Wonderful Life: The Burgess Shale and the Nature of History (18 page)

3.14. Front view of
Marrella
, seen as if walking right toward the reader (Whittington, 1971).

3.15. A pair of biramous appendages from
Marrella:
right and left gill branches above, leg branches below (Whittington, 1971).

Harry worked for four and a half years on
Marrella
, personally preparing, dissecting, and drawing scores of specimens in varying orientations. Efforts of this sort are often left to assistants, but Whittington knew that he must do this basic work himself, over and over again, if he hoped to win a proper “feel” for Burgess preservation and its problems. This labor, however tedious and repetitious at times, also provided more than enough excitement to inspire perseverance. Harry spoke to me about his decision to do all the work himself, a commitment of several precious years in research:

The Burgess studies of Whittington and his team are, for the most part, revisions, not first descriptions of newly found species. They are therefore presented in the context of previous interpretations and stand as evaluations of past work. Walcott had called
Marrella
a trilobite, or at least close enough to share the anatomical signature of the group. Størmer had made
Marrella
a flagship of his Trilobitoidea, the sister-group of trilobites in his larger class Trilobitomorpha. Hence Whittington studied
Marrella
in the primary context of its relationship with trilobites, the subject of his lifelong expertise.

Whittington affirmed that the general form of
Marrella’s
body bears little overall resemblance to trilobites. The single head shield with its two prominent pairs of spines, the subsequent body, with so many uniform segments of gradually decreasing size, and the tiny button of a rear end—all scarcely recalled the “standard” trilobite, with an external skeleton usually shaped as a broad oval, and divided into three basic sections of cephalon, thorax, and pygidium (head, body, and tail for those who shun jargon).

But then, no one had ever invoked overall shape to make claims for
Marrella
’s affinity with trilobites. Størmer had cited a strong similarity in the biramous appendages of the body as a rationale for establishing his concept of Trilobitoidea. However, as Whittington studied hundreds of specimens, he slowly began to discover consistent, and probably fundamental, differences between the appendages of
Marrella
and those of all known trilobites. Whittington admitted, of course, that the basic structures are similar. This overall resemblance had never been doubted, and Whittington quoted Størmer’s own words to emphasize the point: “These appendages are ‘more or less trilobite-like’ (Størmer, 1959, p. 26) in the general sense that there is a segmented walking leg and a filament-bearing gill branch” (Whittington, 1971, p. 21). But the differences began to impress Whittington even more. The walking leg of
Marrella
, with its six sections and terminal spines (see figure 3.15), bears one or two fewer segments than the standard and scarcely varying number in trilobites. Whittington concluded: “Neither branch is like that of any known trilobite, the walking leg having one (or two?) segments less than known in trilobites, the filament-bearing branch being differently constructed” (1971, p. 7).

Walcott’s interpretation of the head shield and its appendages (1912 and 1931) had provided the strongest case for classifying
Marrella
as a trilobite. Trilobites (see inset, page 106) bear a characteristic, almost stereotypical, arrangement of appendages on the cephalon, or head shield—one pair (called antennae) in front of the mouth, and three pairs behind the mouth (older studies argue for four post-oral segments, but later work, especially Whittington’s 1975 monograph on Burgess trilobites, has suggested three as more probable). Walcott reconstructed the head of
Marrella
in perfect conformity with the trilobite plan—one pair of antennae, and three subsequent pairs, which he called mandibles, maxillulae, and maxillae (1931, p. 31). Walcott even published photos (1931, plate 22) purporting to show this arrangement in clear and complex detail. This reconstruction provided a strong reason for linking
Marrella
with trilobites.

But Whittington soon developed doubts that gradually grew into disproof as he studied several hundred specimens. Later authors had not accepted Walcott’s version. (Størmer, for example, who affirmed the link of
Marrella
with trilobites, rejected Walcott’s reconstruction of the head, and relied on similarities in the body appendages.) Whittington found, first of all, that Walcott’s illustrations were products of the retoucher’s art, not fair maps of structures in rocks. On page 13, Whittington explains why his drawings of Walcott’s specimens look so different from Walcott’s 1931 photos: “The originals show that his illustrations were considerably retouched.” By page 20, this measured assessment had yielded to one of the few acerbic remarks in all of Whittington’s writings: “Several are heavily retouched to the point of falsification of certain features, notably the representation of the supposed mandible, maxilla, and maxillula.”

Whittington found only two pairs of appendages,
both
pre-oral—in front of the mouth—attached to the head shield of
Marrella:
the long, many-jointed first antennae (equivalent to Walcott’s “antenna” and interpreted by all in the same way), and a shorter and stouter pair of second antennae (Walcott’s “mandible”), composed of six segments, several covered with setae, or hairs. Whittington could find no trace of Walcott’s maxilla or maxillula, and he concluded that Walcott had confused some crushed and disarticulated legs of the first body segments with structures of the head shield. Walcott himself had admitted that he couldn’t find these supposed appendages on most specimens: “The maxillulae and maxillae were so slender that they are usually absent as the result of having been torn off or crushed between the strong mandibles [Whittington’s second antennae] and the thoracic limbs” (Walcott, 1931, pp. 31–32).

But recognition of two pre-oral (first and second antennae) and no post-oral appendages on the head shield of
Marrella
does not fully answer the anatomical question—for these two appendages could be related in a variety of potential ways, and a decision about taxonomic affinity depends upon the resolution. Whittington faced three major alternatives, all proposed before and each with different implications. First, the two antennae might represent the outer and inner branches of only one ancestral appendage—with the first antenna evolved from the outer gill branch (filaments lost and delicate shaft of numerous segments preserved), and the stout second antenna from the inner leg branch. Second, the two antennae might be truly separate by ancestry, arising as evolutionary modifications of two pairs of limbs on two original segments. Third, the second antenna, which looks so much like a walking leg, might really belong to the first body segment behind the head, and not be attached to the head shield at all. In this case, the head would bear only one pair of appendages—the first antennae.

Whittington wrestled with this issue above all others in resolving the anatomy of
Marrella
. He faced a technical problem because few, if any, specimens reveal the crucial point of connection between the head appendages and the shield itself. (The end of the appendage opposite to the point of attachment with the body—the distal, or farthest, end in technical parlance—is usually well preserved and easily visible because it projects well beyond the central axis of the body. But the end that attaches to the body—called the proximal, or nearest, end—is rarely resolvable because it lies under the axis and becomes inextricably mixed with the jumble of anatomical parts in this central region of the body.)

Whittington had to use all his tricks of analysis to resolve this issue—dissecting through the head shield to search for the limb attachments below, and seeking odd orientations that might reveal the proximal ends of the appendages. Figure 3.16 is a camera lucida sketch of the key specimen that finally drew Whittington to the second interpretation—the two antennae are distinct appendages, both attached to the head shield. This is the only specimen that clearly shows the proximal ends of both antennae, separately attached to the underside of the head shield.

3.16. Camera lucida drawing of the key specimen of
Marrella
that settled the major problem in reconstructing the head anatomy. Only this specimen shows the two pairs of appendages (labeled
a
₁
and
a
₂
) attached separately to the head shield.

Consider now the dilemma that Whittington faced as he began to compose his monograph on
Marrella
. He took for granted the old view that fossils fall within major groups and that life’s history moves toward increasing complexity and differentiation. Yet
Marrella
seemed to belong nowhere. Whittington had found that the legs of the body segments are not sufficiently trilobite-like to warrant classification in this group. He had established a sequence of head appendages—two pre-oral and none post-oral—not only unlike the one pre-oral and three post-oral of trilobites but also completely unknown among arthropods. What was he going to do with
Marrella
?

Today, this situation would cause no problem. Harry would simply smile and say to himself—ah, another arthropod beyond the range of modern groups, another sign that disparity reached its peak at the outset and that life’s subsequent history has been a tale of decimation, not increasing variety in design. But this interpretation was not available in 1971. The conceptual cart could not push this lead horse; in fact, the cart hadn’t even been constructed yet.

In 1971, Harry was still trapped in the concept that Burgess fossils, as old, must be primitive—either generalized members of large groups that later developed more specialized forms, or even more distant precursors that combined features of several groups and could be interpreted as ancestors to all. He therefore toyed with the idea that
Marrella
might be a kind of precursor for both trilobites and crustaceans—trilobites for the vague similarity in leg structure, crustaceans for the characteristic two pairs of pre-oral appendages on the head shield. (A weak argument even in its own terms, for Whittington had shown important differences in detail between the legs of
Marrella
and those of trilobites, while crustaceans also have three post-oral appendages on the head shield, and
Marrella
has none.) Still, Whittington was stuck with a conventional notion of primitivity, and he could offer no more to
Marrella
. He wrote: “
Marrella
is one of the fossils indicating the existence of an early arthropod fauna, characterized by serially uniform, generally trilobite-like limbs … and by a lack of jaws, features associated with particle and detritus feeding” (1971, p. 21).

But Whittington still had to classify
Marrella
. Again, a quandary, for
Marrella
possesses unique features that violate the key characters of every group of arthropods. Harry, on the brink of a transforming insight, chose caution and tradition this one time—and placed
Marrella
in Størmer’s Trilobitoidea, as the title of his monograph proclaims. Yet, as he did so, he felt the pain of betraying his own better judgment. “I had to put something at the top,” he told me, “so I put ‘Trilobitoidea.’” Yet, in the interval between submitting his manuscript and receiving printed copies, Whittington realized that he would have to abandon Trilobitoidea as an artificial group, a “wastebasket” hiding the most interesting story of arthropod evolution. He said to me: “When I saw
Marrella
printed with ‘Trilobitoidea’on top, I knew it was a bust.” But
Marrella
, in fact, had been the beginning of a boom—and the documentation of this anatomical explosion would soon transform our view of life.