Wonderful Life: The Burgess Shale and the Nature of History (16 page)

Just as important, and as crucial to the Burgess story, is the specialization and differentiation of appendages. Each segment of the original, unspecialized, many-segmented arthropod bore a pair of appendages
—
one on each side of the body. Each appendage consisted of two branches, or
rami
(singular
ramus
). These rami are named according to their position
—
the
inner ramus
and the
outer ramus
—
or according to their usual function. Since the outer branch often bears a gill used in respiration or swimming (or both), it is often called the
gill branch
. The inner branch is usually used in locomotion, and may be called the
leg branch
,
walking branch
, or
walking leg
. (The common term “walking leg” may strike readers as amusingly redundant, but “leg” is an anatomical, not a functional term, and not all arthropods use their legs for walking; insect mouth parts, for example, are slightly modified legs.
)

This original structure (figure 3) is called a
biramous
(literally, “two-branched”) limb. (If you retain no other term from this discussion, please inscribe the definition of a biramous limb in your long-term memory. It is the single most important facet of arthropod anatomy in our Burgess discussion.) Specialized arthropods often lose one of the two branches, retaining the other as a
uniramous
(“one-branched”) limb. (Please place “uniramous” next to “biramous” in your long-term memory.) The higher-level taxonomy of arthropods records the different mixes of uniramous and biramous limbs on various parts of the body
.

The walking legs of most marine arthropods perform an additional function that seems odd from our vertebrate-centered perspective. Some marine arthropods feed as we do by seizing food items in front of their head and passing them directly to the mouth. But most use their walking legs to grasp food particles and pass them forward to the mouth along a
food groove
situated in the
ventral
(bottom) midline, between the legs. (The top side of an animal is called
dorsal
.)
Arthropod
means “jointed foot,” and the appendages are composed of several segments. Segments located near the body are
proximal
; those far a way at the ends of the appendage are
distal
. The most proximal segment of the walking leg is called a
coxa
. The edge of the coxa bordering the food groove is often armed with teeth, used to capture and move the food forward (see figure 3) and called a
gnathobase
(literally, “jawed foundation”
).

We form the higher-level taxonomy of arthropods by joining the two principles discussed above: patterns of tagmosis, or fusion of segments, and specialization of appendages by loss of one ramus and differentiation of the other. Beginning with an ancestral arthropod built of many unfused segments, each bearing a biramous limb, the major groups have evolved along different routes of tagmosis and specialization. Consider the four major kinds of arthropods:

1.
Uniramia
. As the name implies, insects and their kin have invariably lost the gill branch of the original biramous limb; they build their appendages (antennae, legs, mouth parts) exclusively from leg branches. (Insects breathe through invaginations of the external body surface, called tracheae.
)

2.
Chelicerata
.
Most modern chelicerates have six uniramous appendages on the prosoma. The first pair—chelicerae
—
are jawlike at the distal end and are used for grasping. (Antennae are absent in this group.) The second pair
—
pedipalps
—
are usually sensory in function. The last four pairs are usually leglike (giving spiders their eight legs). All these anterior appendages have evolved from leg branches. The situation is reversed on the posterior section. The opisthosomal appendages are also uniramous, but have been built from gill branches only. (The “lung-books,” or breathing organs, of spiders are on the abdomen.
)

3.
Crustacea
. Despite an enormous diversity of form, from barnacles to lobsters, all crustaceans are distinguished by their stereotypical pattern of five pairs of appendages on the head (indicating that the head was formed by a tagmosis of at least five segments). The first two pairs, usually called antennae and antennules, are uniramous; they lie in a
pre-oral
position, in front of the mouth, and have sensory functions. The last three lie in a
post-oral
position, behind the mouth, and are usually used in feeding, as mouth parts. Appendages on the trunk often retain the original biramous form
.

4.
Trilobita
. The trilobite head bears one pre-oral pair of appendages (antennae) and three post-oral pairs. Each body segment usually bears a pair of biramous limbs very little modified from the presumed ancestral form
.

The stereotypy of these patterns is, perhaps, the most striking phenomenon in modern arthropods. Of nearly a million described species of insects, none has a biramous appendage, and nearly all have exactly three pairs of limbs on the thorax. Marine crustacea display incredible diversity of form, but all have the same pattern of tagmosis in the head—
two pre-oral and three post-oral pairs of appendages. Apparently, evolution settled upon just a few themes or ground plans for arthropods and then stuck with them through the greatest story of diversification in the entire animal kingdom.

The story of the Burgess Shale ranks as perhaps the most amazing in the history of life largely in relation to this phenomenon of later restriction in arthropod ground plans
—
for in addition to early representatives of all four later groups, the Burgess Shale, one quarry in British Columbia, contains fossils of more than twenty additional basic arthropod designs. How could such disparity originate so quickly? Why did only four basic designs survive? These questions form the primary subject of this book
.

But the Burgess was too beautiful and variegated a plum to resist. Besides, everybody knew that its arthropods—the focus of Whittington’s proposed work—posed no major taxonomic dilemmas. Harry told me that when he first decided to work on the Burgess, he “expected to spend a year or two describing some arthropods—full stop.” In England, a “full stop” is a period—ending the sentence, and ending the project.

It was not to be. Harry Whittington spent four and a half years just writing his first monograph on the genus
Marrella
. Surprise cascaded upon surprise, starting slowly with doubts about the identity of certain arthropods, and accelerating until a new interpretation jelled in the mid-1970s. This view blossomed to guide all subsequent work toward a new conception for the history of early life. As I read the taxonomic monographs in chronological order, I came to see this story as a classical drama in five acts. No one was killed; few people even got angry. But just as Darwin let his theory gestate for twenty-one basically quiet years between formulation and publication, the similar time for the reevaluation of the Burgess Shale has produced, behind a placid exterior, an intellectual drama of the highest order.

THE CONCEPTUAL WORLD THAT WHITTINGTON FACED

Harry Whittington is, by nature, a cautious and conservative man. To this day, though he served as midwife to a major transformation of thought, he views himself as an empiricist, with skill in the meticulous description of arthropod fossils. His favorite motto exhorts his younger colleagues to place fact and description before theory, for “one should not run before one can walk.”

Whittington began, as would any paleontologist who believes in cranking up slowly and deliberately, with the genus
Marrella
, the most common organism in the Burgess Shale.
Marrella splendens
overwhelms anything else in the Burgess by sheer abundance. Walcott collected more than 12,000 specimens. Whittington’s party gathered another 800, and I am custodian to 200 more, collected by Percy Raymond in 1930. Many Burgess species are known from fewer than ten specimens, some from only one. But with nearly 13,000 potential views, one need hardly worry about destroying unique evidence by dissection, or failing to find a crucial orientation.

Marrella splendens
is the first Burgess organism that Walcott found and drew; it virtually identifies the Burgess Shale. When Walcott described
Marrella
formally in 1912, he recognized that his “lace crab” was not a conventional trilobite, but still placed
Marrella
in the class Trilobita, order previously unknown. Following his need to view Burgess organisms as primitive members of later successful groups, he wrote: “In
Marrella
the trilobite is foreshadowed” (1912, p. 163).

Not all of Walcott’s colleagues were convinced. In the Smithsonian archives, I found some interesting correspondence with Charles Schuchert, celebrated Yale paleontologist and codifier of the canonical legend about Walcott’s discovery of the Burgess Shale. After reading his friend Walcott’s paper on the Burgess arthropods, Schuchert wrote to him on March 26, 1912:

Yet Schuchert, as committed as Walcott to the larger theme that all Burgess creatures belong in known groups, never suggested uniqueness for
Marrella
, but only hinted at a different home among well-known arthropods.

To give some idea of the conceptual barriers that Whittington faced when he began to redescribe the arthropods of the Burgess Shale, I must now exemplify what I shall call, throughout this volume, “Walcott’s shoehorn”—his decision to place all Burgess genera in established major groups. Most readers will need to consider these pages in conjunction with the insets on taxonomy and arthropod anatomy (pages 98 and 102). I am asking some investment here from readers with little knowledge of invertebrate biology. But the story is not difficult to follow, the conceptual rewards are great, and I shall try my best to provide the necessary background and guidance. The material is not at all conceptually difficult, and the details are both beautiful and fascinating. Moreover, you can easily retain the thread of argument without completely following the intricacies of classification—as long as you realize that Walcott and all students of the Burgess before Whittington placed these organisms in conventional groups, and that Whittington slowly weaned himself away from this tradition, and toward a radical view about the history of life’s diversification.