SHOWDOWN ON THE BURGESS SHALE. Simon Conway Morris; Stephen Jay Gould. Natural History, Dec 1998 v107 i10 p48(1) Abstract: Stephen Jay Gould published "Wonderful Life: The Burgess Shale and the Nature of History." His observations on the diversity of fossils found in Canadian shale and made conclusions on evolution. However, many of Gould's interpretations may be flawed. A participant in the research reevaluates and challenges some confusions. Almost a decade ago, Harvard paleontologist and Natural History columnist Stephen Jay Gould published Wonderful Life: The Burgess Shale and the Nature of History (W. W. Norton and Company, 1989). In addition to chronicling ongoing work on the Burgess creatures, Gould used these fascinating fossils to exemplify his view of evolution. A few months ago, in The Crucible of Creation: The Burgess Shale and the Rise of Animals (Oxford University Press, 1998), invertebrate paleontologist Simon Conway Morris, of Cambridge University, a key player in Burgess research, challenged Gould's interpretations. We invited Conway Morris to summarize his argument, which we publish here, along with Gould's reply. The Challenge Few books on paleontology have achieved the wide readership of Stephen Jay Gould's Wonderful Life, which popularized research spearheaded by Harry Whittington at Cambridge on the 520-million-year-old Burgess Shale, found between two peaks in the Canadian Rockies near Banff. But Gould did much more than chronicle discoveries concerning these Cambrian fossils; he also set forth his own deeply held views on the mechanisms and nature of evolution--and even on humankind's place in the universe--as the "lessons" to be drawn from the Burgess Shale. In my new book, The Crucible of Creation, I argue that the major premises and conclusions of Wonderful Life must be seriously challenged. Let me begin with some matters of interpretation of patterns in the fossil record, then move on to paleontological particulars, and finally offer different "lessons" on what the Burgess Shale means in the larger reading of evolutionary history. Gould emphasizes, above all, the apparent weirdness and diversity of the Burgess fossils. How, he asks, was such an extraordinary range of anatomies produced, and all apparently in a blink of geological time? He hints at a special mechanism at work--some unusual genetic happenstance gone wild--that might account for the production of so many biological novelties in such a startlingly short period of time, perhaps only a few million years. And what if we could "rerun the tape" so that the subsequent history of this maelstrom of diversification would have taken a different course? We would still have a planet full of life, he argues, but surely one utterly different from our familiar world. Notably, this new trajectory of evolution would probably not have led to the human species and its unique form of consciousness and self-awareness, which emerged through a series of contingent accidents in a unique, unrepeatable sequence. Gould also charges that Charles D. Walcott, the discoverer of the Burgess Shale, was ill equipped to appreciate how diverse these novel phyla of sea creatures really were. Committed to the orthodox view that the range of life-forms must become ever greater over time, Walcott, in Gould's view, was unprepared to confront a world in which the proliferation of different kinds of life-forms (phyla) was much greater in the distant past than in, say, the age of dinosaurs or the more recent age of mammals. Therefore, he argues, Walcott attempted to "shoehorn" a range of previously unknown creatures into a few familiar categories to fit his preconceptions. Gould asserts that paleontologists have only just begun to appreciate the ever-expanding catalog of bizarre "dead-end experiments" conducted by nature in ancient seas. Looking back at the Cambridge group's classifications of the Burgess Shale, undertaken many decades after Walcott's pioneering work, my colleagues and I can see that we made some mistakes. Too often, we thought we had stumbled across yet another novel body plan (phylum, if you will), and in a few crucial instances, we did not realize that seemingly unrelated fossils were actually fragments of a single organism. With the benefit of hindsight, we can see that we had exaggerated the diversity of these supposedly bizarre fossils and needed to reconsider their evolutionary relationships. Recent discoveries in southern China (Yunnan) and northern Greenland (Peary Land) have provided links that join several of these previously unconnected fossils and establish them in recognizable phyla. Let's begin with the animal Wiwaxia. In Wonderful Life, it is described as "another Burgess oddball, perhaps closer to the Mollusca than to any other modern phylum ... but probably not very close." Ironically, the first breakthrough in establishing Wiwaxia's affinities came from a postgraduate paleontologist at Harvard who was inspired by Gould's lectures a decade or so ago. This young researcher, Nick Butterfield, managed to extract pieces of scalelike armor from the fossilized creature. When Butterfield studied their microstructure, he noticed immediately that it was the same as that of the chitinous bristles (chaetae) that project from the bodies of such modern annelids as earthworms. His conclusion, published in 1990, was that Wiwaxia was not a mollusk at all but an annelid. Yet this was what Walcott had claimed in 1911. In at least this case, Butterfield concluded, Walcott was not "shoehorning" bizarre animals into familiar phyla, as Gould had charged; Walcott had got it right the first time. Another recent discovery, in which I was fortunate to play a role, sheds further light on the place of certain Burgess animals in evolutionary history. On July 9, 1989, I was with a team in northern Greenland, collecting at a site containing fauna from Sirius Passet, a regional variant of the Burgess Shale. It was our first day at the site, and almost immediately we found an extraordinarily complete fossil of a halkieriid--an armored slug with a trig shell at either end. We wondered whether this organism--with such a weird anatomy, apparently so different from any other animal's--represented yet another new phylum. But that was only at first sight. Until then, halkieriids had been known only from the evidence of isolated scales; with our discovery of this and other complete specimens, however, we were able to confirm that the creature was in reality closely related to Wiwaxia. In making that connection, we were moving toward resolving a fundamental problem in evolution: How are body plans constructed, and how do new phyla actually emerge? To get from halkieriids, well represented as Lower Cambrian fossils, to Wiwaxia, which thrived in the Middle Cambrian, there is no need to postulate macroevolutionary jumps or some sort of genetic revolution. The halkieriids are not only older than Wiwaxia but also clearly more primitive. In life, halkieriids crawled across the seabed, their scales forming a beautifully arranged protective armor. Wiwaxia looked somewhat similar, but as Butterfield showed, its scales evolved into chaetae. So is Wiwaxia an annelid? It is really a matter of definition, but in my opinion, Wiwaxia is a member of the annelid stem group--a creature still in the process of becoming an annelid. Once scrutinized, the wiwaxiids and the halkieriids, despite their seemingly great differences, are closely related. They may be connected by two simple steps: the scales of halkieriids are transformed into wiwaxiid chaetae, and lobate, leglike extensions develop so that the style of locomotion changes from crawling to a kind of stepping. In recent years, the techniques of molecular biology have profoundly influenced paleontology in ways that hear on Gould's premise that the Burgess Shale was a seemingly inexplicable explosion of hundreds of bizarre life-forms, unrelated to anything familiar. One major surprise concerns the evolutionary position of the phylum Brachiopoda, a group with bivalved shells. Molecular data, quite unexpectedly, shows brachiopods to he closely related to annelids. Functional morphology also indicates that the shells of brachiopods must have originated as two separate valves; clams, in contrast, derived their familiar double shells from an ancestor with a single plate, across which developed a narrow zone of weakness, which became the hinge. Projecting from the margin of both valves of a brachiopod are delicate, chitinous bristles--identical to those of annelids Halkieriids also have two prominent shells. In the pre-brachiopods, I believe, the two shells were probably close to each other, back to back. To produce a true brachiopod, all that was necessary was to fold one shell beneath the other. And, interestingly, exactly this process can he seen in the embryological development of certain primitive, living brachiopods. So what was once a worm is transformed into a bivalved animal, the familiar brachiopod. Nor does the story finish here. If the scales of halkieriids can become chaetae, surely they can also evolve into the structurally identical chitinous bristles of a brachiopod. Of course, the origin of brachiopods is not so simple, but such transformations are functionally plausible and historically believable. Although constrained by genetic possibilities, they are products of convergent evolution. Similar environmental selection pressures, acting on differing anatomies, can create convergent or parallel adaptations. New discoveries and interpretations have altered our view of arthropod evolution as well. The biggest surprise is Hallucigenia, exemplar of the bizarre. Or is it? Recent finds from the Chinese deposit of Chengjiang reveal that my original reconstruction of this odd-looking, spiky animal had but one simple mistake: I had envisioned it upside down. Hallucigenia (a name coined by a colleague and me in an attempt to capture its dreamlike appearance) may still look strange, but with new discoveries, especially from southern China, Hallucigenia is now seen to belong to a group of primitive arthropods.