Wonderful Life: The Burgess Shale and the Nature of History (25 page)

Seven pairs of sharply pointed spines—not jointed, arthropod-like appendages, but single discrete structures—connect to the sides of the trunk, near the bottom surface, and extend downward to form a series of struts. These spines do not articulate to the body, but seem to be embedded within the body wall, which extends as a sheath for a short distance along the top of each spine. Along the dorsal midline of the body, directly opposite the spines, seven tentacles with two-pronged tips extend upward. The seven tentacles seem to be coordinated with the seven pairs of spines in an oddly displaced but consistent way: the first tentacle (nearest the “head”) corresponds to no spine below. Each of the next six tentacles lies directly above a pair of spines. The last pair of spines has no corresponding tentacle above. A cluster of six much shorter dorsal tentacles (perhaps arranged as three pairs) lies just behind the main row of seven. The posterior end of the trunk then narrows into a tube and bends upward and forward.

3.35. Original reconstruction of
Hallucigenia
by Conway Morris (1977c).

How can a taxonomist proceed in interpreting such a design? Simon decided that he must first try to figure out how such an animal could operate; then he might gain some further clues to its anatomy. Searching for analogies, Simon noted that some modern animals rest upon, and even move with, spines attached to their bottom sides. “Tripod” fish support themselves upon two long pectoral spines and one tail spine. The elasipods, a curious group of deep-sea holothurians (sea cucumbers of the echinoderm phylum), move in groups along the bottom, supported by elongate, spiny tube feet (Briggs and Conway Morris, 1986, p. 173). In
Hallucigenia
, the two spines of each pair meet at an angle of some seventy degrees, an excellent arrangement for a series of struts supporting the body in fair stability. Conway Morris therefore began by supposing that the seven pairs of spines permitted
Hallucigenia
to rest on a muddy substrate. This assumption defines both a mode of life and an orientation: “Dorsal and ventral surfaces are identified on the assumption that the spines were embedded in the bottom sediments” (Conway Morris, 1977c, p. 625).

So far, so good;
Hallucigenia
could rest on the bottom in fair stability. But the animal couldn’t stand there in perpetuity like a statue; bilaterally symmetrical creatures with heads and tails are almost always mobile. They concentrate sensory organs up front, and put their anuses behind, because they need to know where they are going and to move away from what they leave behind. How in heaven’s name could
Hallucigenia
move on a set of spikes fixed firmly into the body wall? Conway Morris did manage to suggest a plausible model, in which strips and bands of muscle anchor the proximal end of the spine to the inner surface of the body wall. Differential expansion and contraction of these bands could move the spines forward and back. A coordinated wave of such motion along the seven pairs might propel the animal, if a bit clumsily. He was not thrilled with the prospects for such a mode of locomotion, and suggested that “
Hallucigenia sparsa
probably did not progress rapidly over rocks or mud, and much of its time may have been spent stationary” (1977c, p. 634).

If the spines are hard to interpret, what about the tentacles above—where prospects for modern analogues are dimmer. The pincers at their tips could have captured food, but the tentacles don’t reach the head region, and passage of food from one tentacle to another toward a frontal mouth offers little promise of efficient eating. Noting a possible connection between a hollow tube within each tentacle and a gut within the trunk (neither well enough preserved to inspire confidence), Conway Morris offered a fascinating alternative. Perhaps
Hallucigenia
had no frontal mouth at all. Perhaps each tentacle gathered food independently, passing the collected particles down its own personal gullet into the communal gut. You have to consider bizarre solutions when you work with such a strange animal.

Yet
Hallucigenia
is so peculiar, so hard to imagine as an efficiently working beast, that we must entertain the possibility of a very different solution. Perhaps
Hallucigenia
is not a complete animal, but a complex appendage of a larger creature, still undiscovered. The “head” end of
Hallucigenia
is no more than an incoherent blob in all known fossils. Perhaps it is no head at all, but a point of fracture, where an appendage (called
Hallucigenia
) broke off from a larger main body (yet undiscovered). This prospect may seem disappointing, since
Hallucigenia
by itself forms such a wondrous beast. Hence, I am rooting for Conway Morris’s interpretation (but if forced to bet, I would have to place my money on the appendage theory). But then, the prospect of
Hallucigenia
as only an appendage may be even more exciting—for the whole animal, if ever discovered and reconstructed, might be even more peculiar than
Hallucigenia
as now interpreted. It has happened before in the Burgess.
Anomalocaris
(see Act 5) was once viewed as an entire arthropod, and a fairly dull crustacean at that. Then Whittington and Briggs (1985) resolved it as a feeding appendage of an animal ranking just behind
Hallucigenia
in Burgess oddity. We have surely not seen the last, and perhaps not the greatest, of Burgess surprises.

I must begin with an apology to Derek Briggs for an invisible slight arising from both ignorance and thoughtlessness. I made a bad mistake when I first laid out this chronological centerpiece of the book—that is, before I read the monographs in detail. I saw the Burgess transformation as a dramatic interplay between Harry Whittington, the conservative systematist who started it all, and Simon Conway Morris, the young and radical man of ideas who developed a revolutionary interpretation and dragged everyone else along. I have already indicated my error in reading this interaction according to the conventional script.

Let me now confess another mistake, one that I should not have made. This is the classic error of those who write about science without an intuitive feel for its daily procedures; those who do the work should know better. The journalistic tradition so exalts novelty and flashy discovery, as reportable and newsworthy, that standard accounts for the public not only miss the usual activity of science but also, and more unfortunately, convey a false impression about what drives research
*

A project like the Burgess revision has potentially flashy and predictably less noticeable aspects. Both are necessary. A conventional reporter will convey only the hot ideas and the startling facts—
Hallucigenia
gets ink; the Burgess trilobites get ignored. But the Burgess oddballs mean little in isolation. When placed in an entire fauna, filled with conventional elements as well, they suggest a new view of life. The conventional creatures must be documented with just as much love, and just as assiduously—for they are every bit as important to the total picture.

Derek Briggs drew the bivalved arthropods as his subject—the apparently most conventional group in the Burgess fauna. He produced an elegant series of monographs on these animals, finding some surprises, but also confirming some expectations. I had not appreciated the central role that Briggs’s work on bivalved arthropods played in the Burgess transformation. As I read Derek’s monographs, I recognized my error with some shame, and grew to understand Harry, Derek, and Simon as a trio of equals, each with a distinct and necessary role in the total drama.

Walcott and others had described about a dozen genera of arthropods with a bivalved carapace (usually enclosing the entire head and front part of the body). Several of these genera cannot be classified with certainty, for only the carapaces have been found, not the soft parts. The other genera have always, and without any doubt or hesitation, been identified as crustaceans—as are all modern arthropods with a bivalved carapace. Derek Briggs began his project without any conscious doubts: “There were some redescriptions to be done. I assumed I would be dealing with a bunch of crustaceans.”

Briggs described two outstanding discoveries in his first monographs on the bivalved arthropods of the Burgess Shale. Put these together with Simon’s oddballs and Harry’s orphaned arthropods, and you have, by 1978, both a fully articulated and completely new account of how multicellular animal life evolved.

1.
Branchiocaris
, the first discovery. The Crustacea are an enormous and diverse group—from the nearly microscopic ostracodes with bivalved carapaces covering the entire body like a clamshell, to giant crabs with leg spreads of several feet. Yet all are built upon a stereotyped ground plan, with a definite signature in the structure of the head The crustacean head is an amalgam of five original segments plus eyes. Five pairs of appendages are therefore present—and in a definite arrangement: two pre-oral (usually antennae) and three post-oral (usually mouth parts).
*
Since all modern bivalved arthropods are crustaceans, Briggs assumed that he would find this frontal signature in his Burgess subjects. But the Burgess soon provided yet another surprise.

Back in 1929, Charles E. Resser, Walcott’s right-hand man at the Smithsonian, had described a single Burgess specimen as the crustacean
Protocaris pretiosa
. The genus
Protocaris
had been established in 1884, by none other than Charles Doolittle Walcott in his pre-Burgess days, for a Cambrian arthropod from the Parker Slate of Vermont. Resser considered the Burgess animal as sufficiently close for inclusion in the same genus. Briggs disagreed and established the new genus
Branchiocaris
.

Briggs managed to amass a total of five specimens—Resser’s original, three more from the Walcott collection, and a fifth whose part was found by Raymond in 1930, but whose counterpart remained on the Burgess talus until collected by the Royal Ontario Museum expedition in 1975, as recounted in the heart-warming tale earlier in this chapter. The bivalved carapace of
Branchiocaris
covers the head and anterior two-thirds of the body (figure 3.36). The body itself contains some forty-six short segments, with a two-pronged telson behind. The appendages are not clearly distinguishable in the limited number of available fossils, but may have been biramous, with a short segmented branch (presumably homologous to the walking leg of most biramous arthropods, but too reduced for such a function in
Branchiocaris
), and a larger bladelike process, probably used for swimming near the sea floor.

But the head of
Branchiocaris
provided the big surprise. Two pairs of short antenna-like appendages, pointing forward, could clearly be seen—the first more conventional in form, uniramous with many segments; the second more peculiar, stout and composed of few segments, perhaps with a claw or pincer at the end. Briggs called this second pair the “principal appendage”—just as Whittington, stumped by an analogous structure in
Yohoia
, had spoken of a “great appendage.”

These appendages attached to the upper and lateral surfaces of the head. On the ventral side, three pairs of additional appendages should have followed the mouth. Briggs found nothing. The mouth stood all alone on an unadorned ventral surface.
Branchiocaris
, with two and only two pairs of appendages on the head, was not a crustacean. “It apparently defies classification within any group of Recent arthropods,” Briggs concluded (1976, p. 13).

3.36. Reconstruction of
Branchiocaris
by Briggs (1976). (A) Side view. (B) Bottom view, showing the ventral surface of the animal surrounded by the two valves of its carapace. Note in particular the pairs of uniramous appendages, especially the unique principal appendage (labeled
lpa
and
rpa
). And note also the absence of any appendages on the head behind the mouth; this arrangement is unknown in any modern arthropod group.

Thus, the bivalved arthropods—the group that seemed most promising as a coherent set of evolutionary cousins—also formed an artificial category hiding an unanticipated anatomical disparity. What order could possibly be found among the Burgess arthropods? Each one seemed to be built from a grabbag of characters—as though the Burgess architect owned a sack of all possible arthropod structures, and reached in at random to pick one variation upon each necessary part whenever he wanted to build a new creature. Could a biramous limb of trilobite type adorn any kind of arthropod body? Could a bivalved carapace cover any anatomy? Where was order, where decorum?