Wonderful Life: The Burgess Shale and the Nature of History (27 page)

This change may seem small, a shift from one well-known group to another, and therefore an event of little conceptual interest in the midst of so much Burgess turmoil and discovery. Not so. The classification of
Naraoia
is a satisfying final piece of a puzzle, proving that the basic Burgess pattern—anatomical disparity beyond the range of later times—applies at all levels. Simon’s weird wonders had established the pattern at the highest level of phyla, the basic ground plans of animal life. Whittington’s monographs had told the same story at the next lower level of disparity within phyla—group after group of orphaned arthropods spoke of Burgess anatomy far beyond the range of any later time, despite a vast increase in arthropod species, including a modern insect fauna of nearly a million described species. Now Harry had demonstrated the same pattern again at the lowest level of disparity within major groups of a phylum. He had discovered an apparent contradiction in terms—a soft-bodied trilobite with a carapace of two valves. (In 1985 he would describe a second soft-bodied trilobite,
Tegopelte gigas
, one of the largest Burgess animals at nearly a foot in length, so
Naraoia
is no lone oddity among trilobites.) The Burgess pattern seems to display a “fractal” character of invariance over taxonomic scales: crank up the telescope, or peer down the microscope, and you see the same picture—more Burgess disparity, followed by decimation and diversification within fewer surviving groups.

The monograph on
Naraoia
marked a conceptual watershed for Whittington. He finally sank the class Trilobitoidea officially, as an artificial wastebasket with no evolutionary validity. He had finally freed himself to view the Burgess arthropods as a series of unique designs, beyond the range of later groups.

Harry’s next monograph, on
Aysheaia
, begins with his most explicit recognition of the new view: “The animals in this community include an astonishing variety of arthropods as well as bizarre forms, such as those described by Whittington and Conway Morris which, like
Aysheaia
, are not readily placed in Recent higher taxa” (1978, pp. 166–67).
Aysheaia
was perhaps the most famous and most widely discussed of Burgess organisms—for an interesting reason rooted in the two
p’s
, “primitive” and “precursor.” Walcott (1911c) had described
Aysheaia
as an annelid worm, but colleagues soon pointed out with excitement that the creature could hardly be distinguished, at least superficially, from a small group of modern invertebrates called the Onychophora and represented primarily by a genus with the lovely name
Peripatus
. The Onychophora possess a mixture of characters recalling both annelids and arthropods; many biologists therefore regard this group as one of the rare connecting forms (“nonmissing links,” if you will) between two phyla. But modern Onychophora are terrestrial, while the actual transition from annelid to arthropod, or the derivation of both from a common ancestor must have occurred in the sea. In addition, modern Onychophora have undergone more than 550 million years of evolution since the supposed linkage of annelid and arthropod, and could not be viewed as direct models of the transition. A marine onychophoran from the Cambrian would be a creature of supreme evolutionary importance—and
Aysheaia
, generally so interpreted (Hutchinson, 1931), became a hero of the Burgess. The great ecologist G. Evelyn Hutchinson, who had done important work on the taxonomy of
Peripatus
in South Africa, and who, looking back on a rich career from his ninth decade, still places his study of
Aysheaia
among his most significant (interview of April 1988), wrote:

Aysheaia
has an annulated, cylindrical trunk, with ten pairs of annulated limbs attached at the sides near the lower surface, and pointing down, presumably for use in locomotion (figures 3.41 and 3.42). The anterior end is not separated as a distinct head. It bears a single pair of appendages, much like the others in form and annulation but attached higher on the sides and pointing laterally. The terminal mouth (smack in the middle of the front surface) is surrounded by six or seven papillae. The head appendages bear three spinelike branches at their tip, and three additional spines along the anterior margin. The body limbs end in a blunt tip carrying a group of up to seven tiny, curved claws. Larger spines emerge from the limbs themselves. These spines are absent on the first pair, point forward on pairs 2–8, and backward on 9–10.

3.41.
Aysheaia
, probably an onychophoran. Drawn by Marianne Collins.

Whittington combined this anatomical information with other data to reconstruct an interesting and unusual life style for
Aysheaia
. On or near six of the nineteen
Aysheaia
specimens he found remains of sponges—an association hardly ever encountered with other Burgess animals. Whittington conjectured that
Aysheaia
might have fed on sponges and lived among them for protection as well (figure 3.43). The tiny terminal claws of the limbs would not have worked on mud, but might have helped in climbing upon sponges and holding on. The anterior appendages could not have swept food directly into the mouth, but they might have lacerated sponges with their spines, permitting the animal to lap up nutritious juices and soft tissues. The backward-facing claws and spines of the posterior body limbs might have functioned as anchors to keep the animal in place at odd angles.

3.42. Reconstruction of
Aysheaia
by Whittington (1978). (A) Top view. (B) Side view: the ring of tentacles surrounding the terminal mouth is visible at the top; the dorsal surface is at the right.

3.43. Reconstruction by Whittington (1978), showing
Aysheaia
living and feeding on sponges.

But was
Aysheaia
an onychophoran? Whittington admitted some impressive similarities in the anterior appendages, the short, uniramous body limbs with terminal claws, and the annulations on body and limbs. But he also cited some differences, including lack of jaws (possessed by modern onychophorans) and the termination of the body at the last pair of limbs (the body extends farther back in modern onychophorans).

In Whittington’s judgment, these differences raised sufficient doubts to debar
Aysheaia
from the Onychophora and to recognize this genus, albeit tentatively, as a unique and independent group. Citing the lessons of other genera, he wrote: “Thus
Aysheaia
, like other Burgess Shale animals as
Opabinia, Hallucigenia
, and
Dinomischus
, does not fit readily into any extant higher taxon” (1978, p. 195).

I regard these words as momentous, and (symbolically, at least) as the completion of the Burgess transformation. I say this, ironically, because I think that for once, Harry was probably wrong about
Aysheaia
. I believe that, on the balance of evidence,
Aysheaia
should be retained among the Onychophora. The similarities are impressive and anatomically deep, the differences superficial and not of great evolutionary import. Of Harry’s two major differences, jaws may simply have evolved later. Structures can be added in evolution provided that ancestral anatomies do not preclude their development. Just such an event occurred in at least one prominent Burgess group. Burgess polychaetes have no jaws, but jaws evolved by Ordovician times and have persisted ever since. As for the extension of the body beyond the last pair of limbs, this strikes me as an easy evolutionary change, well within the capacity of a broad group like the Onychophora. The American paleontologist Richard Robison, who developed a much longer list of distinctions between
Aysheaia
and modern onychophorans, agrees nonetheless that
Aysheaia
belongs in the group, and writes of Whittington’s second major difference:

Why then did Whittington separate
Aysheaia
from the Onychophora and assert its taxonomic uniqueness? Since this conclusion came from a man who, for years, had been resisting the temptation to separate Burgess organisms from well-known groups, and who had made such divisions only when forced by weight of evidence, we would naturally assume that he had been compelled to this uncomfortable conclusion by new data direct from
Aysheaia
. But read the 1978 monograph carefully. Whittington did not upset any of Hutchinson’s basic statements about
Aysheaia
. Harry had listed and discussed the same differences; he had essentially affirmed, in much greater and more elegant detail to be sure, Hutchinson’s excellent work. But Hutchinson had classified
Aysheaia
as an onychophoran—on the very same data that Whittington later used to reach the opposite conclusion.

What then had prompted Whittington’s reversal, if not the anatomy of
Aysheaia?
We have a reasonably well-controlled psychological experiment here. The data had not changed, so the reversal of opinion can only record a revised presupposition about the most likely status of Burgess organisms. Obviously, Whittington had come to accept, and even to prefer, the idea of taxonomic uniqueness for animals of the Burgess Shale. His conversion was complete.

Many fascinating genera still awaited description; the halfway point had not even been reached. But Whittington’s 1978 monograph on
Aysheaia
marks the codification of a new view of life. What a dizzying few years between 1975 and 1978—from the disturbing discovery that
Opabinia
is neither an arthropod, nor anything else ever known before, through the cascade of Simon’s weird wonders, to the full acceptance of taxonomic uniqueness as a preferred hypothesis. Three short years and a new world!

The seven short years from
Marrella
(1971) to
Aysheaia
(1978) had produced an extraordinary shift of perspective—from a project designed to redescribe some arthropods classified in familiar groups, to a new conception of the Burgess Shale and the history of life.