Wonderful Life: The Burgess Shale and the Nature of History (32 page)

Now who ever dreamed about a connection between the rear end of a shrimp, the feeding appendage of
Sidneyia
, a squashed sea cucumber, and a jellyfish with a hole in the center? Of course, no one did. The amalgamation of these four objects into
Anomalocaris
came as an entirely unanticipated shock. Moreover, the successful resolution did not emerge from this unimproved initial chaos. Several intermediate efforts, all basically erroneous but each supplying an important link in a developing story, preceded the successful conclusion.

Anomalocaris
has been the nemesis of recent Burgess research. This creature eventually yielded its secret, but not until both Simon Conway Morris and Derek Briggs had committed their biggest mistakes in coping with its various parts. One cannot hope to do anything significant or original in science unless one accepts the inevitability of substantial error along the way. Three steps, however, did inch matters forward toward a resolution, whatever the longer lateral errors.

3.62. The best-known reconstruction of the Burgess Shale, drawn for the 1979
Scientific American
article by Conway Morris and Whittington. Note priapulid worms in their burrows, and several Burgess oddballs—including
Dinomischus
(17),
Hallucigenia
(18),
Opabinia
(19), and
Wiwaxia
(24). In a major error, two jellyfish (10) are shown swimming in like pineapple slices from the west. This structure is actually the mouth of
Anomalocaris
. (From “The Animals of the Burgess Shale,” by Simon Conway Morris and H. B. Whittington. Copyright © 1979 by Scientific American, Inc. All rights reserved.)

1. In 1978, Conway Morris applied Whittington’s new techniques for distinguishing three-dimensional structure to
Laggania
, now regarded as a sponge rather than a holothurian. He took a dental microdrill to the counterpart of the unique specimen, and uncovered a pineapple slice of
Peytoia
, where Walcott had identified the indistinct mouth. Conway Morris stood on the threshold of the proper interpretation, but he guessed wrong. He considered the possibility that the “sponge” called
Laggania
was not a distinct creature, but a body attached to
Peytoia
, which would then become the centerpiece of a strange medusoid. But Conway Morris rejected this reconstruction because he regarded almost all Burgess organisms as discretely preserved, rather than disaggregated into parts. He wrote: “The vast majority of Burgess Shale fossils are preserved complete and it may reasonably be concluded that the body of
Laggania cambria
is not an integral part of
Peytoia nathorsti
, but an extraneous addition to the medusoid which is interpreted here as a sponge” (1978, p. 130). He argued that the association was simply an accident of deposition from the Burgess mudslide: “The association of the medusoid and sponge is presumably by chance. The phyllopod bed was deposited as a series of turbidites, and it is likely that after transport the two specimens settled together” (1978, p. 130).

Conway Morris guessed wrong about the reasons for a link between
Peytoia
and
Laggania
, but he had uncovered (literally) a key association, joining the first two of four pieces that would form
Anomalocaris
.

2. In 1982, Simon tried to grapple with the strangeness of
Peytoia
(Conway Morris and Robison, 1982). He called
Peytoia
“one of the most peculiar of Cambrian medusoids” (1982, p. 116), and even used the word “enigmatic” in his title. Simon did not correctly resolve this beast, but he cast doubt upon its affinity with medusoids, and thus kept the channels of questioning wide open. Writing about the central hole, Conway Morris and Robison concluded: “This feature is unknown in either living or fossil cnidarians and may indicate that
Peytoia nathorsti
is not a cnidarian. Its relationship with any other phylum would seem to be even more obscure” (1982, p. 118).

3.
Anomalocaris
itself, Whiteaves’s original rear end of a shrimp, had been allocated to Derek Briggs in the original divvying up of the Burgess Shale. It was, after all, supposed to be the body of an arthropod with a bivalved carapace.

In 1979, Briggs published a provocative reconstruction of his assignment. He made two outstanding observations that contributed to the resolution of
Anomalocaris:

First, he recognized that
Anomalocaris
was an appendage with paired spines on its inner borders, not an entire body with appendages on its ventral edges. If
Anomalocaris
was the trunk of an entire organism, then some of the more than one hundred specimens should show traces of a gut, and at least a few would be found with arthropod joints on their supposed appendages.

Second, he argued that
Anomalocaris
and appendage F (Walcott’s feeding limb of
Sidneyia
) were variants of the same basic structure, and probably belonged together. This conclusion, as we shall see, was not quite correct, but Briggs’s argument did properly unite two more pieces of the
Anomalocaris
puzzle.

Beyond these important insights, Briggs’s reconstruction was basically erroneous, though spectacular. He continued to view both
Anomalocaris
and appendage F as parts of an arthropod, conjecturing that
Anomalocaris
was a walking leg, and appendage F a feeding structure, of a single giant creature, probably more than three feet long! He called his paper “
Anomalocaris
, the Largest Known Cambrian Arthropod.”

But Briggs was scarcely convinced by his own reconstruction. So many mysteries remained. He puzzled over the failure to find any sign, even fragmentary, of the giant body that supposedly held these appendages. Could a structure three feet long be entirely absent from a soft-bodied fauna? Briggs conjectured that such pieces might exist as organic sheets and films, thus far ignored for their lack of distinguishable structures. He wrote: “Large, previously unidentified, relatively featureless fragments of the body cuticle of
Anomalocaris canadensis
almost certainly await discovery on the scree slopes of Mt. Stephen” (1979, p. 657). Little did Derek realize that the body of
Anomalocaris
had been known and named since Walcott’s time, but masquerading as the “holothurian”
Laggania
, later interpreted as a sponge with a jellyfish on top.

The Geological Survey of Canada expedition had discovered an odd specimen in the Raymond quarry, just above Walcott’s phyllopod bed. Whittington had taken this large, ill-defined, and virtually featureless fossil and placed it in a drawer—hoping, I think, to bury it by the old cliché: Out of sight, out of mind. But he kept thinking about this peculiar fossil of a creature so much larger than anything else in the Burgess Shale. “I used to open the drawer and then close it,” Harry explained to me. One day in 1981, he decided to excavate the fossil in the hope that some details of structure might be resolved. He dug into one end of the creature and, to his astonishment, found a specimen of
Anomalocaris
apparently attached and in place (figure 3.63). Harry told Derek Briggs about his discovery, and Derek simply couldn’t believe it. The excavated object was surely
Anomalocaris
, but, like Simon’s interpretation of the jellyfish
Peytoia
on the sponge
Laggania
, perhaps this specimen of
Anomalocaris
had been accidentally entangled with a large sheet of something else as the mudslide coalesced.

Soon afterward, Whittington and Briggs were studying a suite of specimens borrowed from the Walcott collections. These slabs showed relatively featureless blobs and sheets that had never attracted much attention, including the body of
Laggania
with
Peytoia
on top. On a single momentous day—the positive counterpart (in the vernacular, not technical, sense) of another key Burgess moment, nearly a decade before, when Whittington had cut through the head and sides of
Opabinia
and found nothing underneath—they excavated and found
both Peytoia
and appendage F as organs of a larger creature.

As they assimilated this greatest of all Burgess surprises, and kept finding
Peytoia
and appendage F in the same association on other slabs, Harry and Derek realized that they had resolved a forest of problems into one creature.
Peytoia
was no jellyfish, but the mouth of the large beast, attached to the ventral surface near the front. Appendage F was not one member of a large sequence of repeated limbs on an arthropod; rather, two appendage F’s formed a single pair of feeding organs attached, in front of the mouth, to the bottom end of the new animal.

3.63. The specimen dissected by Harry Whittington that revealed the true nature of
Anomalocaris
. In this camera lucida drawing, the mouth misidentified by Walcott as the jellyfish
Peytoia
is at top center (labeled
Pp
); the oblique line (
ve
) just above it represents a crack in the rock. The structure originally named
Anomalocaris
is the curved feeding appendage just to the left of the mouth with its middle segment labeled
j5
. Also visible is the trace of the central gut, or alimentary canal (
al
).

But Whittington’s specimen back in England bore
Anomalocaris
, not appendage F, in this frontal position (see figure 3.63). When he dissected this specimen more fully, he found traces of both the
Peytoia
mouth and a second
Anomalocaris
, forming a pair of feeding organs in the same position as the appendage-F pairs on the specimens in Washington (figure 3.64).

3.64. The key specimen of
Anomalocaris
further dissected to reveal parts of both feeding appendages. This is the other slab, and therefore a mirror image, of part of the specimen represented in figure 3.63. Note the mouth (labeled
p
) and the first discovered appendage (
j1–j14
). But now a trace of the second feeding appendage has been excavated at the lower left, just below the oblique line representing the crack in the rock.

All the pieces had finally come together. From four anomalies—a crustacean without a head, a feeding appendage that didn’t fit, a jellyfish with a hole in the middle, and a squashed sheet that had bounced from one phylum to another—Whittington and Briggs had reconstructed two separate species of the single genus
Anomalocaris. Laggania
was a squashed and distorted part of the body;
Peytoia
, the mouth surrounded by a circlet of toothed plates, not a series of lobes with hooks;
Anomalocaris
the pair of feeding organs in one species (
Anomalocaris canadensis);
appendage F a feeding organ in the second species (
Anomalocaris nathorsti
, borrowing the old trivial name of
Peytoia
). The uncompromising rules of nomenclature, honoring oldest first, required that the entire genus be called
Anomalocaris
, to recognize Whiteaves’s original publication of 1892. But what a happy and appropriate imposition in this case—an “odd shrimp” indeed!

Since the organ originally named
Anomalocaris
can be up to seven inches in length when extended, the entire animal must have dwarfed nearly everything else in the Burgess Shale. Whittington and Briggs estimated the biggest specimens as nearly two feet in length, by far the largest of all Cambrian animals! A recent reconstruction of the whole fauna (Conway Morris and Whittington, 1985), basically an update of the 1979
Scientific American
version, has replaced the pineapple-slice
Peytoia
that used to angle in from the west (see figure 3.62) with a large and menacing
Anomalocaris
, purposefully advancing from the east (figure 3.65).