Burgess Shale and Ediacaran Faunas

The Burgess Shale was one of the most famous and important fossil localities known at the end of the twentieth century. Charles Walcott, who at the time was secretary of the Smithsonian Institution, discovered this fossil-rich rock bed in 1909 while exploring the Canadian Rockies of British Columbia. The Burgess Shale, which is now a part of Yolo National Park, is famous for the wide diversity of fossils of soft-bodied marine animals that are embedded in it. These fossils are approximately 530 million years old, and represent an array of life-forms present during the early-middle years of the Cambrian period (545-495 million years ago). The area originally quarried by Walcott is surprisingly small given the array of unique animal forms found in it. His initial survey covered an area that is only 10 feet tall and 200 feet long, the length of one city block.

The fossils of the Burgess Shale are significant for a number of reasons. The quality of their preservation provided the first opportunity to examine, in astonishing detail, the morphology (form and structure) of early soft-bodied life forms. In addition, these fossils are an early record of the novel body plans that were created during the Cambrian Explosion (approximately 570-540 million years ago), a geologically abrupt time period during which multicellular forms organized in a variety of new ways. The Burgess Shale does not depict the Cambrian Explosion itself, but the aftermath. The fossils are impressive not only for the novelty of the body plans represented, but also for the diversity of body plans.

Fossils of soft-bodied forms are rare because the process that creates fossils works better at preserving bones and hard structures. Before the discovery of the Chingjiang fossils in Yunnan Province, China, in the late 1980s, the Burgess Shale fossils provided the only evidence of the early soft-bodied animals that appeared during the Cambrian Explosion. The quality of these fossils indicates that they were created under anoxic (low oxygen) conditions. Many millions of years ago, the site of the Burgess Shale was underwater and located near the equator. Soft-bodied marine animals were carried by strong currents from surrounding highly oxygenated areas to the site of the Burgess Shale and were buried in an underwater mudslide. The low oxygen content of these waters killed the animals and protected their remains from decay.

Classifying the Burgess Animals

Approximately 120 species are found in the Burgess Shale, including familiar forms as well as several species belonging to previously unknown phyla. Some of the fossilized species are members of groups (phylum) that still exist. These species can be categorized as members of the phyla Porifora (sponges), Annelida (segmented marine flatworms), Arthropoda (insects, crabs, and trilobites), and Echinodermata (sea urchins, sea fans, and sea lilies), and one species is the earliest representative of the phylum Chordata (which includes vertebrates). Most of these animals were scavengers, and a few were predators. Of these Burgess Shale animals, the aptly named Hallucigenia (phyla Annelida) is probably one of the most famous for its bizarre morphology. Seven pairs of stiltlike legs support its long, cylindrical body. It is hard to tell for certain which end is "head" and which end is "tail," but most scientists designate the head end by the bulbous projection that is prominent on one end of its body.

The previously unseen animal forms found within the Burgess Shale include a number of wormlike and segmented organisms, some of which were assigned to novel phyla (phyla Priapulida and Onychophora) while others remain "unclassified to this day." Opabinia was a five-eyed, 3-inch-long creature with a frontal "nozzle" that was presumably used in its search for worms and other fossorial (living in burrows) prey. Anomalocaris ("unusual shrimp") was a fierce, 2-foot-long predator with robust forelimbs for grasping its prey and a square-shaped mouth rimmed with multiple rows of sharp teeth. Because Anomalocaris existed in the Burgess Shale only as separate pieces, Walcott first reconstructed it as two animals: a bivalved (having two symmetrical, shelled parts joined by a hinge) arthropod and a jellyfish. Fossils of related species later found in China reached a length of up to 6 feet! Wiwaxia, a spike-covered, sluglike animal, was a bottom feeder that was protected from hungry predators by its scaly back.

In attempting to fit these new forms into the preexisting classification scheme, which included only phyla Porifora, Annelida, Arthropoda, Echinodermata, and Chordata, Walcott erroneously classified these animals as worms and arthropods. His categorization of the Burgess animals as ancestors of modern-day animals conformed to the idea that the diversity of life-forms arose in a manner resembling the shape of an inverted cone, with the large number of species that exist today arising from a small number of ancient organisms.

It was not until H. B. Wittington of Cambridge University examined the fossils forty years later that these forms were placed into unique phyla. Whittington's reclassification caused a major upheaval in the way people thought about the origin of animals. Instead of the popular view that a small number of general body plans originated during the Cambrian period and gave rise to all the animals seen today, Whittington contended that the body plans evident today represent only some of the novel forms that were created during the Cambrian period. He argued that many different body plans were created then, a number of which went out of existence while the remainder continued and gave rise to the forms we see today. This premise formed the basis for his reclassification of the Burgess animals.

Ediacaran Fauna

Before the Cambrian Explosion and the associated appearance of new animal forms, there existed the Ediacaran Fauna (also known as Vendian Biota), a group of multicellular organisms with relatively simple body plans. Geologist Reginald Sprigg first discovered the fossil traces of these organisms in 1946 while exploring the Ediacara Hills of Australia. Since the initial discovery of the Australian fossils, additional Ediacaran fossils have been found on every continent except Antarctica. The age of the rocks containing these fossils range from 600 million to 544 million years old. Before the discovery of the Ediacarans, it was believed that animals did not exist before the Cambrian Period (before 545 million years ago).

In contrast to the Burgess Shale fossils, most of the Ediacaran fossils are burrows and trace fossils—casts and molds of the organisms they depict. The fossil traces of these simple animals can be broadly divided into those that are radially symmetric and those that are segmented. The radially symmetric traces are believed to have been formed by polyplike and disk-shaped organisms. The more complex, segmented forms are traces of tubelike units. The shape of these soft-bodied forms was preserved during rapid burial under sand on the ancient marine floor bed.

The classification of the Ediacarans as animals remains controversial. The superficial similarity that some of the Ediacaran forms bear towards sea anenomes and jellyfish led some scientists to conclude that they are true animals, precursors to the animals that exist today. Various Ediacarans have also been mistakenly classified in the past as algae, lichens, or giant protozoans.

However, some scientists believe that the Ediacarans were not animals as we know them, and they did not evolve into such animals. These scientists focus upon characteristics of the Ediacarans that are not found in the body plans that evolved during the Cambrian. Based on information gleaned under close examination of the fossil traces, they concluded that the Ediacarans underwent a set of embryonic/morphological development processes that differs radically from the normal pattern of development experienced by true animals.

see also Cambrian Explosion; Cambrian Period; Geological Time Scale.

Judy P. Sheen

Bibliography

Erwin, Douglas, Jim Valentine, and David Jablonski. "The Origin of Animal Body Plans." American Scientist 85 (1997):126-137.

Knoll, Andrew H., and Sean B. Carroll. "Early Animal Evolution: Emerging Views from Comparative Biology and Geology." Science 284 (1997):2129-2137.

Gould, Stephen Jay. Wonderful Life: The Burgess Shale and the Nature of History. New York: W. W. Norton and Co., 1989.

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.

Burgess Shale An horizon from the Cambrian of British Columbia which has yielded an exceptionally preserved fauna. It was originally discovered by C. D. Walcott in 1909. Apparently, the fauna was deposited in deep water on or near a submarine fan. Many of the animals are Arthropoda, but other groups are also represented, some of them bizarre. Their identification has been aided by the discovery of other Cambrian faunas, especially those in China, Greenland, and Australia. Some forms, including Anomalocaris and Opabinia, form a metazoan group between the Onychophora and arthropods, but the fauna as a whole represents a rapid radiation and was probably a typical Cambrian fauna, unusual only in the preservation of soft-bodied forms.

Burgess Shale A horizon from the Cambrian of British Columbia which has yielded an exceptionally preserved fauna. Originally discovered by C. D. Walcott in 1909, it has been redescribed by H. B. Whittington and other authors (1967–8). Apparently the fauna was deposited in deep water on or near a submarine fan. Arthropods of various types account for more than 30% of the fauna, but other groups are also represented, many of them bizarre. It is probable that in terms of its diversity (140 species in 119 genera) this was a typical Cambrian fauna, but it is unusual in that the softbodied forms are preserved.

Burgess shale Layer of siltstone in a quarry in Yoho National Park, e British Columbia, Canada. Discovered in 1909 by US scientist Charles Walcott, it contains a large number of animal fossils from the Cambrian period. The silt has preserved traces of many of the soft bodies of sea creatures. The fossils include the oldest known chordate – a forerunner of all animals with backbones. They also include a number of kinds of animals that have completely vanished, and apparently do not belong to any of the 32 or so phyla of animals we know today.

http://www.nmnh.si.edu/paleo/shale

Burgess Shale A Middle Cambrian (540 Ma ago) rock horizon in British Columbia, Canada, that has yielded many exceptionally well-preserved fossils of metazoans, the remains of which were deposited in deep water or near a submarine fan. First discovered in 1909 by C. D. Walcott, the fossil fauna has been described many times. The fossils reveal a high level of endemism, with many taxa that subsequently became extinct. The discovery of other well-preserved Cambrian faunas in China, Greenland, and Australia has helped with the interpretation of the Burgess fossils.

Burgess Shale a stratum of sedimentary rock exposed in the Rocky Mountains in British Columbia, Canada. The bed, dated to the Cambrian period (about 540 million years ago), is rich in well-preserved fossils of early marine invertebrates, many of which represent evolutionary lineages unknown in later times.