Wonderful Life: The Burgess Shale and the Nature of History (10 page)

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.

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:

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:

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 night, 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:

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.

On September 3, Walcott was even more successful: “Found a fine lot of Phyllopod crustaceans and brought in several slabs of rock to break up at camp.” In any event, he continued to collect, and put in a full day for his last hurrah on September 7: “With Stuart and Mr. Rutter went up on fossil beds. Out from 7
A.M.
to 6:30
P.M.
Our last day in camp for 1909.”

If I am right about his discovery of the main bed in 1909, then the second part of the canonical tale—the week-long patient tracing of errant block to source in 1910—must be equally false. Walcott’s diary for 1910 supports my interpretation. On July 10, champing at the bit, he hiked up to the Burgess Pass campground, but found the area too deep in snow for any excavations. Finally, on July 29, Walcott reported that his party set up “at Burgess Pass campground of 1909.” On July 30, they climbed neighboring Mount Field and collected fossils. Walcott indicates that they made their first attempt to map the Burgess beds on August 1: “All out collecting the Burgess formation until 4
P.M.
when a cold wind and rain drove us into camp. Measured section of the Burgess formation—420 feet thick. Sidney with me. Stuart with his mother and Helen puttering about camp.” “Measuring a section” is geological jargon for tracing the vertical sequence of strata and noting the rock types and fossils. If you wished to find the source of an errant block that had broken off and tumbled down, you would measure the section above, trying to match your block to its most likely layer.