Discovery and Study
The Burgess Shale is a unit in the Stephen Formation, located in the Yoho National Park in the Rockies of British Columbia. It is named after the nearby Burgess Pass close to Mt. Burgess. It is Middle Cambrian in age, currently dated about 515 Ma (Parker 1998) to 520 Ma (Conway Morris 1998).
The fossils of the Burgess Shale were discovered by Charles Walcott, Director of the U.S. Geological Survey and Assistant Secretary of the Smithsonian Institution, in 1909. The area was quarried by Walcott during the summer collecting seasons of 1910-1913, and again in 1917. He named the most fruitful part of the 6-m quarry, the lowest 2 m, the "Phyllopod Bed" after the animals with leaf-shaped legs that it contained. A total of over 60,000 fossils was collected from the Phyllopod Bed and the rest of the quarry; most of the collection is kept at the Smithsonian Institution's National Museum of Natural History.
In 1930, Percy Raymond, a Harvard University professor of paleontology, and several of his students went to the site. They reopened Walcott's quarry and made a new, less productive one about 21 m above Walcott's. The fossils from this expedition are located in the Museum of Comparative Zoology at Harvard University.
Around the mid-1960s, the Geological Survey of Canada was mapping the Rockies of southern Alberta and British Columbia. They commissioned a team of scientists, led by Harry Whittington of Cambridge University and J. D. Aitken of the Geological Survey, to reopen the two Burgess quarries during the collecting seasons of 1966 and 1967.
Further collections from the talus of the quarries were made by Des Collins of the Royal Ontario Museum in 1975, 1981, and 1982. Many of these specimens are on display at the Royal Ontario Museum and at other museums in Canada.
Walcott and others published some papers on the fossils, but their descriptions were far from exhaustive. Relatively little work had been done on this huge collection of fossils by the mid-1960s; Raymond's collection at Harvard was especially neglected. After the collections of the 1960s, Whittington, an expert on trilobites, led a project to redescribe the Burgess animals, assisted by his colleagues David Bruton and Christopher Hughes and graduate students Simon Conway Morris and Derek Briggs. This research has sparked the current excitement among paleontologists over the Burgess Shale, which is still strong after 25 years (Whittington 1980, Whittington 1985, Gould 1989, Conway Morris 1998).
These fossils comprise what paleontologists call a Lagerstätte, or "mother lode." Most fossils preserve only hard parts, meaning that animals without hard structures, such as calcite skeletons or shells, are lost. Soft-bodied fossils are preserved only very rarely, although they are more common to the Lower and Middle Cambrian than to later eras (Conway Morris 1998).
The area where the Burgess animals lived half a billion years ago was near the base of a huge escarpment up to 160 m high. This structure was a reef made by calcareous algae and branching bryozoa; their original limestone has converted to the dolomite of the Cathedral Formation. The name Cathedral Escarpment comes from the name of the formation. The Stephenson Formation's shales, including the Burgess Shale, were deposited alongside this cliff, with layers of sediment eventually overtopping the reef (Whittington 1985).
The animals were preserved after the sediments where they lived failed and flowed down the slope; there may have been more than 50 flows. The falling sediment lost cohesion, as shown by the separation by sediment of different parts of the bodies of the animals (eg. the branches of a biramous appendage) and by the variable orientations of the specimens (Conway Morris et al. 1982).
Many workers have thought that the animals of the Burgess experienced very little decay, indicating very rapid burial (eg. Gould 1989). Others suggest that the "dark stains" associated with many specimens is the result of decaying matter that seeped from the animals' bodies (eg. Whittington 1980). In any case, there is general consensus that the animals were moved from their aerobic habitat to an anaerobic area, where they were buried, because there is no evidence of scavenging and the total amount of decay is small (Whittington 1980).
The Burgess fossils look like shiny silver smears on the rocks. Whittington (1980) speculates that the preservation of the fossils may be due to a mineralizing solution that permeated the soft tissues. Analysis of the fossils, however, shows that the original material of the organism's body is still present in the fossils (Orr et al. 1998).
About 150 species in about 120 genera are found in this fauna (Whittington 1980), and not all have been described. Of those that have been studied since the revisions of the past few decades, many are difficult to place in known animal groups. This section deals only with a few of the more prominent and problematic species.
Many animals from this fauna are highly enigmatic in their affinities. For example, the arthropod Marrella splendens, one of the most common Burgess animals, belongs to none of the four major groups of arthropods (trilobites, chelicerates, crustaceans, and uniramians). Others could not even be placed in any known phylum by the researchers studying them (Gould 1989).
One such enigmatic animal was Opabinia regalis, interpretted by Walcott as an arthropod. This five-eyed predator had an anterior trunk- or nozzle-like appendage terminating in a claw-like apparatus, which it almost certainly used for grasping prey and conveying it to the ventral mouth. It possessed flap-like appendages, probably used for swimming, and possibly lobopodian legs (Conway Morris 1998). Once thought to possibly be a member of a previously unknown phylum (Gould 1989), it is now thought to belong to an extinct group of arthropods (Collins 1996).
Another Burgess enigma was Wiwaxia corrugata, a small animal which grazed the sea floor like some modern echinoderms and gastropods (Conway Morris 1998). This animal, which Walcott interpretted as a polychaete annelid, was armored with sclerites and spines. It had some possible molluscan affinities, moving on a structure much like a mollusc's foot and using a feeding apparatus similar to the molluscan radula, but there are significant differences. In his monograph on this species, Conway Morris could not place it in any extant phylum (Gould 1989), but it is now considered to be a polychaete of some sort after all. This affinity is inferred from the structural similarities between the sclerites of Wiwaxia and the chaetae of a Burgess polychaete, Canadia spinosa (Conway Morris 1998). Wiwaxia may even have ties to the brachiopods, a most unexpected relationship (Conway Morris 1998).
Even among those organisms redescribed by Whittington, Briggs, and Conway Morris, several glaring errors were made that were only corrected when better-preserved specimens came to light. Two particularly outstanding mistakes were Hallucigenia sparsa (Gould 1993; Conway Morris 1998), and Anomalocaris (Collins 1996, Conway Morris 1998).
Hallucigenia.-- Hallucigenia was originally interpretted by Conway Morris as a bilaterally symmetrical animal which walked or stood on seven pairs of rigid spines (Gould 1989). Along its back was a single row of tentacles, each terminating in a pincer-like apparatus. This bizarre organism, named for its "dream-like" appearance, was unlike any animal ever described before and was at first considered to belong to a new phylum. Gould (1989) and others speculated that it might have even been a piece broken from a larger animal.
Some of the surprise surrounding this strange creature was resolved by Ramsköld and Hou (1991) when they described a new animal from the Chengjiang Lagerstätte in China. Microdictyon was described as an armored lobopod, similar to extant onychophorans, which possessed a spine-bearing plate over each leg. Further, Hallucigenia was proposed to be a related taxon that was interpretted upside down by Conway Morris. The tentacles of Hallucigenia, if paired, bear a strong resemblance to lobopod legs. The fact that only a single row is apparent is due to a rotation of up to 45° of the animal's trunk, causing the legs and trunk to overlap proximally with the opposing row located under the body.
When Hallucigenia was inverted, its affinities to other lobopods were obvious. It is now considered by some workers to be a lobopodian arthropod of some sort (Conway Morris 1998).
Anomalocaris.-- Described in 1892, Anomalocaris canadensis was first found on Mount Stephen. It was determined to be the abdomen of a crustacean, whose appendages were highly peculiar in being unjointed -- hence its name, meaning "strange shrimp."
Meanwhile, Walcott's examination of the Burgess fossils revealed a strange-looking jellyfish which he named Peytoia nathorsti. Conway Morris and Whittington later described this creature as looking like a slice of pineapple.
In 1981, Whittington began preparing an unidentified specimen from Raymond's quarry and discovered that the animal had Anomalocaris appendages. More extraordinarilly, further preparation revealed that its mouth was a Peytoia. Thus the true nature of this predator was revealed, after having been described variously as a holothurian, a polychaete worm, and a composite fossil of a jellyfish superimposed as a sponge (Collins 1996).
Anomalocaris, like many other Burgess species, was at first considered to belong to a previously unknown phylum (Gould 1989). It is now proposed to belong to a new class of arthropods, the Dinocarida, along with Opabinia. At least two genera of the family Anomalocaridae are recognized, Anomalocaris and Laggania (Collins 1996).
The Burgess Shale comes from a time shortly after an event known as the Cambrian Explosion. This event, which took place over a very short period of geologic time, resulted in a sudden increase in the amount of diversity as well as of disparity of the world's fauna.
It has been suggested that this sudden radiation of phyla is connected to the onset of predation. According to this hypothesis, the selection pressure exerted on both predators and prey species stimulated rapid evolution.
This hypothesis has found some support. For example, the Ediacaran faunas, which predate the Burgess fauna and which are characterized by relatively low disparity, were probably free from predators. No animals with structures for seizing prey are known from these faunas, and there is no evidence in Edicaran fossils of damage from the attacks of predators. Both of these have been found in the Burgess fauna, and it has been suggested that the first skeletons arose as protection from predators (Conway Morris 1998).
Lending further support to this hypothesis is the fact that diffraction gratings were found in the external surfaces of the Burgess species Wiwaxia corrugata, Canadia spinosa (a polychaete), and Marrella splendens. These gratings were shown to cause the sclerites of W. corrugata, the setae of C. spinosa, and the outer prolongations on the head shield of M. splendens to be iridescent when light fell upon them. Because the gratings were present on presumably defensive structures, it was proposed that their function was aposematic, providing a visual warning against predators such as Anomalocaris and Opabinia (Parker 1998). Thus skeletons may have provided not only physical protection, but behavioral protection as well, by deterring predators.
Gould (1989) suggested that if the evolutionary history of animals were to be represented graphically, it should show a large number of branches initially, with most of those snipped off early in history, leaving relatively few of the original phyla to diversify until the present. He further suggests that the phyla which survived were not "more fit" than those which went extinct, but rather that chance events determined which groups died out. On the basis of this concept of the role of contingency in the history of life, he suggested that if the "tape" of earth's history were rewound and run again, a whole new set of phyla would have become dominant, and chordates might never have endured past the early Paaleozoic, let alone evolved into a riot of species including humans. History is unpredictable due to the effects of random, contingent events.
Conway Morris (1998) criticizes Gould's argument. He proposes that even in the light of contingency, events are predictable, noting that the same ecological niches exist and can only be fulfilled by certain body plans. Ecologies and anatomies follow predictable patters, even if some of the trivial details differ.
Gould's argument is based partly on the fact that at the time, it was thought that many of the Burgess species belonged to entirely novel phyla. In this scenario, the phyla that survived to the present day would be only a few of the total that had ever existed. The recent classification of many species into extant phyla casts doubt on this idea. If most or all of the Burgess organisms belonged to known groups, then their survival is not due to their being "in the right place at the right time." Gould also fails to propose a mechanism for the early extinction of most phyla.
The Burgess Shale is a valuable glimpse into earth's history. Its remarkable preservation allows a more accurate look at the past than those faunas which preserve only hard skeletons. The specimens it contains tell stories not only of earth's history by showing what happened, but of the nature of earth's history as well by hinting at why it happened. This remarkable fauna and others like it are key to understanding the history of life not only in the Cambrian, but in all eras.
Aguinaldo, A. M. A., Turbeville, J. M., Linford, L. S., Rivera, M. C., Garey, J. R., Raff, R. A., Lake, J. A. 1997. Evidence for a clade of nematodes, arthropods and other moulting animals. Nature 387:489-493.
Collins, D. 1996. The "evolution" of Anomalocaris and its classification in the arthropod Class Dinocarida (nov.) and Order Radiodonta (nov.). Journal of Paleontology 70:280-293.
Conway Morris, S., Whittington, H. B., Briggs, D. E. G., Hughes, C. P., Bruton, D. L. 1982. Atlas of the Burgess Shale. Special publication of the Palaeontological Association.
Conway Morris, S. 1998. The Crucible of Creation: The Burgess Shale and the Rise of Animals. Oxford: Oxford University Press, 242 p.
Gould, S. J. 1989. Wonderful Life: The Burgess Shale and the Nature of History. New York: W. W. Norton & Company, 347 p.
------. 1993. The reversal of Hallucigenia. Pages 342-352 in Eight Little Piggies: Reflections in Natural History. New York: W. W. Norton & Company, 479 p.
Hou, XG., and Bergström, J. 1995. Cambrian lobopodians -- ancestors of extant onychophorans? Zoological Journal of the Linnean Society 114:3-19.
Orr, P. J., Briggs, D. E. G., and Kearns, S. L. 1998. Cambrian Burgess Shale animals replicated in clay minerals. Science 281:1173-1175.
Parker, A. R. 1998. Colour in Burgess Shale animals and the effects of light on evolution in the Cambrian. Proceedings of the Royal Society of London Series B 265:967-972.
Ramsköld, L., and Hou, XG. 1991. New Early Cambrian animal and onychophoran affinities of enigmatic metazoans. Nature 351:225-228.
Waggoner, B. M. 1996. Phylogenetic hypotheses of the relationships of arthropods to Precambrian and Cambrian problematic fossil taxa. Systematic Biology 45:190-222.
Whittington, H. B. 1980. The significance of the fauna of the Burgess Shale, Middle Cambrian, British Columbia. Proceedings of the Geologists' Association 91:127-148.
------. 1985. The Burgess Shale. New Haven: Yale University Press, 151 p.
© 1997, 1998 by Robyn Conder Broyles. All rights reserved.
E-mail comments to Robyn Conder Broyles at ginkgo100@yahoo.com
Geology and Preservation
Descriptions of Selected Burgess Organisms
Overview
Recent Mistakes
Implications
The interest generated by the Burgess Shale stems largely from the novelty of the bizarre organisms it contains, but the fauna's significance goes beyond the enigmas of the anatomy of its species. The diversity and disparity of animals at this point in the earth's history raises questions about the history and pace of evolution. It has raised many questions among paleontologists, who hope that their attentions will produce some answers.
The Cambrian Explosion and the Onset of Predation
Contingency
Conclusion
Literature Cited
