Wonderful Life: The Burgess Shale and the Nature of History (13 page)

3.2. The best unretouched photo ever taken of a Burgess Shale organism. Des Collins took this photograph of a
Naraoia
, preserved in side view. This specimen does not come from Walcott’s quarry, but from one of the dozen additional localities for soft-bodied fossils recently found by Collins in the same area. Specimens from Walcott’s quarry do not photograph this well.

1.
Excavation and dissection
. If Walcott had been right, all anatomy would be compressed into a single film, and the task of reconstruction would be akin to reviving a cartoon character squashed flat by a steam-roller. But what works for Sylvester the cat in a world of fantasy cannot be duplicated for a slab of shale.

Fortunately, the Burgess fossils do not usually lie on a single bedding plane. Engulfed by the mud that buried them, the animals settled into their tombs at various orientations. The mud often infiltrated and sorted their parts into different microlayers, separated by thin veils of sediment—carapace above gills, and gills above legs—thus preserving some three-dimensional structure even when the muds became compressed later on.

By using small chisels or a very fine vibro-drill, not much different from the model in your dentist’s office, upper layers can be carefully removed to reveal internal parts beneath. (As these layers are often but microns thick, this delicate work can also be done by hand and with needles, grain by grain or flake by flake.)

3.3. Reconstruction of
Sidneyia
from a three-dimensional model built in sections by Bruton. (A) The entire animal. (B) The model in six segments, starting from bottom left—the head with its ventral covering plate below, the body in three sections, and the tail piece. (C) The head and front part of the body connected, with the head in the background and to the right. Note the biramous appendages with their walking legs below and gill branches above.

Some arthropods are fairly flat, but
Sidneyia
, as the reconstruction shows (figure 3.3), possessed considerable relief; its carapace, or outer covering, formed an arched semicylinder over the soft parts beneath.
*
In some specimens the underlying gills and legs protrude through a broken carapace, for natural 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:

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, and then passing countless hours “rotating the damned thing in my mind” from the position of one drawing to the different angle of another, until every specimen could be moved without contradiction from one stance to the next. Then he finally knew that nothing major was missing or out of place.

3.7. This specimen of
Sidneyia
reveals its last meal, a tiny trilobite preserved in the rear end of the alimentary tract. The trilobite lies in the small exposed portion of the gut (labeled
al
), just above the first abdominal segment (ab1).

3.8. Two figures from Whittington (1978), illustrating the preservation of
Aysheaia
in various positions. (A) The conventional orientation: we look down on the dorsal, or top, side; the appendages are splayed out in both directions. (B) A much less common orientation: the animal was buried on its side, and the resulting fossil shows one flank, with the appendages of both sides compressed together.

Most specimens of
Sidneyia
are preserved in full, flattened view—as if we were looking down from above (as in figure 3.5). This orientation reveals, better than any other, the basic dimensions of body parts, but must leave several questions unresolved, particularly the degree of relief, or rounding, of the body. In this orientation, we can’t tell whether
Sidneyia
was a pancake or a tube. Frontal views are needed to reconstruct the basic shape, and to determine some crucial aspects of anatomy not well seen “from above”—the form of the legs in particular.

Figure 3.9, a view from the front, shows the rounded shape of the head, and the positions of insertion for the single pair of antennae and the eyes. Figure 3.10, a head-on view from farther back, illustrates both the rounded shape of the body and a sequence of legs, with their numerous spiny segments all well preserved. We also note the dimensions of the central food groove, running between the coxae, the first segments of the legs, on each side. The gnathobases, the spiny edges of the coxae, border the food groove and give us some appreciation for the probable predatory or scavenging habits of this largest Burgess arthropod. We must assume that large pieces of food were passed forward to the mouth—no wimpy filtrate for this creature. Figure 3.11 shows a close-up of a walking leg, also in frontal orientation.