Plate tectonic activity explains why a trove of sea creatures were found fossilized in the mountains of Yoho National Park rather than along a seafloor somewhere. But there’s still the question of why so many different types of marine invertebrates, including soft-bodied ones, were so unusually well preserved. Paleontologists think they know the answer. They think the marine animals that were later fossilized in the Burgess Shale lived near the bottom of an ancient sea in front of an underwater cliff or escarpment. Due to tectonic activity, blocks at the edge of this underwater cliff began to break off. These blocks slumped, creating underwater mudflows in their wake. These slumps and flows transported the Burgess animals several kilometers into deeper waters where they were buried in such a way as to leave them not only undamaged, but also protected from scavengers and bacteria. Very probably, the mudflows were highly turbulent, for paleontologists found the creatures dumped and preserved in a variety of angles in relation to the bedding. The speed and pressure of these mudflows quickly produced a preservation-friendly, oxygen-free environment. Then the turbulent and muddy currents pressed fine silt and clay into the crevices of the bodies at just the right consistency and pressure to fossilize them without tearing their delicate appendages, an ideal set of circumstances for ensuring later observation by future paleontologists.23
Due in part to the unusual circumstances under which these fossils were preserved, there’s now little doubt about the unparalleled disparity of the Cambrian fauna. Based on available evidence from the Burgess Shale and other sites around the world, the Cambrian period witnessed the appearance of, arguably, more disparate body plans than ever before or since. And this disparity arose at a most unexpected time, assuming Darwinian theory, namely, right at the dawn of animal life.
The Artifact Hypothesis
Walcott grasped these difficulties, and had a deep enough professional commitment to Darwinism, to search for a solution. He realized that the Precambrian fossil record could, in principle, assist in explaining the pattern of the Cambrian fossil record. The discovery of a rich Precambrian fossil history detailing variations accumulating little by little would serve to cast the pattern of the Burgess in a different light. Yet the Precambrian strata of his day showed no signs of providing any obvious transitional forms, much less a well-articulated bottom-up pattern of animals representing lower taxa proliferating into forms exemplifying higher and higher taxonomic categories. Nevertheless, an idea occurred to Walcott that gave him fresh hope.
Perhaps his awareness of the dramatic ways that the surface of the earth had changed over geologic time, making preservation of the Burgess fauna itself possible, inspired him. Finding marine animals so high above sea level no doubt made Walcott acutely aware of the way in which continents and seas had changed locations over the course of geological time. And so, Walcott, ever the geologist, proposed an ingenious geological solution to the biological problem of the origin of animal life. He noted that the Precambrian period was a period of dramatic continental uplift. He then suggested that the ancestors of the trilobites first evolved at a time when the Precambrian seas had receded from the landmasses. Then, at the beginning of the Cambrian, the seas again rose, covering the continents and depositing recently evolved trilobites. Thus, according to Walcott, ancestral precursors to the trilobites and other distinctive Cambrian forms had existed, but they were not fossilized in sediments that would later be elevated above sea level until early in the Cambrian; instead, before the Cambrian, during a period when sea levels were lower, trilobites and their ancestral forms were being deposited offshore in what are now only deep-sea sediments.24 Walcott named this cryptic period of time in which trilobites and other animals were rapidly evolving offshore as the “Lipalian interval.” (The term “Lipalian” is derived from the Greek word for lost.) In this view, the abrupt appearance of the Cambrian body plans in the geological column was merely an “artifact” of incomplete sampling of the fossil record and, indeed, the inability to access the undersea sedimentary layers where the ancestors of the Cambrian fauna presumably lay encased. In short, the transgression and regression of ancient seas made the ancestral precursors of the Cambrian fauna inaccessible to discovery.
His artifact hypothesis (also known as the “Lipalian interval” hypothesis) was a distinct advance over Darwin’s unadorned claim that the fossil ancestors of the Cambrian animals had not yet been discovered. Walcott’s hypothesis had the advantage of accounting for the sudden appearance of the trilobites and the absence of ancestral and transitional forms by reference to known geological processes. It also could be tested, at least once offshore drilling technology advanced to allow for the sampling of the buried offshore sedimentary rocks.
Although Walcott conceded that his hypothesis was essentially a negative argument that attempted to explain away the absence of evidence, he insisted that it was a sensible inference from his broad sampling of the paleontological data. “I fully realize that the conclusions above outlined are based primarily on the absence of a marine fauna in Algonkian [Precambrian] rocks,” he wrote, “but until such is discovered I know of no more probable explanation of the abrupt appearance of the Cambrian fauna than that I have presented.”25
Lumping and Splitting
Walcott used another strategy for squaring the Burgess Shale with Darwin’s theory of evolution. Taxonomists, tasked with identifying and naming distinct groups of life-forms, have been divided into two types: “lumpers” and “splitters.” “Lumpers” tend to group disparate organisms together in the same large classificatory categories and then make distinctions between them at lower taxonomic levels. “Splitters” tend to separate similar organisms into numerous higher taxonomic divisions. Walcott favored lumping, and his doing so with the Burgess fossils seemingly minimized the difficulties associated with the sudden proliferation of so many new Cambrian forms.
On his return to the Smithsonian, he placed all of the exotic forms of the Burgess into modern phyla. One of his efforts at lumping placed Marrella splendens not only in the same phylum, but also in the same class (Trilobita) as the trilobites, despite obvious morphological differences. He justified this classification by arguing that the organism foreshadowed the trilobite (compare Figs. 1.4 and 2.3). Gould later criticized Walcott’s method of classification as “shoehorning.” He noted that even one of Walcott’s fellow lumpers, Yale paleontologist Charles Schuchert, called the classification of Marrella into question.26 Gould also noted that Walcott used this strategy to minimize the challenge posed by the morphological disparity of the Burgess forms.27
Some paleontologists today reject Gould’s criticism of Walcott’s inclusion of so many Burgess animal forms into existing taxonomic categories.Nevertheless, few paleontologists think Walcott’s use of lumping explained away the Cambrian explosion. Most, for example, classify Marrella splendens within an existing modern phylum, namely, Arthropoda, even if they also classify it within a new and separate class, Marrellomorpha. Yet, whether Marrella, for example, falls within a novel phylum or class, matters less than explaining why so many clearly novel forms, and the novel structures these forms exhibit, first arose with such apparent suddenness.
Resolution—for a Time
Walcott thought that he had solved the mystery of the Cambrian explosion, as did many other Darwinists who gratefully adopted both his taxonomy and his version of the artifact hypothesis. And since Walcott’s approach held out hope of one day uncovering evidence of a Precambrian trunk for the animal phyla along with its primary limbs, adherents could not be accused of moving the paleontological case for Darwinism into the realm of untestable dogma. They had only to wait for the technologies of seafloor drilling to emerge and hope that nature had seen fit, unmolested under the ocean deeps, to leave concrete evidence of the gradual emergence of the major Cambrian body plans.
Walcott’s theoretical accomplishment was no mean feat. His discovery of the Burgess Shale was like a defense attorney with absolute faith in his client stumbling upon a room s
tuffed with clues that would seem to discredit him. Through his grouping of disparate body types into existing phyla and his ingenious version of the artifact hypothesis, Walcott had found an elegant way to explain all this seemingly uncooperative evidence in a Darwinian way.
In defending Walcott for overlooking significant features of the Burgess fossils, Gould points out that Walcott’s multiple and growing administrative demands hardly left time to revisit the foundational categories of animal taxonomy. How much less, then, was Walcott likely to revisit the most fundamental assumption of all—the assumption that animals originated gradually in a Darwinian way as the result of natural selection acting on small incremental variations? Consideration of that possibility would come decades later, only after Walcott’s version of the artifact hypothesis had itself exploded.
3
Soft Bodies and Hard Facts
In the spring of 2000, Discovery Institute, where I do my research, sponsored a lecture at the University of Washington geology department by renowned Chinese paleontologist J. Y. Chen (see Fig. 3.1). As the result of his role in excavating a new discovery of Cambrian-era fossils in southern China, Professor Chen’s standing in the scientific world was on the rise. The discovery, near the town of Chengjiang in the Kunming Province, revealed a trove of early Cambrian animal forms. After Time magazine mentioned the Chengjiang discovery in a 1995 cover story about the Cambrian explosion,1 interest in the fossils surged. When he came to Seattle, Professor Chen had already published numerous scientific papers about this profusion of novel life-forms and had established himself as one of the foremost experts on the fossils in this unique geological setting.
Not surprisingly, Chen’s visit generated considerable interest among the University of Washington faculty. He came bearing intriguing photographs and samples of the oldest and most exquisitely preserved Cambrian fossils in the world from an exotic site halfway around the globe, a site, moreover, that was now widely acknowledged to surpass even the legendary Burgess Shale as the most extensive and significant Cambrian-era locality.
The fossils from the Maotianshan Shale near Chengjiang (see Fig. 3.2) had established an even greater variety of Cambrian body plans from an even older layer of Cambrian rock than those of the Burgess, and they did so with an almost photographic fidelity. The Chinese fossils also helped to establish that the Cambrian animals appeared even more explosively than previously realized.
FIGURE 3.1
J. Y. Chen.
FIGURE 3.2
Figure 3.2a (left) shows the Moatianshan Shale outcrop. Courtesy Illustra Media. Figures 3.2b and c (center and right) show a Precambrian-Cambrian boundary marker at the Moatianshan site. Courtesy Paul Chien.
So there was little doubt about the significance of the discoveries that Chen came to report that day. What was soon in doubt, however, was Chen’s scientific orthodoxy. In his presentation, he highlighted the apparent contradiction between the Chinese fossil evidence and Darwinian orthodoxy. As a result, one professor in the audience asked Chen, almost as if in warning, if he wasn’t nervous about expressing his doubts about Darwinism so freely—especially given China’s reputation for suppressing dissenting opinion. I remember Chen’s wry smile as he answered. “In China,” he said, “we can criticize Darwin, but not the government. In America, you can criticize the government, but not Darwin.”
Nevertheless, those in the audience that day soon learned that Professor Chen had good reasons for questioning Darwin’s picture of the history of life. As Chen explained, the Chinese fossils turn Darwin’s tree of life “upside down.” They also cast doubt on a surviving version of Charles Walcott’s artifact hypothesis, a crucial prop in the case for Darwinian gradualism.
Burgess Revisited
By the time Charles Walcott finished his last excavation of the Burgess Shale in 1917, he and his team had collected over 65,000 fossil specimens, all of which were shipped to the Natural History Museum at the Smithsonian Institution for cataloguing. In 1930, another American paleontologist, Harvard professor Percy Raymond, initiated another investigation of the Burgess. His specimens were also eventually stored in the United States.
As a result of these two prominent American-led excavations, there were initially no collections of Burgess fossils on public display in Canada. Many Canadian scientists regarded this as a national embarrassment, so in the 1960s the Canadian Geological Survey commissioned a British team to resume digging at the Walcott quarry, in order to “repatriate the Burgess Shale” by keeping most of the newly discovered fossils on permanent display in Canada.2 The team was led by paleontologist Harry Whittington (see Fig. 3.3), of the University of Cambridge, who was assisted by two of his graduate students, Simon Conway Morris and Derek Briggs, both of whom would eventually distinguish themselves as internationally renowned experts on the Burgess Shale.
FIGURE 3.3
Harry Whittington. Courtesy Archives of the Museum of Comparative Zoology, Ernst Mayr Library, Harvard University.
As Whittington analyzed the Cambrian fauna at the Burgess, he realized that Walcott had grossly underestimated the morphological disparity of this group of animals. Many of the creatures in the assemblage featured unique body designs, unique anatomical structures, or both.3 Opabinia, with its five eyes, fifteen distinct body segments, and a claw at the end of a long proboscis, exemplified the unique forms on display in the Burgess. But so did Hallucigenia, Wiwaxia, Nectocaris, and many other Burgess animals. To this day, paleontologists describing Nectocaris, for example, can’t decide whether it more closely resembles an arthropod, a chordate, or a cephalopod (a class of mollusk; see Fig. 3.4).
Whittington found that grouping such forms within well-established taxonomic categories, even higher taxonomic categories such as the class or phylum, strained the limits of these classifications. Even many of those animals that fell easily into existing phyla represented clearly unique subphyla or classes of organisms. Anomalocaris (literally, “abnormal shrimp”) and Marrella, for example, both had hard exoskeletons and clearly represent either arthropods or creatures closely related to them. Yet each of these animals possessed many distinct anatomical parts and exemplified different ways of organizing these parts, thus clearly distinguishing themselves from better-known arthropods such as the previous staple of Cambrian paleontological studies, the trilobite.
Whittington, a trilobite expert, understood this as well as anyone. In 1971, he published the first comprehensive taxonomic review of the Burgess biota. In his review, he broke decisively with Walcott’s previous attempt to lump all Cambrian forms into a few preexisting taxonomic categories.
FIGURE 3.4
Figure 3.4a (left): Artist rendering of Nectocaris. Figure 3.4b (middle, right): Photograph of Nectocaris fossil. Reprinted by permission from Macmillan Publishers Ltd.: Nature, Martin R. Smith and Jean-Bernard Caron, “Primitive Soft-Bodied Cephalopods from the Cambrian,” Nature, 465 (May 27, 2010): 469–72. Copyright 2010.
In so doing, he reemphasized the morphological disparity present in the Burgess animal biota and, in the process, deprived evolutionary biologists of one part of Walcott’s two-part strategy for minimizing the Cambrian problem. By lumping all Burgess animals into existing phyla and classes, Walcott had seemingly diminished the problem of disparity by reducing the number of novel phyla for which connecting intermediates were required. By recognizing the disparity clearly on display in the Burgess, Whittington partially undercut Walcott’s solution to the Cambrian mystery and highlighted what would become its central unsolved problem: the origin of novel biological form.
Sounding the Seafloor
Although Whittington, and later Gould, rejected Walcott’s early attempt to “shoehorn” all the animals of the Burgess Shale into preexisting taxonomic categories, many paleontologists now also reject Stephen Jay Gould’s characterization of many Burgess Shale creatures as being so exotic as to defy affinity in classification with any modern groups.4 Many of these paleontologists would also recognize fewer total phy
la first appearing in the Cambrian than Gould did, and perhaps even as few as Walcott did. As discussed in the previous chapter, still other paleontologists now favor “rank-free” approaches to classification.
Regardless, most paleontologists recognize that the Burgess Shale attests to an extraordinary profusion of new animal forms—including many manifesting unique anatomical structures and arrangements of body parts. Thus, whatever differences of opinion exist about how to classify these animals—and any five-year-old child can distinguish them from each other and from all previously known forms of life—their origin still requires explanation. Thus, as noted, Walcott’s use of “lumping” did not solve the Cambrian mystery.5
But what of the second part of Walcott’s proposal, the artifact hypothesis? To evaluate this hypothesis, Walcott devised a more clear-cut and less subjective test. Recall that Walcott argued that the ancestral precursors of the Cambrian animals were missing from the Precambrian fossil record because of the transgression and regression of seas. He posited an interval of geologic time in which the ancestors of the Cambrian fauna were evolving offshore in a Precambrian ocean and being deposited only in layers of marine sedimentary rock. In this hypothesis, only after the ancient ocean rose and covered the continent were the remains of Cambrian sea animals preserved in sediment that today is above sea level.
When Walcott proposed his ingenious geological scenario, it could not yet be tested. But with the development of offshore drilling technology in the 1940s, 1950s, and 1960s, oil companies began to drill through thousands of feet of marine sedimentary rock.6 As geologists evaluated the contents of these drill cores, they did not find Walcott’s predicted Precambrian fossils.
Instead, an even more fundamental problem for the hypothesis arose. At the time that Walcott proposed his version of the artifact hypothesis, geologists considered the oceanic and continental plates to be essentially stable and fixed with respect to one another. Mountain building, faulting, and other geologic processes were attributed to worldwide changes in sea level, accumulating troughs of sediment called geosynclines, rising mounds of igneous rocks from beneath the earth’s crust, and even a shrinking earth.7
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