Burgess Shale fauna

Burgess Shale fauna The Burgess Shale is a lens-shaped mass of muddy sediments in the basinal shales of the Middle Cambrian Stephen Formation near Field, British Columbia. It has become exceptionally well known because of the extensive soft-bodied fossil fauna that it contains, making it an important example of a conservation Lagerstätten or accumulation of unusually well-preserved fossils. The importance of such accumulations cannot be denied: recent work on the history of lineages has shown that 20 per cent of major groups are known exclusively from their presence in the three great Palaeozoic Lagerstätten: the Burgess Shale, the Devonian Hunsrückschiefer, and the Carboniferous Mazon Creek fauna.

The Burgess Shale fauna was discovered in 1909 by Charles Walcott, then head of the U.S. Geological Survey and Secretary of the Smithsonian Institution, while mapping in the Canadian Rockies. He opened up a small quarry and collected for the next eight years, eventually amassing a collection of 80 000 specimens at the Smithsonian Institution. Administrative duties prevented him from publishing thorough descriptions of the fauna, and that work was taken up in the 1960s by Harry Whittington of Cambridge University and his students Simon Conway Morris, also of Cambridge, and Derek Briggs of Bristol University. Through their work a clear picture is now developing of the range of organisms present and the way in which the accumulation was formed. The Burgess Shale was deposited in relatively deep water seaward of an enormous algal reef (Fig. 1a). The reef had a vertical face hundreds of metres high and the organisms lived in and on the muds that accumulated at its base. Patches of mud slumped downslope periodically, carrying with them the carcasses of dead organisms together with live inhabitants of the sediment, and all these remains were deposited together when the mud settled out. The animals appear to have been carried into an anaerobic (low in oxygen) environment that inhibited decay and were preserved as flattened films in which the organic material has been replaced by calcium aluminosilicates. In order to study the compressed specimens, layers are carefully removed by hand using needles, and reconstructions are then built up. As the fossils consist of black films on black shale, photographs are taken with ultraviolet light, or with the specimens under water or ethyl alcohol to exploit the reflectivity of the fossils.

The fauna itself includes over 120 species, representing major groups such as arthropods, molluscs, brachiopods, cnidarians, polychaetes (bristle worms), priapulid worms, echinoderms, chordates, and many forms that cannot be attributed to known phyla. The species are not equally abundant: some are represented by thousands of individuals, others by only a few. The largest proportion (37 per cent of the organisms) are arthropods, which are extremely abundant, both in numbers and diversity. The trilobites include specimens of Olenoides in which the appendages are preserved, a rare circumstance providing valuable information on these structures. Although the non-trilobite arthropods were originally classed together, subsequent work has shown that a great diversity of groups is present, including early representatives of crustaceans, phyllocarids, merostomes, and other forms with puzzling combinations of characters that make their evolutionary relationships difficult to understand. The polychaetes are represented by five genera and are, therefore, only minor constituents of the fauna; the priapulids, however, appear to have been important infaunal carnivores. One significant organism is Pikaia, which appears to show the notochord and chevron-shaped muscle blocks that characterize the phylum Chordata and is thus the first-known representative of the phylum that includes the vertebrates. The most interesting parts of the fauna are those organisms that do not fit into known phyla. Of these, Wiwaxia was a hemispherical animal that seems to show affinity with molluscs, although it is covered by most unmollusc-like scales and spines. Opabinia is superficially arthropod-like with a long segmented body; each segment, however, had a flexible lateral lobe rather than jointed legs and the head bore five eyes on short stalks, and a long flexible process armed with a terminal claw. The aptly named Hallucigenia was originally reconstructed as an animal that walked on seven pairs of spines, but in later reconstructions it was turned upside down and reinterpreted as an onychophoran, a group similar to both annelid worms and arthropods. The largest animal in the fauna, Anomalocaris (Fig. 1b), is also of unknown affinity. The large segmented appendages, mouth with cutting teeth, and segmented lobed body show that it was clearly adapted for life as a predator on active benthonic invertebrates such as trilobites.

The significance of the Burgess Shale fauna lies in its ability to show us the diversity of organisms during the ‘Cambrian explosion’, the initial period of development of invertebrate phyla during which many new body plans were developed. Evolution since then has progressed by the refinement of the body plans that survived. In addition, an analysis of the feeding habits of the organisms within this fauna show that the fundamental trophic structure of marine organisms had already been established at this time. The study of this fauna has also led palaeontologists into a new dimension of evolutionary thinking. When Walcott initially described the organisms, he ‘shoehorned’ them into established categories, but later work by Whittington and his colleagues showed how much greater a diversity of body plans was present, and this observation enabled them to document the explosion of diversity that occurred at the beginning of the Phanerozoic.

David K. Elliott

Bibliography

Gould, S. J. (1989) Wonderful life. W. W. Norton and Company, New York.
Conway Morris, S. (1998) The crucible of creation. Oxford University Press.
Whittington, H. B. (1985) The Burgess Shale. Yale University Press, New Haven.