The Burgess Shale Fauna and Fossils

When most people picture paleontologists, they see scientists sweating in the summer's heat to wrest treasures from rocks. From this image, some might think that the paleontologists recognize the greatest discoveries the moment they are uncovered.

In fact, the implications of a find are often not obvious, and are only revealed much later in the laboratory, after the fossil is prepared and studied. Finding time and funding to study all the fossils uncovered during fruitful collecting seasons is sometimes impossible, so often the "great discoveries" are not appreciated for years.

This is the case with the Burgess Shale. Collected and preliminarily described in the early twentieth century, these fossils began to rock the world of paleontology only in the late 1960s to early 1970s. Their fruits are still being discovered, filling journals on geology, paleontology, and zoology with a large number of articles. This paper takes a brief look at the fossils of the Burgess Shale and their significance.

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 to 520 Ma.

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 re-describe 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 many years.

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.

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.

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.

Many workers have thought that the animals of the Burgess experienced very little decay, indicating very rapid burial. Others suggest that the "dark stains" associated with many specimens is the result of decaying matter that seeped from the animals' bodies. 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.

The Burgess fossils look like shiny silver smears on the rocks. Whittington 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.