Wonderful Life: The Burgess Shale and the Nature of History (14 page)

3.
Part–counterpart
. When you split a rock to find a fossil, you get two for the price of one—the fossil itself (called the part) and the impression of the organism forced into layers above (called the counterpart)—thumb and thumbprint, if you will. The part, as the actual fossil, has been favored by scientists and collectors; the counterpart, as an impression, has less to offer in traditional evaluations. Walcott worked almost exclusively with parts, and frequently didn’t bother to keep the counterparts at all. (When he did collect counterparts, he often didn’t catalogue them with the matching parts. They ended up in different drawers or relegated to the spoil heaps of less interesting material. Some he even gave away in trade with other museums.)

For a traditional fossil, coherently made of a single piece—the shell of a clam or snail, for example—the distinction between part and counterpart is obvious. The specimen is the part; the mold on the upper surface, the counterpart. Under Walcott’s view of Burgess organisms as single films, the same clear difference applies—the film itself is the part; its impression, the less interesting counterpart.

But when Whittington revealed the three-dimensional nature of the Burgess fossils, this easy distinction and differential rating disappeared. An arthropod contains hundreds of articulating pieces; since these are preserved on several adjacent layers in the Burgess Shale, splitting a rock at a bedding plane cannot yield a clear division, with the entire organism (the part) on one surface, and only the impression (the counterpart) on the other. Any split must leave some pieces of the organism on one side, other bits on the opposite block. In fact, the distinction between part and counterpart ultimately breaks down for the Burgess fossils. You can only say that one surface preserves more interesting anatomy than the other. (By convention, the Burgess workers finally decided to designate the top view upon the organism as the part, and the view looking up as the counterpart. By this scheme, for an animal like
Sidneyia
, eyes, antennae, and other features of the external carapace are often preserved on the counterpart, legs and internal anatomy on the part.)

All expeditions from 1966 to the present have rigorously collected both part and counterpart (when preserved), keeping and cataloguing them together. Some of the greatest Burgess discoveries of the past twenty years have occurred at the Smithsonian when a Walcott counterpart, sometimes uncatalogued, sometimes even classified in a different phylum, was recognized and reunited with its part. Can you top this for a heart-warming tale, more satisfying (since less probable) than the reunion of Gabriel with Evangeline? In 1930, the Raymond expedition found a specimen of
Branchiocaris pretiosa
, an exceedingly rare arthropod with fewer than ten known examples. In 1975 (when Derek Briggs had already submitted his monograph on this species for publication), the Royal Ontario Museum expedition found the counterpart of this specimen, still lying on the talus slope in British Columbia where Raymond and his party had spurned it forty-five years before!

Obviously, if both part and counterpart contain important bits of anatomy, we must study them together if we strive for tolerable completeness in reconstruction. (In their camera lucida drawings, Whittington and colleagues have followed the convention of including information from both part and counterpart in the same figure.) Reassociation of part with counterpart has resolved a puzzle in the study of
Sidneyia
. Based on an isolated specimen, Walcott had suggested a peculiar reconstruction for the gills of
Sidneyia
. But Bruton, examining both Walcott’s part and the “counterpart which Dr. D. E. G. Briggs observantly found among uncatalogued material in the Walcott Collection” (Bruton, 1981, p. 640), discovered that the supposed gill did not belong to
Sidneyia
at all. Conway Morris later identified this fossil as a decayed and folded specimen of the priapulid worm
Ottoia prolifica
.

3.9. Camera lucida drawing of a specimen of
Sidneyia
preserved in an unusual orientation. We are looking at the front end head on, and therefore can appreciate the convexity of the animal—information that we cannot get in the usual orientation. Note in particular the positions of insertion for the antennae (labeled
Ra
and
La
) and for the eye (
e
).

3.10. A specimen of
Sidneyia
in an unusual orientation that reveals the arrangement of the legs. We are looking head on at a cross section through the front end of the body, just behind the head, and can see the first four legs on the animal’s right side, compressed together (labeled
Rl
₁
–Rl
₄
). The alimentary canal (
al
), in the center of the body, is also visible.

3.11. Camera lucida drawing of a walking leg of
Sidneyia
. Note the strong spines (labeled
gn
, for “gnathobase”) at the point of insertion for the leg into the body. This array of spines bordering the food groove suggests that the animal was a predator. The leg is so well preserved that we can count the segments and infer the orientation in life.

These three procedures—excavation, odd orientations, and part-counterpart—are guides to the three dimensional reanimation of squashed and distorted fossils. They don’t tell us much about other aspects of life among Burgess organisms—how they moved and ate, and how they grew, for example. Unfortunately, for all its virtues in preserving anatomy, the Burgess Shale, as a transported assemblage buried in a mud cloud, does not provide other kinds of evidence that more conventional faunas often include. We have no tracks or trails, no burrows, no organisms caught in the act of eating their fellows—in short, few signs of organic activity in process. For some reason not well understood (and most unfortunately), the Burgess Shale includes almost no juvenile stages of organisms.

Still, some procedures beyond those already noted have been useful in particular cases; they will be discussed in turn as the organisms enter our story. I have already mentioned the gut contents of
Sidneyia
. Other organisms have also been identified as carnivores by a study of their alimentary tracts. For example, in the gut of a priapulid worm Conway Morris found smaller members of the same species—the world’s earliest example of cannibalism—and numerous hyolithids. He also used varying degrees of decay to resolve the anatomy of the priapulid worm
Ottoia prolifica
. Bruton (for
Sidneyia, Leanchoilia
, and
Emeraldella
) and Briggs (for
Odaraia
) made three-dimensional models from composites of their drawings and photographs. Conway Morris has used injuries and patterns of growth to understand the habits of the enigmatic
Wiwaxia
. He argues (1985) that in a unique Burgess example of growth caught in the act, one specimen was buried in the process of molting—casting off an old garment for an entirely new outer coat of plates and spines.

What do scientists “do” with something like the Burgess Shale, once they have been fortunate enough to make such an outstanding discovery? They must first perform some basic chores to establish context—geological setting (age, environment, geography), mode of preservation, inventory of content. Beyond these preliminaries, since diversity is nature’s principal theme, anatomical description and taxonomic placement become the primary tasks of paleontology. Evolution produces a branching array organized as a tree of life, and our classifications reflect this genealogical order. Taxonomy is therefore the expression of evolutionary arrangement. The traditional medium for such an effort is a monograph—a descriptive paper, with photographs, drawings, and a formal taxonomic designation. Monographs are almost always too long for publication in traditional journals; museums, universities, and scientific societies have therefore established special series for these works. (As noted before, most Burgess descriptions have appeared in monographs published by the Royal Society of London in their
Philosophical Transactions
—a series for long papers.) These monographs are expensive to produce and have strictly limited circulation, mostly to libraries.

This situation has engendered the unfortunate condescension expressed toward monographs and their authors by many scientists from other disciplines. These works are dismissed as exercises in “mere description,” a kind of cataloguing that could as well be done by clerks and drones. At most, some credit may be given for care and attention to detail—but monographs do not emerge as the vanguard of creative novelty.

Some monographs are pedestrian, of course—the description of a new brachiopod or two from a well-known formation deposited during the heyday of the group’s success will raise few eyebrows—but then a great deal of workaday physics and chemistry is also dial-twirling to iterate the obvious. The best monographs are works of genius that can transform our views about subjects inspiring our passionate interest. How do we know about Lucy, the “ape-man of Java,” our Neanderthal cousins, the old man of Cro-Magnon, or any of the other human fossils that fire our imagination as fully as an Apollo landing on the moon, except by taxonomic monographs? (Of course, in these cases of acknowledged “newsworthiness,” highly touted preliminary reports long precede any technical publication, usually providing, as the cliché goes, much heat with little light.)

This famous couplet, from
A Child’s Garden of Verses,
expresses the chief delight of our natural world and the primary result of evolution
—
incredible and irreducible variety. Since the human mind (in its adult version, at least) craves order, we make sense of this variety by systems of classification. Taxonomy (the science of classification) is often undervalued as a glorified form of filing
—
with each species in its folder, like a stamp in its prescribed place in an album; but taxonomy is a fundamental and dynamic science, dedicated to exploring the causes of relationships and similarities among organisms. Classifications are theories about the basis of natural order, not dull catalogues compiled only to avoid chaos
.

Since evolution is the source of order and relationship among organisms, we want our classifications to embody the cause that makes them necessary. Hierarchical classifications work well in support of this aim because the primary topology of life’s tree
—
the joining of twigs to branches, branches to limbs, and limbs to trunks as we trace species back to ever earlier common ancestors
—
can be expressed by a system of ever more inclusive categories. (People join with apes and monkeys to make primates; primates with dogs to make mammals; mammals with reptiles to make vertebrates; vertebrates with insects to make animals, and so on. Since Linnaeus and other pre-Darwinians also used hierarchical systems, evolution is not the only possible source of order expressed by this form; but evolution by diversification does imply branching from common ancestry, and such a topology is best rendered by hierarchical classification.
)