Wonderful Life: The Burgess Shale and the Nature of History (30 page)

3.53. Camera lucida drawings of two specimens of
Leanchoilia
. The great appendages are labeled
Lga
and
Rga
, and their major segments are numbered.(A) The great appendages are folded back, presumably in the swimming position; the right appendage is flat against the body, with the left just below. A trace of the gut, or alimentary canal (
al
) and the tail spine (
tsp
) are visible. (B) The appendages extend forward, in the feeding position.

3.54. Two views of
Leanchoilia:
top, in swimming position, with the great appendages folded back and the whiplike tentacles extending beyond the length of the body; and bottom, with the great appendages extending forward to aid the animal in resting on the bottom. Drawn by Marianne Collins.

Bureaucratic entanglement provides one possible benefit amidst its own distinctive and inimitable brand of frustration. You sometimes get so angry that you do something useful as an end run around intransigence. As the old motto goes, Don’t get mad, get even. When Des Collins, after sublime patience and deep entanglement, was denied permission to excavate at Walcott’s quarry and allowed only to gather specimens from the talus slope (under further restrictions and nearly endless delays), he realized that he would have to shift his Burgess interests elsewhere.
*

Collins therefore began to search for Burgess equivalents in surrounding areas, where collection and excavation might be permitted. He succeeded abundantly, finding soft-bodied fossils at more than a dozen additional nearby localities. Most of these assemblages contain the same species as Walcott’s quarry, but Collins made a few outstanding discoveries of his own. At a locality five miles south of Walcott’s quarry (Collins, 1985), and one hundred feet below in stratigraphic sequence, Collins made the find of the decade—a large arthropod with so many spiny appendages on its head that Collins, following an old tradition of field work, gave it a nickname. As Walcott had called
Marrella
the “lace crab,” Collins dubbed his discovery “Santa Claws.” Working with Derek Briggs, Collins has now formalized and honored this name in his technical description (Briggs and Collins, 1988). “Santa Claws” is now, officially,
Sanctacaris
, which means almost the same thing.

Sanctacaris
has a bulbous head shield, wider than long and extending laterally as a flat, triangular projection on each side (figure 3.55). The body bears eleven broad segments, the first ten with a pair of biramous appendages. A wide, flat telson caps the rear end. The combination of large lamellate gill branches on the body appendages and a broad telson well designed for stabilization and steering indicates that
Sanctacaris
probably favored swimming over walking.

The striking suite of head appendages identifies this relatively large Burgess arthropod (up to four inches long) as a carnivore specialized for direct pursuit. The first five pairs make a coordinated and formidable array that inspired Collins’s field name. They are biramous, with the outer branches reduced to antenna-like projections (not gills) and the inner branches arranged as a fierce-looking set of jointed feeding appendages with sharp spines on the inner borders. These feeding branches gain in length from front to back, starting with four segments on the first pair, and increasing to eight or more on the fifth. The sixth pair, different in both form and position, lies behind the first five and well to the side. The outer branch is, again, similar to an antenna in form, but much larger than the corresponding branch of the five feeding appendages. The inner branch is short, but terminates in an impressive fringe of radiating spines.

One might think at first assessment, Oh, just another of those Burgess “merostomoids”—with a forest of head appendages as its distinctive specialization, just as
Habelia
has its tubercles,
Sidneyia
its stout walking legs, and
Leanchoilia
its great appendage. Interesting, but not my advertised “find of the decade.”

Not so. The difference between
Sanctacaris
and the others is taxonomic, and conceptually stunning:
Sanctacaris
seems to be a genuine chelicerate, the first known member of a line that eventually yielded horseshoe crabs, spiders, scorpions, and mites.
Sanctacaris
bears the requisite six pairs of appendages on its head. None of these appendages has been specialized to form the distinctive claw, the chelicera, that defines and names the group, but the absence of a structure early in the geological run of a group may simply mean that such a specialization has not yet evolved.

Briggs and Collins (1988) have also identified other derived chelicerate characters (including the differentiation of head from body appendages, and the position of the anus), thus corroborating the status of
Sanctacaris
by more than a single feature. They state:

3.55.
Sanctacaris
. Drawn by Marianne Collins.

The limbs of modern chelicerates are uniramous, with the outer branch lost on the head appendages (yes, the walking legs of spiders are all on the prosoma, or head portion), and the inner branch lost on the trunk (yes again, spider gills are on the opisthosoma, or body portion).
Sanctacaris
, by preserving the full set of possibilities before selective elimination in later specialized lines, serves as an interesting structural precursor for its great group.

But the chief excitement of
Sanctacaris
lies in its key role in completing the fundamental argument for Burgess arthropods. With the discovery of
Sanctacaris
, we now have, in the Burgess, members of
all four
great arthropod groups—trilobites in fair abundance, crustaceans represented by
Canadaspis
, uniramians by
Aysheaia
*
(accepting Robison’s interpretation, as I do), and chelicerates by
Sanctacaris
. They are all there—but so are at least thirteen other lineages (and perhaps as many again yet to be described) of equal morphological uniqueness. Some of these thirteen are among the most specialized (
Leanchoilia
) or, at least by numbers, the most successful (
Marrella
) of Burgess arthropods. I challenge any paleontologist to argue that he could have gone back to the Burgess seas and, without the benefit of hindsight, picked out
Naraoia, Canadaspis, Aysheaia
, and
Sanctacaris
for success, while identifying
Marrella, Odaraia, Sidneyia
, and
Leanchoilia
as ripe for the grim reaper. Wind back the tape of life, and let it play again. Would the replay ever yield anything like the history that we know?

The last decade, so satisfying for arthropods, has also witnessed the resolution of two additional weird wonders—unique and independent anatomies that would merit classification as separate phyla if we felt comfortable about bestowing so high a taxonomic rank on a single species (see Briggs and Conway Morris, 1986, for a list of such Burgess creatures still unstudied). These two works may be the most elegant and persuasive in the entire Burgess canon. They stand as a fitting end to my play, for they combine the greatest intellectual and aesthetic satisfaction with an assurance that this particular drama has no foreseeable end.

When I asked Simon Conway Morris why he had chosen to work for many years on so complex a beast as
Wiwaxia
, he replied, with welcome frankness, that Harry and Derek had both done their “blockbusters,” and he wanted to prove that he could also write a “strict monograph in the tradition of the others.” (I regard this statement as overly modest. Simon’s 1977 and 1979 works on priapulids and polychaetes are true and extensive monographs. But each treats several genera, and therefore cannot give the exhaustive treatment to any one species that Whittington provided for
Marrella splendens
, or Briggs for
Canadaspis perfecta
.) Perhaps Simon felt unfulfilled in choosing such rare creatures for his first run through the weird wonders that he could write only short, separate papers on five examples. In any case, his monograph on
Wiwaxia
is a thing of beauty, and the original source of my interest in writing about the Burgess Shale (Gould, 1985b)—for which, Simon, my greatest thanks once again.

Wiwaxia
is a small creature, shaped as a flattened oval (a well-rounded pebble in a stream comes to mind), about an inch long, on average, with a two-inch maximum. The simple body is covered with plates and spines called sclerites—except for the naked ventral surface that rested on the substrate as
Wiwaxia
crawled across the sea floor. Walcott had shoehorned
Wiwaxia
into the polychaete worms, mistaking these sclerites for superficially similar structures in a well-known marine worm, whose technical and common names convey such different impressions—
Aphrodita
, the sea mouse. But
Wiwaxia
has no body segmentation and no true setae (the hairlike projections of polychaetes)—and therefore lacks both defining traits of the group. Like so many Burgess animals,
Wiwaxia
is an anatomy unto itself.
Wiwaxia
is also inordinately difficult to reconstruct, because the sclerites spread over the rock surface in a horribly confused jumble as the fossil compressed on its bedding plane. In figure 3.56, a camera lucida drawing of the most coherent specimen in the most convenient orientation provides a good idea of the problems involved. Simon’s resolution of
Wiwaxia
is one of the great technical achievements of the Burgess research program.

The sclerites of
Wiwaxia
, the key to this reconstruction, grew in two different styles; flattened scales, ornamented with parallel ridges, cover most of the body, while two rows of spines emerge from the top surface, one on each side of the central axis (figures 3.57 and 3.58). The scales display a symmetrical and well-ordered tripartite pattern: (1) a field of overlapping plates, on the top surface, arrayed as six to eight parallel rows (figure 3.57A); (2) two regions on each side (figure 3.57B), with two rows of plates pointing upward and two rows pointing backward; (3) a single bottom row of crescent-shaped sclerites forming a border between the ornamented upper body and the naked belly.

3.56. (A) Camera lucida drawing of a complete specimen of
Wiwaxia
. Note the complex intermingling of the compressed sclerites. The labels, which need not concern readers here, identify individual sclerites. For example,
R.d.sl
.
₁
(top right) is a right, dorsal sclerite (
sl.
) of the first row.
L.sp
.
₁
(top left) is the first spine on the left side. (B) Enlargement of one particularly interesting sclerite (located in A at the lower left, next to the label
br
.). A small brachiopod (
br.
) affixed itself to the sclerite during the life of this
Wiwaxia
specimen. Using such evidence, we can reconstruct the life style of this animal. It could not have lived by burrowing under the substrate, for such a habit would have killed the brachiopod.