Wonderful Life: The Burgess Shale and the Nature of History (31 page)

The two rows of seven to eleven elongate spines arise from the upper row of sclerites on each side, near the border with the plates of the top surface. The spines project upward and presumably acted as protection against predators, as indicated by their breakage in several specimens (during the animal’s life, not after burial).

Simon could see little of
Wiwaxia’s
internal anatomy beyond a straight gut near the ventral surface—further evidence, combined with the naked belly and spines pointing upward, for the animal’s orientation in life. But one internal feature may be crucial both for understanding
Wiwaxia
and for a general interpretation of the Burgess fauna. About five millimeters from the front end, Conway Morris found two arc-shaped bars, each carrying a row of simple, conical teeth directed toward the rear (figure 3.59). The front bar bears a notch at its center, marking a toothless area between the side regions, each with seven or eight teeth. The rear bar has a more curved but smoother front margin, and teeth all along the back edge. These structures were probably attached to the bottom of the gut. In view of their form and their position near the animal’s front end, their interpretation as feeding devices—“jaws,” if you will—seems secure.

3.57. Reconstructions of
Wiwaxia
by Conway Morris (1985). (A) Top view: one of the two rows of spines has been omitted (note the blackened areas of their insertion) so that the sclerites can be seen better. (B) Side view: the front end is at the left.

In attempting to gather and integrate all the evidence, Conway Morris proceeded as far as possible beyond the basic anatomy of
Wiwaxia
, probing for hints wherever he could extract some precious information—from growth, from injury, from ecology, from preservation. Small specimens either carry relatively small spines or lack them entirely—thus providing a rare Burgess example of change in form with growth. Two juxtaposed specimens seem to represent an act of molting by one individual, not two animals accidentally superimposed by the Burgess mudslide: the smaller specimen is shrunken and elongate, as if the large body had just crawled out, leaving its old skin behind as “a vacated husk.” Small brachiopod shells, occasionally found attached to a sclerite, indicate that
Wiwaxia
crawled along the top of the sediment, and did not burrow underneath, where the permanent hitchhikers could not have survived. Patterns of breakage in spines point to the activity of predators and to the possibility of escape. Small spines occasionally found in an otherwise large and uniform row indicate the possibility of regeneration after breakage, or of orderly patterns in replacement (as in the shedding and cycling of teeth in vertebrates without a permanent dentition). The presence of “jaws” suggests a life spent scraping algae or gathering detritus on the substrate.

3.58.
Wiwaxia
as it might have crawled on the sea floor. Drawn by Marianne Collins.

3.59. The jaw apparatus of
Wiwaxia
(Conway Morris, 1985).

Put all these bits and pieces together, and
Wiwaxia
emerges as a complete, working organism—a herbivore or omnivore, living on small items of food collected from the sediment surface as it crawled along the sea floor.

But if all these guides had enabled Conway Morris to reconstruct
Wiwaxia’s
mode of life, he could find no similarly persuasive clues to homology, or genealogical relationship with any other group of organisms. With no setae or appendages and no segmentation,
Wiwaxia
is neither an arthropod nor an annelid. The jaw displays an intriguing similarity to the feeding apparatus of mollusks, called a radula, but nothing else about
Wiwaxia
even vaguely resembles a clam, snail, octopus, or any other mollusk living or dead.
*
Wiwaxia
is another Burgess oddball, perhaps closer to the Mollusca than to any other modern phylum, if its jaw can be homologized with the molluscan radula—but probably not very close.

I could not have made up a better story to illustrate the power and extent of the Burgess revision than the actual chronicle of
Anomalocaris
—a tale of humor, error, struggle, frustration, and more error, culminating in an extraordinary resolution that brought together bits and pieces of three “phyla” in a single reconstructed creature, the largest and fiercest of Cambrian organisms.

The name
Anomalocaris
, or “odd shrimp,” predates the discovery of the Burgess Shale, for this is one of the few soft-bodied Burgess creatures endowed with parts solid enough for preservation in ordinary faunas (the spicules of
Wiwaxia
are another example). The first
Anomalocaris
were found in 1886 at the famous
Ogygopsis
trilobite beds, exposed on the next mountain over from the Burgess Shale. In 1892, the great Canadian paleontologist J. F. Whiteaves described
Anomalocaris
in the
Canadian Record of Science
as the headless body of a shrimplike arthropod. Walcott accepted the standard view that this fossil represented the rear end of a crustacean, with the long axis as the trunk and the ventral spines as appendages (figure 3.60). Charles R. Knight followed this tradition in his famous painting of the Burgess fauna (see figure 1.1), where he constructed a composite organism by attaching
Anomalocaris
to
Tuzoia
, one of the bivalved arthropod carapaces that lacked associated soft parts and was therefore a good candidate for the cover of
Anomalocaris
’s unknown head.

But this official name-bearer of
Anomalocaris
provides only one piece of our story. Three other structures, all named by Walcott, play central roles in this complex tale.

3.60. The fragment of a segmented creature originally called
Anomalocaris
in 1886 (Briggs, 1979). For many years this fossil was considered to represent the trunk and tail of an arthropod. It has now been correctly identified as one of a pair of feeding appendages from the largest of all Cambrian animals.

3.61. Reconstruction of appendage F by Briggs (1979). Walcott originally described this structure as a feeding limb of
Sidneyia
. Briggs reinterpreted it as an appendage of a giant arthropod. Recent research shows that appendage F is actually one of a pair of feeding organs from the largest known Cambrian animal.

1. The head of
Sidneyia
, the arthropod that Walcott named for his son Sidney and then described first among Burgess creatures (1911a), bears a pair of antennae and no other appendages. Walcott also found a large isolated arthropod feeding limb, later (1979) called “appendage F” (for feeding) by Derek Briggs (figure 3.61).
Sidneyia
was, in Walcott’s judgment, the only Burgess creature large enough to carry such an appendage; its rapacious character also fitted well with Walcott’s concept of
Sidneyia
as a fierce carnivore. So Walcott made the marriage without direct evidence, and joined appendage F to the head of
Sidneyia
. Bruton (1981) later determined that
Sidneyia
’s head shield does not contain enough space to accommodate such a structure.

2. Walcott’s second paper (1911b), on the supposed jellyfish and holothurians (sea cucumbers of the echinoderm phylum) from the Burgess Shale, does not rank among his more accurate efforts. He described five genera.
Mackenzia
is probably a sea anemone and therefore a coelenterate in the same phylum as jellyfish, but Walcott placed this genus in his other group, the holothurians. A second creature turned out to be a priapulid worm (Conway Morris, 1977d). A third,
Eldonia
, still ranks as a peculiar floating holothurian in the latest reconstruction (Durham, 1974), but I’ll wager a reasonable sum that it will finally end up as another Burgess oddball.

Walcott named a fourth genus
Laggania
, and identified this fossil as a holothurian, on the basis of one specimen. He noted a mouth, and thought that it might be surrounded by a ring of plates. Poor preservation had effaced all the distinctive features of holothurians. Walcott admitted: “The body of the animal is so completely flattened that the tube feet are obscured, the outline of the ventral sole lost, and the concentric bands almost obliterated” (1911b, p. 52).

3. As a fifth and last genus, Walcott named the only Burgess jellyfish
Peytoia
. He described this peculiar creature as a ring of thirty-two lobes around a central opening. This series of lobes could be divided into four quadrants, with a larger lobe at each of the four corners of the squared-off ring, and seven smaller lobes between the corners in each quadrant. Walcott noted two short points on each lobe, projecting inward toward the central hole. He interpreted these structures as “points of attachment of the parts about the mouth, or possibly oral arms” (1911b, p. 56). Except for radial symmetry, Walcott found no trace of the defining characters of a jellyfish—no tentacles or concentric muscle bands.
Peytoia
, looking more like a pineapple slice than a medusa, made an awfully odd jellyfish. No true member of the group has a hole in the center. Nonetheless, Walcott’s interpretation prevailed. The best-known modern reconstruction of the Burgess fauna, published in
Scientific American
several years after Whittington and colleagues began their revisions (Conway Morris and Whittington, 1979), shows
Peytoia
as a kind of Frisbee
cum
flying saucer
cum
pineapple slice, entering the scene from the west (figure 3.62).