Wonderful Life: The Burgess Shale and the Nature of History (26 page)

2.
Canadaspis
, the second discovery. Consider the story of Burgess arthropods as published by the end of 1976.
Marrella
, a supposed relative of trilobites, was an orphan.
Yohoia
, with its great appendage, was uniquely specialized and unaffiliated, not a precursor of anything.
Burgessia
, namesake of the fauna, was another orphan. Even
Branchiocaris
, firm candidate for a crustacean, sported a unique anatomy under its bivalved carapace. Moreover, these four orphans showed no propensity for coherence among themselves; each reveled in its own peculiarities. Would any Burgess arthropod ever accept the allegiance to a modern group that Walcott, wielding his shoehorn, had once forced upon all?

Canadaspis
is the second most common animal in the Burgess Shale. It is large by Burgess standards (up to three inches in length) and tends to be preserved with a conspicuous reddish color. It has a bivalved carapace, but as Briggs soon discovered, an underlying anatomy very different from
Branchiocaris
.

In a short paper of 1977, Briggs placed two bivalved species in the new genus,
Perspicaris
. His reconstructions suggested something exciting, but the rarity of specimens and their poor preservation precluded any firm conclusion. He couldn’t prove the affiliation, but nothing about these two species precluded membership in the Crustacea. Had a representative of a modern group finally been found?

In 1978, Briggs resolved this issue with elegance and finality. His long monograph on the well-preserved, superabundant
Canadaspis perfecta
finally placed a Burgess creature in a successful modern group.
Canadaspis
was not only a crustacean, but its home within the Crustacea could be established.
Canadaspis
is an early malacostracan—a representative of the great group of crabs, shrimp, and lobsters. Briggs found all elements of the intricate malacostracan stereotype in the anatomy of
Canadaspis:
a head bearing five pairs of appendages, and built of five segments plus eyes; a thorax (middle section) of eight segments, and an abdomen (back section) of seven segments plus a telson. Further, the head appendages are arranged just right, with two pairs of short, uniramous antennae in front of the mouth, and three pairs of ventral appendages behind the mouth.
*
The abdominal segments bear no appendages, but each thoracic segment carries a pair of standard biramous appendages, with an inner leg branch and a broad outer gill branch (figures 3.37 and 3.38).

3.37. Reconstruction of
Canadaspis
by Briggs (1978). This animal has the typical structure of a true crustacean of the malacostracan line: two pairs of appendages in front of the mouth (labeled
an1
and
an2
), three pairs of appendages behind the mouth (
ma, mx1
, and
mx2
), a thorax of eight segments (beginning with the segment labeled
t1
), and an abdomen of seven segments (
ab1-ab7
). Each thoracic segment bears a pair of biramous appendages.

The brevity of this description is no denigration of the importance of
Canadaspis
in the Burgess reformulation. A weird animal needs a longer write-up to explain its uniqueness; a familiar creature can simply be characterized as “like Joe whom everyone knows.” But
Canadaspis
is both a key and an anchor to the Burgess story, a creature every bit as important as any of Simon’s weird wonders. Suppose that every Burgess animal were a bizarre denizen of a lost world. What then would we make of the assemblage? A failed experiment, a washout, a first attempt totally bypassed by a reconstituted modern fauna, and therefore offering no clues and no connection to the origin of later life. But the presence of
Canadaspis
, and other creatures of modern design, suggests a different and more enlightening view. The Burgess fauna does include modern prototypes, and, in this key respect is an ordinary Cambrian fauna; but the vastly broader range of designs that disappeared may reveal the most important of all patterns in life’s early history.

3.38. The true crustacean
Canadaspis
. The five head segments bear two pairs of antennae and three pairs of appendages behind the mouth, the last two of which are continuous with, and similar in form to, the biramous appendages of the body. Drawn by Marianne Collins.

As Derek resolved
Canadaspis
, Simon had left behind his whirlwind of wonders to work on the main subjects of his project, the true Burgess worms. His results, published in two monographs (1977 and 1979), beautifully affirmed the lesson of
Canadaspis
. Some Burgess organisms, even among soft-bodied members of the fauna, fit comfortably into modern groups—thus accentuating and highlighting the importance of the oddballs as additions to normality. In 1977, Conway Morris recognized among forms that Walcott had scattered across three phyla (as polychaetes, crustaceans, and echinoderms) six or seven genera of priapulid worms. The Priapulida form a small phylum of ten genera or so in today’s oceans, but they dominated the worm fauna of the Burgess Shale. (The Burgess priapulids form a major part of my story in chapter V.)

In 1979, Conway Morris sorted out one of Walcott’s greatest confusions—the Burgess polychaetes. Walcott had used the Polychaeta (marine representatives of the phylum Annelida, or segmented worms) as a dumping ground for many Burgess oddities. Within Walcott’s polychaetes, Conway Morris found two genera of priapulids and four genera of weird wonders. But Walcott had also identified some true polychaetes. From this mixture, Conway Morris identified and established six genera of Burgess polychaetes. This group, so dominant in today’s seas, was overshadowed by priapulids (with the same number of genera, but many more specimens) in Burgess times. But both groups proclaimed the same general message. The Burgess fauna contained both ordinary and unique anatomies in abundance.

After such an extended third act, we need a sparer fourth to make a largely symbolic point amidst the resolution of two important Burgess genera distinguished by more than their maximally unpronounceable, vowel-laden names.

Harry Whittington had started this drama by orphaning some arthropods that everyone had previously placed in established groups (Act 1). He had upped the ante by showing that
Opabinia
was not an arthropod at all, but a creature of strange and unique anatomy (Act 2). His students and associates then converted these anomalies into a generality about the Burgess and its time by documenting the same pattern throughout the fauna (Act 3). When Harry Whittington finally accepted the new interpretation, and began to view anatomical oddity as a preferred hypothesis
a priori
, rather than a last resort, the story had reached its logical end; the Burgess transformation had been completed (Act 4). In conceptual terms, the rest would be mopping up, but with the best of all particular stories still to be told (Act 5).

Naraoia
added the last substantial piece to the logical structure of the new view. This old Burgess standby, described by Walcott as a branchiopod crustacean, has a carapace composed of two flat, smooth, oval valves, meeting at straightened borders one behind the other. These valves, discrete and shiny on most fossils, make
Naraoia
one of the most striking and attractive of Burgess organisms, but they also impose a severe problem in interpretation. They cover almost all the soft anatomy; most specimens show only the distal tips of the appendages, protruding out beyond the edge of the carapace (figure 3.39). Since the proximal (and invisible) ends of the appendages provide the primary taxonomic basis for identifying arthropod groups—both by their form and by their pattern of insertion into the body—
Naraoia
could never be properly interpreted.

Whittington resolved this dilemma with his discovery of three-dimensional structure in the Burgess fossils. He realized that he could dissect through the firm carapace to reveal the proximal ends of the appendages, and their points of insertion. When he cut through the carapace of
Naraoia
(figure 3.40), he uncovered enough of the appendages to count their segments and resolve their proximal ends, including gnathobases and food grooves. Whittington also received one of the great surprises of his professional life. He was looking at a leg branch of the animal he knew best—a trilobite. But beyond a vague similarity in general outline, the carapace, with its two valves, hardly resembles the exoskeleton of a trilobite. Most trilobites have a threefold division, into head, thorax, and pygidium. (Contrary to popular belief, this division, stem to stern, is not the source of the name “trilobite,” or “three-lobed.” Trilobation refers to the threefold side-to-side division into a central axis and two side regions, called pleurae.)

3.39. Camera lucida drawing of an excellent specimen of
Naraoia
(Whittington, 1977). The two valves of the carapace cover almost all the soft anatomy, and only the ends of the appendages protrude beyond them.

3.40. Determination of the taxonomic affinity of
Naraoia
by dissection. (A) A complete specimen before dissection. (B) The same specimen, dissected to reveal the legs at their point of attachment to the body. (C) Camera lucida drawing of the dissected specimen. Since the legs are of typical trilobite form,
Naraoia
is identified as the first known bivalved trilobite.

Whittington also found other key trilobite characters in
Naraoia
, notably the defining segmentation of the head, with one pair of uniramous pre-oral antennae and three pairs of ventral post-oral appendages.
Naraoia
, despite its curious outer covering, was surely a trilobite. Whittington therefore described this genus as a new and separate class within the Trilobita. He wrote with barely disguised joy and an uncharacteristic personal touch—and why not, for Harry is the world’s expert on trilobites. These are his babies, and he had just given birth to a stunning and different child: