Wonderful Life: The Burgess Shale and the Nature of History (19 page)

On his cautious journey through the Burgess arthropods, Whittington meant to proceed in order of abundance.
Canadaspis
stood next in line, but Harry wanted a research student to handle the entire group of arthropods with bivalved carapaces (Derek Briggs would do this work with brilliant results, as Act 3 will show).
Burgessia
and
Waptia
, the two genera that Størmer had united as his subclass Pseudonotostraca, followed in terms of abundance. But Whittington had allocated these genera to his colleague Chris Hughes (who published a study of
Burgessia
in 1975, but has yet to finish his work on
Waptia
). Hence, Whittington tackled the next most abundant arthropod (with some four hundred specimens)—the interesting genus
Yohoia
, namesake of the national park that houses the Burgess Shale.

Whittington’s second monograph, his 1974 study of
Yohoia
, marks a subtle but interesting transition in his thinking, a necessary step toward the major transformation to come. Whittington had struggled with
Marrella
, and had come to the correct empirical conclusion—that this most common Burgess genus fits into no known group of arthropods. But he lacked the conceptual framework for thinking of Burgess organisms as anything other than primitive or ancestral—and he certainly had no inclination to construct a new guidepost for only one example that might not be typical. But one is an oddity, and two a potential generality. With
Yohoia
, Whittington made his first explicit move toward a new view of life.

Yohoia
is a very peculiar animal. It looks “primitive” and uncomplicated at first glance (figure 3.17)—an elongate body with a simple head shield, and no funny spines or excrescences. Walcott had placed
Yohoia
among the branchiopods, Størmer as an uncertain genus tacked to the end of Trilobitoidea. Yet, as Whittington proceeded, he became more and more puzzled. Nothing about
Yohoia
fitted with any known group.

The preservation of
Yohoia
left much to be desired by Burgess standards, and Whittington had difficulty resolving the order and arrangement of appendages—a crucial factor in arthropod taxonomy. He finally decided that the head probably bears three pairs of uniramous walking legs—nothing unconventional here, since this is the standard trilobite pattern and consistent with Størmer’s placement in Trilobitoidea. But the most curious anomaly of all stands just in front—the large pair of grasping appendages, composed of two stout segments at the base and four spines at the tip. This design is unique among arthropods, and Whittington found no name in the panoply of available jargon. With elegant simplicity, he opted for the vernacular and called this structure the “great appendage.”
*

3.17. Reconstruction of
Yohoia
by Whittington (1974). Note the unique great appendage (labeled
rga
and
lga
) attached to the head.

Yohoia
bears no other appendages on its head shield—no antennae,
†
no feeding structures (the so-called jaws and mouth parts of insects and other arthropods are modified legs—the main source for our feelings of bizarreness or discomfort when we view films of enlarged insects eating). The first ten body segments behind the head bear lobate appendages fringed with setae, or hairlike extensions (figure 3.18; see also figure 3.17). The appendage on the first segment may have been biramous, including a walking leg as well—but Whittington was not able to resolve the appendages satisfactorily due to poor preservation. Segments 11–13 are cylindrical and carry no appendages, while the last, or fourteenth, segment forms a flattened telson, or tail. Again, this arrangement of segments and appendages departs strongly from the standard trilobite pattern of biramous limbs on each body segment.
Yohoia
, with its great appendage in front, and curious arrangement of limbs behind, was an orphan among arthropods.

Whittington (interview of April 8,1988) remembers his study of
Yohoia
as a turning point in his thinking. He had assimilated
Marrella
, despite its uniquenesses, under the two reigning
p’
s—” primitive” and “precursor.” But
Yohoia
forced a different insight. This basically simple, elongate animal with many segments did have a primitive look in some respects. “This animal,” he wrote, “resembles Snodgrass’hypothetical primitive arthropod in that the alimentary canal extended the length of the body” (1974, p. 1). But Whittington did not shunt the uniquenesses aside, particularly the form of the great appendage. He had attempted a reconstruction of
Yohoia
as a working animal—showing how the lobate body appendages with their setal fringes might have been used for swimming, for breathing (as gills), and for transporting food particles; and how the great appendage might have captured prey with its spiny tips and then folded back to bring food right to the mouth.

All these features were unique anatomical specializations that probably helped
Yohoia
to work in its own well-adapted way. This animal was not a precursor with a few oddities, but an entity unto itself with a mixture of primitive and derived characters. “In the exoskeleton and appendages,” Whittington wrote, “
Yohoia tenuis
is clearly specialized” (1974, p. 1).

Thus, as the crucial year 1975 dawned, Whittington had completed monographs on two Burgess arthropods with the same curious result.
Marrella
and
Yohoia
didn’t fit anywhere—and they were specialized animals apparently living well with their unique features, not simple and generalized creatures from the dawn of time, ripe for replacement by more complex and competent descendants.

3.18.
Yohoia
. Drawn by Marianne Collins.

3.19. The fateful first expression of doubt. Whittington (1974, p. 4) still placed
Yohoia
in the Trilobitoidea, but expressed his doubt about the status of Størmer’s group.

Whittington remained too cautious to translate these suspicions into hard taxonomy. He still, and for one last time, placed
Yohoia
in Trilobitoidea, but with two crucial differences. He did not use Størmer’s category in the title of his monograph, and he inserted a fateful question mark after the designation in his formal taxonomic chart (1974, p. 4)—the first overt sign of challenge to the old order (figure 3.19). Whittington wrote: “I am doubtful whether
Yohoia
should be placed in Trilobitoidea” (1974, p. 2). Never doubt the conceptual power of a question mark.

3.20. Reconstruction of
Opabinia
by Whittington (1975). (A) Top view, showing the five eyes on the dorsal surface of the head. (B) Side view: note the orientation of the tail fins relative to the body; the dorsal surface is at the right.

In 1912, Walcott had described
Opabinia
as yet another branchiopod crustacean. Its curious design, particularly the bizarre frontal nozzle (figure 3.21), had made
Opabinia
a center of Burgess attention. Many different reconstructions had been attempted, but all authors had found a place for
Opabinia
within a major group of arthropods.
Opabinia
, as the most puzzling of all Burgess arthropods, stood as a challenge and a logical next step for Harry Whittington after two monographs on common genera (
Marrella
and
Yohoia
), and one on the structure of trilobite limbs (1975b).

3.21.
Opabinia
, showing the frontal nozzle with terminal claw, five eyes on the head, body sections with gills on top, and the tail piece in three segments. Drawn by Marianne Collins.

Whittington began his study of
Opabinia
without any doubt about its status as an arthropod. He soon received the surprise of his life, though the lesser oddities of
Marrella
and
Yohoia
had prepared him for astonishment from the Burgess. Whittington presented his first reconstruction of
Opabinia
to the annual meeting of the Palaeontological Association
*
in Oxford in 1972.