Burgess shale
Encyclopedia : B : BU : BUR : Burgess shale
The Burgess Shale is a black shale exposure named after Mount Burgess, close to where it was found, high up in the Canadian Rockies in Yoho National Park near the town of Field, British Columbia. Fossils were first found in the Burgess Shale in 1909 by Charles Doolittle Walcott, who 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.
History and significance
The significance of the finds was not realised at the time of discovery. A reinvestigation of the fossils in the 1980s by Harry Blackmore Whittington, Derek Briggs, and Simon Conway Morris of the University of Cambridge, however, revealed that the fauna represented were 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. Gould suggests that the extraordinary diversity of the fossils indicate that life forms at the time were much more diverse than those that survive today and that many of the unique lineages were evolutionary experiments that became extinct. He suggests that this interpretation supports his hypothesis of evolution by punctuated equilibrium. However the widely accepted reclassifiction by Briggs and Fortey contradicts this account and both those authors have criticised Gould for what they believe is a hasty and incomplete analysis used to support Goulds' own ideas and which has since entered the popular public conciousness.
The diversity and exotic nature of the Burgess fauna (Middle Cambrian, 505 mya) has caused a great deal of controversy in paleontology with regard to the reasons for and nature of an earlier event in the history of life that has come to be called the Cambrian Explosion (Early Cambrian, 542-530 mya).
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 turbidite 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.
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 (phylum Onychophora)
- Ctenorhabdotus (Ctenophora)
Species assigned to a group of extinct taxa
Species of uncertain classification
- Amiskwia
- Anomalocaris
- Dinomischus
- Nectocaris
- Odontogriphus
- Opabinia
- Wiwaxia
See also
- Body form
Further reading
- Simon Conway Morris, The Crucible of Creation: The Burgess Shale and the Rise of Animals, Oxford University Press, Oxford, 1998 (paperback 1999) ISBN 0198501978 (hbk), ISBN 0192862022 (pbk)
- Richard Fortey, Trilobite: Eyewitness to Evolution, Flamingo, 2001. ISBN 0006551386
- Stephen Jay Gould, Wonderful Life: Burgess Shale and the Nature of History, Vintage, 2000. ISBN 0099273454
Sources
- [The Burgess Shale Geoscience Foundation] - official web site
- [The Burgess Shale - Evolution's Big Bang] - Burke Museum of Natural History and Culture web pages resourcing an exhibition devoted to the Burgess Shale
- [Burgess Shale Fossils]
- [The Cambrian Explosion] - BBC Radio 4 broadcast, In Our Time, 17 February 2005, hosted by Melvyn Bragg (includes links to resource pages)
- [Paleobiology Database The Burgess Shale (skeletonized fauna), Stephen Fm., British Columbia, Canada: St Davids, British Columbia]
- [Paleobiology Database Hanburia gloriosa, Phyllopod Bed, Burgess Shale, Canada - Whittington 1998: St Davids - Merioneth, British Columbia]
From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.