Wonderful Life: The Burgess Shale and the Nature of History Page 6
Clearly, the Burgess pattern of stunning disparity in anatomical design is not characteristic of well-preserved fossil faunas in general. Rather, good preservation has permitted us to identify a particular and immensely puzzling aspect of the Cambrian explosion and its immediate aftermath. In a geological moment near the beginning of the Cambrian, nearly all modern phyla made their first appearance, along with an even greater array of anatomical experiments that did not survive very long thereafter. The 500 million subsequent years have produced no new phyla, only twists and turns upon established designs—even if some variations, like human consciousness, manage to impact the world in curious ways. What established the Burgess motor? What turned it off so quickly? What, if anything, favored the small set of surviving designs over other possibilities that flourished in the Burgess Shale? What is this pattern of decimation and stabilization trying to tell us about history and evolution?
THE SETTING OF THE BURGESS SHALE
WHERE
On July 11, 1911, C. D. Walcott’s wife, Helena, was killed in a railway accident at Bridgeport, Connecticut. Following a custom of his time and social class, Charles kept his sons close to home, but sent his grieving daughter Helen on a grand tour of Europe, accompanied by a chaperone with the improbable name of Anna Horsey, there to assuage grief and regain composure. Helen, with the enthusiasm of late teen-aged years, did thrill to the monuments of Western history, but she saw nothing to match the beauty of a different West—the setting of the Burgess Shale, where she had accompanied her father both during the discovery of 1909 and the first collecting season of 1910. From Europe, Helen wrote to her brother Stuart in March 1912:
They have the most fascinating castles and fortresses perched on the very tops. You can just see the enemy creeping up and up—then being surprised by rocks and arrows thrown down on them. We saw, of course, the famous Appian Way and the remains of the old Roman aqueducts—just imagine, those ruined-looking arches were built nearly 2000 years ago! It makes America seem a little shiny and new, but I’d prefer Burgess Pass to anything I’ve seen yet.
The legends of fieldwork locate all important sites deep in inaccessible jungles inhabited by fierce beasts and restless natives, and surrounded by miasmas of putrefaction and swarms of tsetse flies. (Alternative models include the hundredth dune after the death of all camels, or the thousandth crevasse following the demise of all sled dogs.) But in fact, many of the finest discoveries, as we shall soon see, are made in museum drawers. Some of the most important natural sites require no more than a pleasant stroll or a leisurely drive; you can almost walk to Mazon Creek from down-town Chicago.
The Burgess Shale occupies one of the most majestic settings that I have ever visited—high in the Canadian Rockies at the eastern border of British Columbia. Walcott’s quarry lies at an elevation of almost eight thousand feet on the western slope of the ridge connecting Mount Field and Mount Wapta. Before visiting in August 1987, I had seen many photos of Walcott’s quarry; I took several more in the conventional orientation (literally east, looking into the quarry, figure 2.2). But I had not realized the power and beauty of a simple about-face. Turn around to the west, and you confront one of the finest sights on our continent—Emerald Lake below, and the snow-capped President range beyond (figure 2.3), all lit, in late afternoon, by the falling sun. Walcott found some wonderful fossils on the Burgess ridge, but I now have a visceral appreciation of why, well into his seventies, he rode the transcontinental trains year after year, to spend long summers in tents and on horseback. I also understand the appeal of Walcott’s principal avocation—landscape photography, including pioneering work in the technology of wide-angle, panoramic shots (figure 2.4).
But the Burgess Shale does not hide in an inaccessible wilderness. It resides in Yoho National Park, near the tourist centers of Banff and Lake Louise. Thanks to the Canadian Pacific Railway, whose hundred-car freights still thunder through the mountains almost continuously, the Burgess Shale lies on the border of civilization. The railroad town of Field (population about 3,000, and probably smaller today than in Walcott’s time, especially since the Railway hotel burned down) lies just a few miles from the site, and you can still board the great transcontinental train from its tiny station.
Today you can drive to the Takakkaw Falls campground, near the Whiskey Jack Hostel (named after a bird, not an inebriated hero of the old West), and then climb the three thousand feet up to Burgess Ridge by way of a four-mile trail around the northwest flank of Mount Wapta. The climb has some steep moments, but it qualifies as little more than a pleasant stroll, even for yours truly, overweight, out of shape, and used to life at sea level. A more serious field effort can now employ helicopters to fly supplies in and out (as did the Geological Survey of Canada expeditions of the 1960s and the Royal Ontario Museum parties of the 1970s and 1980s). Walcott had to rely upon pack horses, but no one could brand the effort as overly strenuous or logistically challenging, as field work goes. Walcott himself (1912) provided a lovely description of his methods during the first field season of 1910—a verbal snapshot that folds an older technology and social structure into its narrative, with active sons scouring the hillside and a dutiful wife trimming the specimens back at camp:
2.2. Three views of the Burgess Shale quarries taken during my visit in August 1987. (A) The northern end of Walcott’s quarry, with Mount Wapta in the background. Note the quarry wall with cores drilled for the insertion of dynamite charges, and the debris from blasting on the quarry floor. (B) A similar view of the quarry opened by Percy Raymond in 1930, with yours truly and three avid geologists. This much smaller quarry lies above Walcott’s original site. (C) My son Ethan sitting on the floor of Walcott’s quarry as seen at the southern end.
2.3. The view from Walcott’s quarry. A geologist searches for fossils on the talus slope in the foreground. Emerald Lake lies beyond.
2.4. This reduced version of one of Walcott’s famous panoramic photographs gives a good impression of the technique, but lacks the grandeur of the original, which is several feet long. Walcott took this photograph in 1913. The right-hand side shows the Burgess quarry, with Mount Wapta to the left. Note some collectors and collecting tools within the quarry.
Accompanied by my two sons, Sidney and Stuart … we finally located the fossil-bearing band. After that, for days we quarried the shale, slid it down the mountain side in blocks to a trail, and transported it to camp on pack horses, where, assisted by Mrs. Walcott, the shale was split, trimmed and packed, and then taken down to the railway station at Field, 3,000 feet below.
A year before he discovered the Burgess Shale, Walcott (1908) described an equally charming, rustic technology for collecting from the famous Ogygopsis trilobite bed of Mount Stephen, a locality similar in age to the Burgess, and just around the next bend:
The best way to make a collection from the “fossil bed” is to ride up the trail on a pony to about 2,000 feet above the railroad, collect specimens, securely wrap them in paper, place them in a bag, tie the bag to the saddle, and lead the pony down the mountain. A fine lot can be secured in a long day’s trip, 6:00 AM to 6:00 PM.
The romance of the Burgess has had at least one permanent effect upon all future study of its fossils—the setting of their peculiar names. The formal Greek and Latin names of organisms can sometimes rise to the notable or the mellifluous, as in my favorite moniker, for a fossil snail—Pharkidonotus percarinatus (say it a few times for style). But most designations are dry and literal: the common rat is, for overkill, Rattus rattus rattus; the two-horned rhino is Diceros; the periwinkle, an inhabitant of near-shore, or littoral, waters, is Littorina littorea.
Burgess names, by contrast, are a strange-sounding lot. Decidedly not Latin in their roots, they are sometimes melodious, as in Opabinia, but other times nearly unpronounceable for their run of vowels, as in Aysheaia, Odaraia, and Naraoia, or their unusual consonants, as in Wiwaxia, Takakkawia, and Amiskwia. Walcott, who loved the Canadian Rockies and spent a quarter ce
ntury of summers in its field camps, labeled his fossils with the names of local peaks and lakes,* themselves derived from Indian words for weather and topography. Odaray means “cone-shaped”; opabin is “rocky”; wiwaxy, “windy.”
WHY: THE MEANS OF PRESERVATION
Walcott found almost all his good specimens in a lens of shale, only seven or eight feet thick, that he called the “phyllopod bed.” (“Phyllopod,” from the Latin for “leaf-footed,” is an old name for a group of marine crustaceans bearing leaflike rows of gills on one branch of their legs. Walcott chose this name to honor Marrella, the most common of Burgess organisms. Citing the numerous rows of delicate gills, Walcott dubbed Marrella the “lace crab” in his original field notes. According to later studies, Marrella is neither crab nor phyllopod, but one of the taxonomically unique arthropods of the Burgess Shale.)
At this level, fossils are found along less than two hundred feet of outcrop on the modern quarry face. Since Walcott’s time, additional soft-bodied fossils have been collected at other stratigraphic levels and localities in the area. But nothing even approaching the diversity of the phyllopod bed occurs anywhere else, and Walcott’s original layer has yielded the great majority of Burgess species. Little taller than a man, and not so long as a city block! When I say that one quarry in British Columbia houses more anatomical disparity than all the world’s seas today, I am speaking of a small quarry. How could such richness accumulate in such a tiny space?
Recent work has clarified the geology of this complex area, and provided a plausible scenario for deposition of the Burgess fauna (Aitken and Mcllreath, 1984; and the more general discussion in Whittington, 1985b). The animals of the Burgess Shale probably lived on mud banks built up along the base of a massive, nearly vertical wall, called the Cathedral Escarpment—a reef constructed primarily by calcareous algae (reef-building corals had not yet evolved). Such habitats in moderately shallow water, adequately lit and well aerated, generally house typical marine faunas of high diversity. The Burgess Shale holds an ordinary fauna from habitats well represented in the fossil record. We cannot attribute its extraordinary disparity of anatomical designs to any ecological oddity.
Catch-22 now intrudes. The very typicality of the Burgess environment should have precluded any preservation of a soft-bodied fauna. Good lighting and aeration may encourage high diversity, but should also guarantee rapid scavenging and decay. To be preserved as soft-bodied fossils, these animals had to be moved elsewhere. Perhaps the mud banks heaped against the walls of the escarpment became thick and unstable. Small earth movements might have set off “turbidity currents” propelling clouds of mud (containing the Burgess organisms) down slope into lower adjacent basins that were stagnant and devoid of oxygen. If the mudslides containing Burgess organisms came to rest in these anoxic basins, then all the factors for overcoming Catch-22 fall into place—movement of a fauna from an environment where soft anatomy could not be preserved to a region where rapid burial in oxygen-free surroundings could occur. (See Ludvigsen, 1986, for an alternate view that preserves the central idea of burial in a relatively deep-water anoxic basin, but replaces a slide of sediments down an escarpment with deposition at the base of a gently sloping ramp.)
The pinpoint distribution of the Burgess fossils supports the idea that they owe their preservation to a local mudslide. Other features of the fossils lead to the same conclusion: very few specimens show signs of decay, implying rapid burial; no tracks, trails, or other marks of organic activity have been found in the Burgess beds, thus indicating that the animals died and were overwhelmed by mud as they reached their final resting place. Since nature usually sneezes on our hopes, let us give thanks for this rare concatenation of circumstances—one that has enabled us to wrest a great secret from a generally uncooperative fossil record.
WHO, WHEN: THE HISTORY OF DISCOVERY
Since this book is a chronicle of a great investigation that reversed Walcott’s conventional interpretation of the Burgess fossils, I find it both fitting in the abstract, and beautifully symmetrical in the cause of narrative, that the traditional tale about his discovery is also a venerable legend badly in need of revision.
We are storytelling animals, and cannot bear to acknowledge the ordinariness of our daily lives (and even of most events that, in retrospect, seem crucial to our fortunes or our history). We therefore retell actual events as stories with moral messages, embodying a few limited themes that narrators through the ages have cultivated for their power to interest and to instruct.
The canonical story for the Burgess Shale has particular appeal because it moves gracefully from tension to resolution, and enfolds within its basically simple structure two of the greatest themes in conventional narration—serendipity and industry leading to its just reward.* Every paleontologist knows the tale as a staple of campfires and as an anecdote for introductory courses. The traditional version is best conveyed by an obituary for Walcott written by his old friend and former research assistant Charles Schuchert, professor of paleontology at Yale:
One of the most striking of Walcott’s faunal discoveries came at the end of the field season of 1909, when Mrs. Walcott’s horse slid on going down the trail and turned up a slab that at once attracted her husband’s attention. Here was a great treasure—wholly strange Crustacea of Middle Cambrian time—but where in the mountain was the mother rock from which the slab had come? Snow was even then falling, and the solving of the riddle had to be left to another season, but next year the Walcotts were back again on Mount Wapta, and eventually the slab was traced to a layer of shale—later called the Burgess shale—3000 feet above the town of Field (1928, pp. 283–84).
Consider the primal character of this tale—the lucky break provided by the slipping horse (figure 2.5), the greatest discovery at the very last minute of a field season (with falling snow and darkness heightening the drama of finality), the anxious wait through a winter of discontent, the triumphant return and careful, methodical tracing of errant block to mother lode. Schuchert doesn’t mention a time for this last act of patient discovery, but most versions claim that Walcott spent a week or more trying to locate the source of the Burgess Shale. His son Sidney, reminiscing sixty years later, wrote (1971, p. 28): “We worked our way up, trying to find the bed of rock from which our original find had been dislodged. A week later and some 750 feet higher we decided that we had found the site.”
A lovely story, but none of it is true. Walcott, a great conservative administrator (see chapter IV), left a precious gift to historians in his meticulous habits of assiduous record keeping. He never missed a day in his diary, and we can reconstruct the events of 1909 with fair precision. Walcott found the first soft-bodied fossils on Burgess Ridge on either August 30 or 31. His entry for August 30 reads:
Out collecting on the Stephen formation [the larger unit that includes what Walcott later called the Burgess Shale] all day. Found many interesting fossils on the west slope of the ridge between Mounts Field and Wapta [locality of the Burgess Shale]. Helena, Helen, Arthur and Stuart [his wife, daughter, assistant, and son] came up with remainder of outfit at 4 P.M.
2.5. Walcott in his seventies, during one of his last Western field seasons. He stands with his horse, reminding us of the legend of the discovery of the Burgess Shale.
The next day, they had obviously discovered a rich assemblage of soft-bodied fossils. Walcott’s quick sketches (figure 2.6) are so clear that I can identify the three genera depicted: Marrella (upper left), one of the unclassifiable arthropods; Waptia (upper right); and the peculiar trilobite Naraoia (lower left). Walcott wrote: “Out with Helena and Stuart collecting fossils from the Stephen formation. We found a remarkable group of phyllopod crustaceans. Took a large number of fine specimens to camp.”
What about the horse slipping and the snow falling? If this incident occurred at all, it must have been on August 30, when his family came up the slope to meet him in the late afternoon. They might have turned up the slab as they descended for the ni
ght, returning the next morning to find the specimens that Walcott sketched on August 31. This reconstruction gains some support from a letter that Walcott wrote to Marr (for whom he later named the “lace crab” Marrella) in October 1909:
When we were collecting from the Middle Cambrian, a stray slab brought down by a snow slide showed a fine phyllopod crustacean on a broken edge. Mrs. W. and I worked on that slab from 8 in the morning until 6 in the evening and took back with us the finest collection of phyllopod crustaceans that I have ever seen.
Transformation can be subtle. A previous snowslide becomes a present snowstorm, and the night before a happy day in the field becomes a forced and hurried end to an entire season. But, far more importantly, Walcott’s field season did not finish with the discoveries of August 30 and 31. The party remained on Burgess ridge until September 7. Walcott was thrilled by his discovery, and he collected with avidity every single day thereafter. Moreover, although Walcott assiduously reported the weather in every entry, the diary breathes not a single word about snow. His happy week brought nothing but praise for Mother Nature. On September 1, he wrote: “Beautiful warm days.”
2.6. The smoking gun that disproves the canonical story for the discovery of the Burgess Shale. Walcott sketched three Burgess genera on August 31 and then continued to collect with great success for another week.
Finally, I strongly suspect that Walcott located the source of his stray block during that last week of 1909—at least the basic area of outcrop, if not the phyllopod bed itself. On September 1, the day after he sketched the three arthropods, Walcott wrote: “We continued collecting. Found a fine group of sponges on slope (in situ) [that is, undisturbed and in their original position].” Sponges, containing some hard parts, extend beyond the richest layers of soft-bodied preservation at this site, but the best specimens come from the phyllopod bed. On each subsequent day, Walcott found abundant soft-bodied specimens, and his descriptions do not read like the work of a man encountering a lucky stray block here and there. On September 2, he discovered that the supposed shell of an ostracode had really housed the body of a phyllopod: “Working high up on the slope while Helena collected near the trail. Found that the large so called Leperditia like test is the shield of a phyllopod.” The Burgess quarry is “high up on the slope,” while stray blocks would slide down to the trail.