Wonderful Life: The Burgess Shale and the Nature of History (17 page)
Read Wonderful Life: The Burgess Shale and the Nature of History Online
Authors: Stephen Jay Gould
Walcott presented his complete classification of Burgess arthropods on page 154 of his 1912 paper (reproduced here as table 3.1). He scattered his Burgess genera widely among four subclasses, all placed within his version of the class Crustacea. Walcott defined Crustacea far more broadly than we do today. He included virtually all marine and freshwater arthropods, organisms that span the entire arthropod phylum as defined today. Of his four subclasses, the modern branchiopods (1) are a group of predominantly freshwater crustaceans, including the brine shrimp and the cladocerans, or water fleas; malacostracans (2) form the great group of marine crustacea, including crabs, shrimp, and lobsters; trilobites (3) are, of course, the most famous of fossil arthropods; while merostomes (4), including fossil eurypterids and modern horseshoe crabs, are closely related to terrestrial scorpions, mites, and spiders.
The fate of Walcott’s 1912 chart is a striking epitome of the entire Burgess story. Of his twenty-two genera, only two are legitimate members of their groups.
Nathorstia
(now called
Olenoides serratus
) is an uncontroversial trilobite (Whittington, 1975b);
Hymenocaris
(now called
Canadaspis
) is a true crustacean of the malacostracan line (see Act 3). Three genera (
Hurdia, Tuzoia
, and
Carnarvonia
) are bivalved arthropod carapaces with no soft parts preserved; they cannot be properly allocated to any arthropod subgroup, and remain unclassified today. Three other names do not belong to the story of Burgess arthropods:
Tontoia
, position still unresolved and possibly inorganic, comes from the Grand Canyon, not the Burgess Shale;
Bidentia
is an invalid name, and these specimens belong to the genus
Leanchoilia; Fieldia
, misidentified by Walcott, is a priapulid worm, not an arthropod.
Of the remaining fourteen genera, two (
Opabinia
and
Anomalocaris
) have been reallocated to unique phyla bearing no known relationship to modern groups; they, and at least a dozen others of similar status (classified, for the most part, as annelid worms by Walcott), form the centerpiece of my story. Another eleven have been taken from the known and comfortable homes that Walcott designated, and reclassified as arthropods of unique anatomy, outside the range of any other modern or fossil group. Only
Naraoia
, which Walcott classified as a branchiopod crustacean, belongs in a known group, though Walcott chose the wrong one.
Naraoia
is, in fact, a highly peculiar trilobite (Whittington, 1977).
T
ABLE
3.1.
Walcott’s 1–912 Classification of Burgess Arthropoda
Crustaceaclass
Anostracaorder
Opabinia
Leanchoilia
Yohoia
Bidentia
Notostraca order
Naraoia
Burgessia
Anomalocaris
Waptia
Hymenocaris
[
Canadaspis
]
Hurdia
Tuzoia
Odaraia
Fieldia
Carnarvonia
Marrella
Nathorstia
[
Olenoides serratus
]
Mollisonia
Tontoia
Molaria
Habelia
Emeraldella
Sidneyia
When I state that no one challenged Walcott’s shoehorn until Whittington and colleagues redescribed the Burgess Shale, I do not mean that all paleontologists accepted Walcott’s specific allocations. Articles on Burgess organisms were sparse during the sixty years between Walcott’s descriptions and Whittington’s first monograph—especially considering the importance of the fauna, as acknowledged by all paleontologists
*
—but the limited literature proposed several schemes for taxonomies departing strongly from Walcott’s.
But these alternatives, however varied among themselves, never abandoned a strict allegiance to Walcott’s larger presupposition—the shared, and almost always unstated, view of paleontologists that fossils fall into a limited number of large and well-known groups, and that life’s history generally moves toward increasing complexity and diversity.
Leif Størmer drew the task of describing most Burgess Shale arthropods for the collectively written
Treatise on Invertebrate Paleontology
, and published his results (Størmer, 1959) in a large volume devoted primarily to trilobites. Størmer’s solution was diametrically opposed to Walcott’s. Instead of spreading the Burgess arthropods widely among groups throughout the phylum, he brought most of them together in allegiance with the trilobites themselves. He could not, of course, claim that all these diverse and most untrilobite-like animals
truly
belonged to the class Trilobita proper. But he did neatly (however falsely) resolve the problem of arthropod disparity in the Burgess by placing all the major genera in one supposedly coherent evolutionary group, lying right next to the Trilobita. He called his group the Trilobitoidea (literally, “trilobite-like”).
This solution may seem too pat or arbitrary to be believed. But Størmer had a rationale (invalidated, as we shall see, by later advances in taxonomic theory). He acknowledged, of course, the great range of form among Burgess arthropods, but he forged a taxonomic union because he argued that they all possessed the same kind of “primitive” appendages on body segments behind the head—a biramous, or two-pronged, form with a gill branch above a leg branch (see inset, page 104). Since trilobites also possessed appendages of this form, the Trilobita proper and the Trilobitoidea (the heterogeneous Burgess oddballs) could be grouped together in a larger taxon, called Trilobitomorpha. Størmer presented the following rationale:
The Trilobitomorpha are linked together by the seemingly common basic structure of their appendages. Since the trilobite limb appears to be a characteristic and conservative structure, its presence in fossil arthropods may be interpreted as evidence of close relationship between the many different forms possessing it (1959, p. 27).
Størmer’s classification of the Trilobitoidea is shown in table 3.2. All but two of his sixteen genera reside exclusively in the Burgess Shale (
Tontoia
, as previously mentioned, comes from the Grand Canyon;
Cheloniellon
, from the Devonian
Lagerstätte
of the Hunsrückschiefer). Størmer divided the Burgess genera into three groups: (1)
Marrella
alone; (2) the cluster that Walcott had aligned with the merostomes, or horseshoe-crab group, a superficial similarity that Størmer acknowledged in his name Merostomoidea (“merostome-like”); (3) the genera that Walcott had placed in the Notostraca, a group of branchiopod crustaceans (a superficial similarity honored by Størmer in his chosen name Pseudonotostraca). Yet, try as he might, Størmer could not comfortably squeeze all the Burgess forms into his Trilobitoidea. Four genera stumped him, and he tacked them onto the end of his classification as “subclass Uncertain”—a solution neither elegant nor Latin.
I have presented this detailed contrast of Størmer’s system with Walcott’s original scheme for two reasons. First, the power of the shoehorn can be illustrated by demonstrating that all taxonomic solutions, however divergent in a plethora of details, worked within this unchallenged postulate. Both Walcott’s scattering into a broad range of known groups, and Størmer’s gathering together as the Trilobitoidea remained fully faithful to the rule of the shoehorn—all Burgess genera belong in established groups. Second, Størmer’s interpretation, published in the major compendium of international opinion, was the most up-to-date, standard classification of Burgess arthropods when Whittington started his project. Størmer’s Trilobitoidea was Whittington’s context as he began his monograph on
Marrella
.
T
ABLE
3.2.
Størmer’s 1959 Classification of Trilobitoidea
Trilobitomorpha subphylum
Trilobita class
Trilobitoidea class
Marrella
Sidneyia
Amiella
Emeraldella
Naraoia
Molaria
Habelia
Leanchoilia
Burgessia
Waptia
Opabinia
Cheloniellon
Yohoia
Helmetia
Mollisonia
Tontoia
Harry Whittington’s initial monograph on
Marrella
(1971) scarcely reads like the stuff of revolution—at first glance. It begins with an introduction by Y. O. Fortier, director of the Geological Survey of Canada. Parroting the traditional assumptions of Walcott’s shoehorn and the cone of increasing diversity, Mr. Fortier launched the entire enterprise with the following paragraph:
The Burgess Shale of Yoho National Park, British Columbia, is world famous and unique. It was from these fossiliferous Cambrian beds that Charles D. Walcott … collected and subsequently described … a remarkable and diversified group of fossils that represent the
primitive ancestors of nearly every class of arthropod
as well as several other animal Phyla [my italics].
Whittington’s title contains no hint of the shape of things to come. He followed the standard form of taxon, place and time—what my former student Warren Allmon calls “
x
from the
y
-ity of z-land.” He even adopted—but for the only time, and much to his later regret—Størmer’s name Trilobitoidea: “Redescription of
Marrella splendens
(Trilobitoidea) from the Burgess Shale, Middle Cambrian, British Columbia.”
Marrella
is a small and elegant animal (figure 3.12), fully meriting Walcott’s choice for its specific name—
Marrella splendens
. Specimens measure from 2.5 to 19 mm (less than an inch) in length. The head shield is narrow, with two prominent pairs of spines directed backward (figures 3.13 and 3.14). Behind the head, twenty-four to twenty-six body segments, each bear a pair of biramous (two-branched) appendages (figure 3.15), composed of a lower walking leg and an upper branch bearing long and delicate gills (the source of Walcott’s informal name, “lace crab”). A tiny button, called a telson, caps the rear end. Traces of the gut are preserved on some specimens. The rock surface just adjacent to the fossil itself often shows a characteristic dark stain—probably a remnant of body contents that oozed out beyond the external skeleton after death.
3.12. Side view of
Marrella
. Drawn by Marianne Collins.
3.13. Reconstruction of
Marrella
by Whittington (1971), top view. Note the two pairs of appendages and the two pairs of spines on the head shield. The second pair of spines sweeps back to cover the entire organism. The gill branches are omitted on the animal’s left side, and the leg branches on the right side—all for greater ease in visual resolution. These omissions are standard in scientific illustrations, but can be confusing if you don’t know the tradition.
