Wonderful Life: The Burgess Shale and the Nature of History
Page 11
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).
TABLE 3.1. Walcott’s 1–912 Classification of Burgess Arthropoda
* * *
Crustaceaclass
1.Branchiopodasubclass
Anostracaorder
Opabinia
Leanchoilia
Yohoia
Bidentia
Notostraca order
Naraoia
Burgessia
Anomalocaris
Waptia
2.Malacostracasubclass
Hymenocaris [Canadaspis]
Hurdia
Tuzoia
Odaraia
Fieldia
Carnarvonia
3.Trilobitasubclass
Marrella
Nathorstia [Olenoides serratus]
Mollisonia
Tontoia
4.Merostomatasubclass
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.
TABLE 3.2. Størmer’s 1959 Classification of Trilobitoidea
* * *
Trilobitomorpha subphylum
Trilobita class
Trilobitoidea class
1. Marrellomorphasubclass
Marrella
2. Merostomoideasubclass
Sidneyia
Amiella
Emeraldella
Naraoia
Molaria
Habelia
Leanchoilia
3. Pseudonotostracasubclass
Burgessia
Waptia
4. subclassUncertain
Opabinia
Cheloniellon
Yohoia
Helmetia
Mollisonia
Tontoia
Marrella: FIRST DOUBTS
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 th
e 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.
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:
I think that it was vital. Of course it took hours and hours, but you saw everything yourself, and various things could sink in gradually. I love preparing [paleontological jargon for cleaning and exposing specimens in rock]. It is so exciting to find those hidden things. It is an incomparable thrill to reveal a hidden structure in the rock.
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 resolvi
ng 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.