Burgess shale
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The Burgess shale (named after Mount Burgess, near where the shale was found) is a black shale found high up in the Canadian Rockies in Yoho National Park near the town of Field, British Columbia. Fossils were found in the Burgess Shale by C. D. Walcott in 1909. Walcott returned in the following years to collect additional specimens. The majority of the fossils collected were unique to the site, although some common Middle Cambrian trilobites were also found. The fossils were of substantial interest because they included appendages and soft parts that are rarely preserved.
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History and significance
The significance of these finds was not realised at the time. A reinvestigation of the fossils in the 1980s by Harry Blackmore Whittington, Derek Briggs, and Simon Conway Morris of the University of Cambridge revealed that the fauna represented was much more diverse and unusual than Walcott had recognized. Indeed, many of the animals present had bizarre anatomical features and only the sketchiest resemblance to other known animals. Examples include Opabinia with five eyes and a snout like a vacuum cleaner; Aysheaia which bears an extraordinary resemblance to a minor modern phylum -- the Onychophora; Nectocaris which is apparently either a crustacean with fins or a vertebrate with a shell; and Hallucigenia which was originally reconstructed as walking on bilaterally symmetrical spines. Conway-Morris now reconstructs it as another onychophoran, with the spines on its back. Several poorly understood fossils were found to be body parts of a predatory form known as Anomalocaris. More recent (late 1990s) work by Derek Briggs and Richard Fortey has placed many of the "peculiar" Burgess Shale fossils within the arthropoda, but many animals such as Amiskwia remain enigmatic.
A popular account of the 1980s analysis of the Burgess Shale is given in Wonderful Life by Stephen Jay Gould.
The diversity and exotic nature of the Burgess fauna has caused a great deal of controversy in paleontology with regard to the reasons for and nature of what has come to be called the Cambrian Explosion.
Further investigations showed that the Burgess Shale extends for many miles in isolated outcrops and the various faunas are preserved in different places. The deposits appear to represent small areas of muddy ocean bottom that -- from time to time -- slid down the face of a limestone cliff, carrying their fauna and anything unfortunate enough to be swimming by into oxygen-poor waters in the depths. Six distinct faunal zones have been identified in the Burgess Shale. Now that scientists know what to look for, similar deposits have been identified elsewhere with similar faunas. The most important similar deposits are even older turbide flow deposits created in much the same way as the Burgess shales in Yunnan Province, China. These Maotianshan shales contain fauna quite similar to the Burgess.
Due to its location within Yoho National Park, the shale is part of a UNESCO World Heritage Site, specifically, the Canadian Rocky Mountain Parks. Subsequent exploration has found exposures of the shale over a front of several dozen kilometers and has identified at least six fossiliferous lagerstätten within the formation.
Partial species list
Species assigned to a group of extant taxa
- Thaumaptilon (a type of sea pen)
- Aysheaia (phylum Onychophora)
- Sidneyia (arthropod)
- Pikaia (phylum Chordata)
- Canadia (annelid)
- Choia (sponge)
- Ottoia (priapulid worm)
- Canadapsis (arthropod)
- Perspicaris (arthropod)
- Leanchoilia (arthropod)
- Hallucigenia
Species assigned to a group of extinct taxa
- Haplophrentis (phylum Hyolitha)
- Marella (arthropod)
- Olenoides (trilobite)
- Naraoia (trilobite)
Species of uncertain classification
References
- Conway Morris, Simon: The Crucible of Creation: The Burgess Shale and the Rise of Animals, Oxford University Press, Oxford, 1998.
- Fortey, Richard: Trilobite: Eyewitness to Evolution, Vintage Books, New York, 2000.