Wonderful Life: The Burgess Shale and the Nature of History (28 page)

The pathway had not been smooth and direct, clearly marked by the weight of evidence and logic of argument. Intellectual transformations never proceed so simply. The flow of interpretation had meandered and backtracked, mired itself for a time in a variety of abandoned hypotheses (on the primitive status of Burgess oddballs, for example), but finally moved on to explosive disparity.

By 1978, the new conception had settled, as symbolized by Whittington’s interpretation of
Aysheaia
. The period thereafter, and continuing today—Act 5 of my drama—possesses a new calm, in shared confidence about the general status of the Burgess fauna. Yet this final act is no anticlimax in its unaltered conceptual scheme. For confidence has a great practical virtue—you can go forward on specifics without continual worry about basic principles. Hence, Act 5 has witnessed an extraordinary productivity in the resolution of Burgess organisms. Old mysteries have fallen like ranks of tin soldiers—not quite so easily as child’s play (to continue the simile), but with much greater efficiency now that a firm framework guides a coherent effort. The reconstructions of the last decade include some of the strangest and most exciting of Burgess creatures. I can hardly wait to read Act 6.

At the end of 1978, the scorecard for soft-bodied arthropods spoke strongly for uniqueness and disparity. Four genera—
Marrella, Yohoia, Burgessia
, and
Branchiocaris
—had been orphaned within the arthropods. Only
Canadaspis
(and perhaps
Perspicaris
) belonged to a modern group;
Naraoia
had been reclassified as a trilobite, but as a surpassingly odd member of the group, and the prototype of a new order.
Opabinia
had been tossed out of the arthropods altogether, and
Aysheaia
lay in limbo. A good beginning, but not yet imbued with the convincing weight of numbers. As I argued above, the “big” questions of natural history are answered as relative frequencies. More data were required—something approaching a complete compendium of Burgess arthropods. Act 5 has now fulfilled this need, and the revisionary pattern has held, in spades.

In 1981, Derek Briggs continued his dispersion of the bivalved arthropods into a series of orphaned groups (with
Canadaspis
holding increasingly lonely vigil as a true crustacean). Briggs used all twenty-nine specimens to decide the fate of
Odaraia
, the largest bivalved arthropod in the Burgess Shale (up to six inches long). At the front of its head, and extending beyond the carapace,
Odaraia
bears the largest eyes of any Burgess arthropod (figure 3.44). Yet Briggs could find only one other structure on the head—a single pair of short ventral appendages behind the mouth. (This arrangement, with no antennae and only one post-oral pair of appendages, is unique, and would be sufficient in itself to mark
Odaraia
as an orphan among arthropods. But the head is not well preserved under the strong carapace of
Odaraia
, and Briggs was not confident that he had been able to resolve all structures.) The trunk, enclosed by the large carapace for more than two-thirds of its length, contained up to forty-five limb-bearing segments. The limbs, except perhaps for the first two pairs, are typically biramous.

3.44. Reconstruction of the arthropod
Odaraia
by Briggs (1981a). (A) Top view, showing the bivalved carapace as transparent so that the soft anatomy may be revealed beneath. Note the projection of the eyes in front of the carapace, and the arrangement of the three-pronged tail behind. (B) Side view.

3.45.
Odaraia
, swimming on its back. The numerous biramous appendages can be seen through the transparent tubular carapace. Note also the large eyes in front, the curious three-pronged tail behind, and the single pair of feeding appendages behind the mouth. Drawn by Marianne Collins.

Odaraia
also exhibits two unique and peculiar specializations. This animal bears a three-pronged tail (figure 3.45), with two lateral flukes and one dorsal projection—a bizarre structure that evokes images of sharks or whales, rather than lobsters. Nothing similar exists in any other arthropod. Second, the bivalved carapace is not flattened, but essentially tubular. Moreover, Briggs argued that the relatively short appendages did not extend beyond the tube—and furthermore, that the two valves forming the tube probably couldn’t gape widely enough to let the appendages protrude from any ventral opening. Clearly,
Odaraia
did not walk on the sea floor. Briggs wrote: “The combination of an essentially tubular carapace and a telson bearing these large flukes is unique among the arthropods” (1981a, p. 542).

Briggs performed a functional study and united these two peculiarities to infer a mode of life for
Odaraia
. He argued that
Odaraia
swam on its back, using its three-pronged tail for stabilization and steering, and its carapace as a filtering chamber for capturing food. Water could be taken in at one end; the appendages would extract food particles and pass the depleted stream out the other end of the carapace.

Briggs had proven once again that the watchword for Burgess arthropods was “uniquely specialized,” not “primitively simple.” In September 1988, Derek wrote to me, in an assessment of his 1981 monograph: “
Odaraia
turned out to be not only taxonomically unusual but, more importantly in my view,
functionally unique
among the arthropods.”

Also in 1981, David Bruton published his monograph on
Sidneyia
, already discussed on pages 87–96. The resolution of
Sidneyia
set an important milestone in the study of Burgess arthropods for two reasons. First,
Sidneyia
had long acted as a focus or symbol for the fauna. Walcott regarded this genus as the largest of Burgess arthropods (we now know that the soft-bodied trilobite
Tegopelte
and one or two of the bivalved arthropods were bigger). Moreover, he mistakenly assumed that a spine-studded appendage, found separately, fitted onto the head of
Sidneyia
(for he knew nothing else big enough to carry such an appendage). With this addition
Sidneyia
was not only large, but also fierce. Since our culture values these traits,
Sidneyia
attracted attention. (A psychologist friend of mine explains our society’s fascination with dinosaurs by a simple list—“big, fierce, and extinct.”
Sidneyia
, in Walcott’s reconstruction, is all three). In Bruton’s revision,
Sidneyia
is still a predator, but the pair of limbs belongs to
Anomalocaris. Sidneyia
carries no feeding structures on its head.

Second,
Sidneyia
was the first form to be redescribed in the final, potentially coherent group of Burgess arthropods—the so-called “merostomoids.” Hope had surely faltered for placing any major Burgess assemblage in a modern group, but the “merostomoids” represented a last gasp and opportunity for traditionalism. Merostomes are a group of marine arthropods including modern horseshoe crabs and fossil eurypterids. They are united with spiders, scorpions, and mites into one of the four great arthropod groups, the Chelicerata. The basic merostome body plan—more clearly shown by eurypterids, than by horseshoe crabs—includes a strong head shield, a trunk of several broad segments equal in width to the head, and a narrower tail, often forming a spike. Several Burgess genera, including
Sidneyia
, share this basic form.

Bruton dashed the final hope for traditionalism by showing that
Sidneyia
could not be a close relative or ancestor of merostomes. The “merostomoid” body did not define a coherent evolutionary group, but a series of disparate creatures united only by what our jargon calls a symplesiomorphic (or “shared primitive”) trait. Shared primitive traits are ancestral for large groups, and therefore cannot define subgroups within the entire assemblage. For example, rats, people, and ancestral horses do not form a genealogical group within the mammals just because all have five toes. Five toes is an ancestral trait for Mammalia as a whole. Some creatures retain this initial condition; many others evolve modifications. The “merostomoid” body form is a shared primitive trait of many arthropods. True genealogical groups, by contrast, are based on shared
derived
characters—the unique specializations of their common ancestors.

True chelicerates have six pairs of appendages, and no antennae, on their head shield.
Sidneyia
could not be more different in this crucial respect. Its head (figure 3.46) bears one pair of antennae, and no other appendages! Bruton came to regard
Sidneyia
as a curious mosaic of characters. The first four of nine body segments carry uniramous walking legs like those of merostomes. But the five posterior segments bear ordinary biramous appendages, with gill branches and walking legs. The “tail” piece, formed of three cylindrical segments and a caudal fan, looks more crustacean than merostomoid. Bruton found ostracodes, hyolithids, and small trilobites in
Sidneyia
’s gut, and interpreted the animal as a bottom-dwelling carnivore. But with no feeding appendages on the head, and a strong, tooth-lined food groove between the legs,
Sidneyia
presumably fed like most arthropods, by passing food toward the mouth from the rear, not by searching and grasping from the front.

3.46. Two views of
Sidneyia:
top, as seen from below, showing the form of the limbs and the attachment of eyes and antennae; and bottom, as seen from above. Drawn by Marianne Collins.

The year 1981 was pivotal for Burgess arthropods, and for the final dispersal of the last remaining “merostomoid” hope. For, in the same year of
Odaraia
and
Sidneyia
, Whittington published his “mop-up” monograph, “Rare Arthropods from the Burgess Shale, Middle Cambrian, British Columbia.” Most or all of these animals had fallen (or would have fitted, had they been known at the time) into the “merostomoids.” But Whittington could reconstruct not one as a chelicerate. All became orphans, unique arthropods unto themselves.

Molaria
has a deep head shield, shaped like a quarter sphere, followed by eight trunk segments diminishing in size toward the rear, and capped by a cylindrical telson with a very long, jointed posterior spine, extending back more than the length of the body (figure 3.47). This basic form is faultlessly “merostomoid,” but the head bears a pair of short antennae, followed by three pairs of biramous appendages.