Free Wonderful Life: The Burgess Shale and the Nature of History by Stephen Jay Gould

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    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 century 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