Burgess Shale

From Wikipedia, the free encyclopedia

Hallucigenia sparsa, one of the organisms unique to the Burgess Shale.

The Burgess Shale is a black shale fossil bed (Lagerstätte) named after Burgess Pass, 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. Walcott recognized the arthropod fossils were new and unique species, but careful reexaminations showed that many in fact constituted entire new phyla of life, and even today some have proven impossible to classify. The fossils are especially valuable because they include appendages and soft parts that are rarely preserved.

1 History and significance
2 Partial species list
3 See also
4 Further reading
5 Sources

[edit] History and significance

The significance of the finds was not realised at the time of discovery; the trilobites found dated the fossils to the Middle Cambrian period, and Charles Walcott simply placed the unusual new species within the phyla known to exist during that period, a process Stephen Jay Gould dubbed "shoehorning" in his book about the Burgess Shale, Wonderful Life. 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.

Gould's Wonderful Life, published in 1989, popularized the Burgess Shale discovery. 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 consciousness.

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.

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.

[edit] Partial species list

Marella, the most abundant Burgess Shale organism.

[edit] 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)
Canadaspis (arthropod)
Perspicaris (arthropod)
Leanchoilia (arthropod)
Yohoia (arthropod)
Hallucigenia (phylum Onychophora)
Ctenorhabdotus (Ctenophora)
Fasciculus (Ctenophora)
Xanioascus (Ctenophora)

[edit] Species assigned to a group of extinct taxa

Haplophrentis (phylum Hyolitha)
Marella (arthropod)
Olenoides (trilobite)
Naraoia (trilobite)

[edit] Species of uncertain classification

[edit] See also

Body form
List of fossil sites (with link directory)

[edit] 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 0-19-850197-8 (hbk), ISBN 0-19-286202-2 (pbk)
Richard Fortey, Trilobite: Eyewitness to Evolution, Flamingo, 2001. ISBN 0-00-655138-6
Stephen Jay Gould, Wonderful Life: Burgess Shale and the Nature of History, Vintage, 2000. ISBN 0-09-927345-4