments of the gills, and then those to expose the leg. Adjacent to the midline where the limb is attached, all three successive layers, dorsal exoskeleton—gill—leg, lie directly upon each other and it is a matter of hopefully removing an infinitely thin layer of material with the aid of a vibro-chisel (1981, pp. 623–24).

    3.4. Camera lucida drawing of a complete specimen of Sidneyia , showing the outer covering intact.

    3.5. Camera lucida drawing of a Sidneyia specimen, primarily showing the gill branches of the appendages underneath the carapace. The incomplete trace of the gut (center) is indicated by oblique stripes. The gill branches are the delicately fingered structures labeled g (the number that follows identifies the body segment).

    3.6. The walking legs are exposed underneath the gill branches. In this camera lucida drawing, the legs are labeled Rl , for “right leg” (the number that follows identifies the body segment).
    Other rewards lie beneath the outer covering. The alimentary canal runs just beneath the carapace, along the midline. One excavated specimen (figure 3.7) revealed a tiny trilobite right in the canal, near the posterior end—a remnant of Sidneyia’s last meal before the great mudslide.
    2. Odd orientations . Since the phyllopod bed was formed by several fossilized mudslides, animals are entombed in a variety of orientations. The majority were buried in their most stable hydrodynamic position, for the mud settled gradually and animals drifted to the bottom. But some came to rest on one side or at an angle—twisted or turned in various ways. In his monograph on the enigmatic Aysheaia , Whittington illustrated both the “conventional” orientation, with the animal lying flat, its appendages splayed to the sides, and one of the rarer positions, with the animal twisted and sideways, so that appendages from both sides are compressed and jumbled together (figure 3.8).
    Walcott collected specimens in odd orientations, but he tended to ignore them as less informative, and even uninterpretable in their overlapping of different surfaces on a single bedding plane. But Whittington realized that these unusual orientations are indispensable, in concert with specimens in the “standard” position, for working out the full anatomy of an organism. Just as you could not fully reconstruct a house from photos all taken from a single vantage point, “snapshots” at many angles must be combined to reconstruct a Burgess organism. Conway Morris told me that he managed to reconstruct the curious Wiwaxia —an animal with no modern relatives, and therefore no known prototype to use as a model—by drawing specimens that had been found in various orientations,