Wonderful Life: The Burgess Shale and the Nature of History (36 page)

3.74. From Briggs and Whittington, 1985. Additional Burgess arthropods, all drawn to the same scale. (A)
Perspicaris
. (B)
Plenocaris
.(C)
Leanchoilia
.(D)
Branchiocaris
.(E)
Marrella
.(F)
Yohoia
.(G)
Actaeus
.(H)
Canadaspis
.(I)
Waptia
.(J)
Burgessia
.

5.
Nektonic suspension feeders
. This small category—consisting of
Odaraia
and
Sarotrocercus
(figure 3.73A–B)—includes the true swimmers among Burgess arthropods. These genera either had no walking branches (
Sarotrocercus
) or possessed short inner branches that could not extend beyond the carapace (
Odaraia
). They had the biggest eyes among Burgess arthropods, and both probably sought small prey for filter feeding.

6.
Others
. Every classification has a residual category for unusual members.
Aysheaia
(figure 3.73C) may have been a parasite, living among and feeding on sponges.
Alalcomenaeus
(figure 3.73E) bears strong spines all along the inner edges of its walking legs, not only on the first segment, adjoining the food groove. Briggs and Whittington conjecture that
Alalcomenaeus
may have used these spines either to grasp on to algae, or to tear carcasses in scavenging.

Briggs and Whittington include two excellent summary figures in their paper (figures 3.73 and 3.74). Each genus is shown in its probable habitat, and all are drawn to the same scale—so that the substantial differences in size among genera may be appreciated.

Each of the six categories crosses genealogical lines. The ensemble fills a set of ordinary roles for modern marine arthropods. The great anatomical disparity among Burgess arthropods is therefore not a simple adaptive response to a wider range of environments available at this early time. Somehow, the same basic scope of opportunity originally elicited a far greater range of anatomical experimentation. Same ecological world; very different kind of evolutionary response: this situation defines the enigma of the Burgess.

In 1986, a year after his monograph on
Wiwaxia
, Simon Conway Morris published a “blockbuster” of another type—a comprehensive ecological analysis of the entire Burgess community. He began with some interesting facts and figures. About 73,300 specimens on 33,520 slabs have been collected from the Burgess Shale. Ninety percent of this material resides in Washington, in Walcott’s collection; 87.9 percent of these specimens are animals, and nearly all the rest are algae. Fourteen percent of the animals have shelly skeletons; the remainder are soft-bodied.

The fauna contains 119 genera in 140 species; 37 percent of these genera are arthropods. Conway Morris identified two main elements in the fauna: (1) An overwhelmingly predominant assemblage of benthic and near-bottom species that were transported into a stagnant basin by the mudslide. Conway Morris inferred, from abundant algae needing light for photosynthesis, that this assemblage originally lived in shallow water, probably less than three hundred feet in depth. He called this element the
Marrella-Ottoia
assemblage, to honor both the most common substrate walker (the arthropod
Marrella
) and the most common burrower (the priapulid worm
Ottoia
). (2) A much rarer group of permanently swimming creatures that lived in the water column above the stagnant basin, and settled amidst the animals transported by the mudslide. Conway Morris called this element the
Amiskwia-Odontogriphus
assemblage, to honor two of his pelagic weird wonders.

He found that the Burgess genera, despite their odd and disparate anatomies, fall into conventional categories when classified by feeding style and habitat. He recognized four major groups: (1) Deposit-feeding collectors (mostly arthropods)—60 percent of the total number of individuals; 25–30 percent of the genera. (This category includes
Marrella
and
Canadaspis
, the two most common Burgess animals, hence the high representation for individuals). (2) Deposit-feeding swallowers (mostly ordinary mollusks with hard parts)—1 percent of individuals; 5 percent of genera. (3) Suspension feeders (mostly sponges, taking food directly from the water column)—30 percent of individuals; 45 percent of genera. (4) Carnivores and scavengers (mostly arthropods)—10 percent of individuals; 20 percent of genera.

Traditional wisdom, with its progressionist bias and its iconography of the cone of increasing diversity, has viewed Cambrian communities as more generalized and less complex than their successors. Cambrian faunas have been characterized as ecologically unspecialized, with species occupying broad niches. Trophic structure has been judged as simple, with detritus and suspension feeders dominating, and predators either rare or entirely absent. Communities have been reconstructed with broad environmental tolerances, large geographic distributions, and diffuse boundaries.

Conway Morris did not entirely overturn these received ideas of a relatively simple world. He did, for example, find comparatively little complexity in the attacking and maneuvering capacities of Burgess predators: “It seems plausible that the degree of sophistication in styles of predation (search and attack) and deterrence in comparison with younger Paleozoic faunas was substantially less” (1986, p. 455).

Still, his primary message made the ecology of the Burgess Shale more conventional, and more like the worlds of later geological periods. Over and over again, when the full range of this community could be judged by its soft-bodied elements, Conway Morris found more richness and more complexity than earlier views had allowed. Detritus and suspension feeders did dominate, but their niches did not overlap broadly, with all species simply sopping up everything edible in sight. Rather, most organisms were specialized for feeding on particular types and sizes of food in a definitely limited environment. Suspension feeders did not absorb all particles at all levels in the water column; the various species were, as in later faunas, “tiered” in assemblages of complex interaction. (In tiering, various forms specialize, confining themselves to low, medium, or high level of the water column, as communities diversify.) Most surprising of all, predators played a major role in the Burgess community. This top level of the ecological pyramid was fully occupied and functioning. No longer could the disparity of early form be attributed to reduced pressures of an easy world, devoid of Darwinian competition in the struggle for existence, and therefore open to any contraption or jury-rigged experiment. The Burgess fauna, Conway Morris argued, “shows unequivocally that the fundamental trophic structure of marine metazoan life was established early in its evolution” (1986, p. 458).

Conway Morris had reached the same conclusion for the entire Burgess ecology that Briggs and Whittington had established for arthropod life styles. The “ecological theater” of the Burgess Shale had been rather ordinary: “It may transpire,” Conway Morris wrote, “that the community structure of the Phyllopod Bed was not fundamentally different from that of many younger Paleozoic soft-bodied faunas” (1986, p. 451). Why then was the “evolutionary play” of these early times so different?

Nothing breeds scientific activity quite so effectively as success. The excitement generated by recent work on the Burgess Shale has inspired an outburst of interest in soft-bodied faunas and the history of early multicellular life. The Burgess Shale is a small quarry in British Columbia, deposited in Middle Cambrian times, after the celebrated explosion of the Lower Cambrian. As long as its fauna remained geographically confined, and temporally limited to a mere moment after the main event, the Burgess Shale could not tell a story for all of life. The most exciting development of the past decade, continuing and accelerating as I write this book, lies in the discovery of Burgess genera all over the world, and in earlier rocks.

The first and most obvious extension occurred close to home. If a mudslide down an unstable slope formed the Burgess, many other slides must have occurred in adjacent regions at about the same time; some must have been preserved. As previously discussed, Des Collins of the Royal Ontario Museum has pioneered the effort to find these Burgess equivalents, and he has been brilliantly successful; during the 1981 and 1982 field seasons, Collins found more than a dozen Burgess equivalents in areas within twenty miles or less of the original site. Briggs and Conway Morris joined the field party in 1981, and Briggs returned in 1982. (See Collins, 1985; Collins, Briggs, and Conway Morris, 1983; and Briggs and Collins, 1988.)

These additional localities are not mere carbon copies of the Burgess. They contain the same basic organisms, but often in very different proportions. One new site, for example, entirely lacks
Marrella
—the most common species by far in Walcott’s original quarry. The champion here is
Alalcomenaeus
, one of the rarest creatures, with only two known examples, in the phyllopod bed. Collins also found a few new species.
Sanctacaris
, as already noted, is especially important as the world’s first known chelicerate arthropod. Another specimen, a weird wonder, has yet to be described; it is “a spiny animal with hairy legs, of unknown affinities” (Collins, 1985).

Above all, Collins has supplied the most precious themes of diversity and comparison to supplement Walcott’s canonical find. His additional localities include five assemblages sufficiently distinct in mix and numbers of species to be called different assemblages. Significantly, these additional sites include four new stratigraphic levels—all close in time to the phyllopod bed, to be sure, but still teaching the crucial lesson that the Burgess fauna represents a stable entity, not an unrepeatable moment during an early evolutionary riot of change.

A few basically soft-bodied Burgess species have lightly skeletonized body parts that can fossilize in ordinary circumstances—notably the sclerites of
Wiwaxia
and the feeding appendages of
Anomalocaris
. These have long been known from distant localities of other times. But a few bits do not make an assemblage. The Burgess fauna, as a more coherent entity, has now been recognized away from British Columbia, in soft-bodied assemblages in Idaho and Utah (Conway Morris and Robison, 1982, on
Peytoia
; Briggs and Robison, 1984, on
Anomalocaris
; and Conway Morris and Robison, 1986). These contain some forty genera of arthropods, sponges, priapulids, annelids, medusoids, algae, and unknowns. Most have not yet been formally described, but about 75 percent of the genera also occur in the Burgess Shale. Many species once known only for a moment in time, at a dot in space, now have a broad geographic range and an appreciable, stable duration. Writing about the most common Burgess priapulid, Conway Morris and Robison mark the “notable geographic and stratigraphic extensions of a previously unique occurrence.…
Ottoia prolifica
has a range through much of the middle Cambrian (?15 million years) during which time it shows minimal morphological changes” (1986, p. 1).

More exciting still has been the recognition of many Burgess elements in older sediments. The Burgess Shale is Middle Cambrian; the famous explosion that originated modern life occurred just before, during the Lower Cambrian. We would dearly like to know whether Burgess disparity was achieved right away, in the heart of the explosion itself.

Even before the most recent discoveries, a few positive hints were already in hand, notably some Burgess-like elements in the Lower Cambrian soft-bodied Kinzers fauna of Pennsylvania, and a suspected weird wonder from Australia, described as an annelid worm in 1979. Then, in 1987, Conway Morris, Peel, Higgins, Soper, and Davis published a preliminary description of an entire Burgess-like fauna from the mid-to-late Lower Cambrian of north Greenland. The fauna, like the Burgess itself, is dominated by nontrilobite arthropods. The most abundant creature, about a half inch in length, has a semicircular bivalved carapace; the largest, at about six inches, resembles the Burgess soft-bodied trilobite
Tegopelte
. Existing collections are poor, and the area is, as we say in the trade, “difficult of access.” But Simon will be visiting next year, and we can expect some new intellectual adventures. In the meantime, he and his colleagues have made the crucial observation, confirming that the Burgess phenomenon occurred during the Cambrian explosion itself: “The extension of stratigraphic ranges of at least some Burgess Shale–like taxa back into the early Cambrian also suggests that they were an integral part of the initial diversification of metazoans” (1987, p. 182).

Last year, my colleague Phil Signor, knowing of my Burgess interests, sent me a spare reprint from a colleague in China (Zhang and Hou, 1985). I could not read the title, but the Latin name of the subject stood out—
Naraoia
. Chinese publications are notorious for poor photography, but the accompanying plate shows an unmistakable two-valved, soft-bodied trilobite. A key Burgess element had been found half a world away. Far more important, Zhang and Hou date this fossil to the
early
part of the Lower Cambrian.

One creature is tantalizing; but we need whole faunas for sound conclusions. I am delighted to report—for it promises to be the most exciting find since Walcott’s original discovery itself—that Hou and colleagues have since published six more papers on their new fauna. If the djinn of my previous fable (see page 62) had returned five years ago and offered me a Burgess-style fauna at any other place and time, I could not have made a better choice. The Chinese fauna is half a world away from British Columbia—thus establishing the global nature of the Burgess phenomenon. Even more crucially, the new finds seem well dated to a time
deep
in the Lower Cambrian. Recall the general anatomy of the Cambrian explosion: an initial period, called Tommotian, of skeletonized bits and pieces without trilobites—the “small shelly fauna”; then the main phase of the Cambrian explosion, called Atdabanian, marked by the first appearance of trilobites and other conventional Cambrian creatures. The Chinese fauna comes from the second trilobite zone of the Atdabanian—right in the heart, and near the very beginning, of the main burst of the Cambrian explosion!