Burgess Shale

Infobox Rockunit
name = Burgess Shale

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caption = "Marella", the most abundant Burgess Shale organism.
type = Geological formation
prilithology = Shale
otherlithology =
namedfor = Burgess Pass
namedby = Charles Doolittle Walcott
region = Yoho National Park
country = Canada
coordinates =
unitof =
subunits =
thickness =
extent =
area =
age = Middle Cambrian

The Burgess Shale is famous for the exceptional preservation of the fossils found within it, in which the soft parts are preserved. A Cambrian black shale formation, it crops out in the Canadian Rockies of British Columbia, Canada. It is in Yoho National Park, near the town of Field and is named after Burgess Pass.

History and significance

The Burgess Shale was discovered by Charles Walcott in 1909, towards the end of the season's fieldwork.fotbs He returned in 1910 with his sons, establishing a quarry on the flanks of Fossil ridge. The significance of soft bodied preservation, and the range of organisms he recognised as new to science, led to him returning to the quarry almost every year until 1924. At this point, aged 74, he had amassed over 65,000 specimens. Describing the fossils was a vast task, pursued by Walcott until his death in 1927. Walcott, led by scientific opinion at the time, attempted to categorise all fossils into living taxa; as a result, the fossils were regarded as little more than curiosities at the time. It took until 1962 that a first-hand reinvestigation of the fossils was attempted, by Alberto Simonetta. This led scientists to recognise that Walcott had barely scratched the surface of information available in the Burgess Shale, and also made it clear the the organisms did "not" fit comfortably into modern groups. Excavations were resumed at the Walcott quarry by the Geological Survey of Canada under the persuasion of trilobite expert Harry Blackmore Whittington, and a new quarry, the Raymond, was established about 20 metres higher up Fossil ridge. Whittington, with the help of research students Derek Briggs and Simon Conway Morris of the University of Cambridge, began a thorough reassessment of the Burgess Shale, and 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 hose; "Nectocaris", which resembles either a crustacean with fins or a vertebrate with a shell; and "Hallucigenia", which was originally reconstructed as walking on bilaterally symmetrical spines.

With Parks Canada and UNESCO recognising the significance of the Burgess Shale, collecting fossils became politically more difficult from the mid 1970s. Collections continued to be made by the Royal Ontario Museum. The curator of invertebrate palaeontology, Desmond Collins, identified a number of additional outcrops, stratigraphically higher and lower than the original Walcott quarry. These localities continue yield new organisms faster than they can be studied.

Stephen J Gould's book "Wonderful Life", published in 1989, brought the Burgess Shale fossils to the public's attention. 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. Gould's interpretation of the diversity of Cambrian fauna relied heavily on Simon Conway Morris' reinterpretation of Charles Walcott's original publications. Conway Morris strongly disagreed with Gould's conclusions, arguing that almost all the Cambrian fauna could be classified into modern day phyla. ["The Crucible of Creation: The Burgess Shale and the Rise of Animals" , Simon Conway Morris]

Geological setting

The fossiliferous deposits of the Burgess Shale belong to the Stephen formation, a collection of sightly calcareous dark mudstones, about 505 million years old. The beds were deposited at the base of a cliff about 160m tall. This vertical cliff was composed of the calcareous reefs of the Cathedral formation, which probably formed shortly before the deposition of the Burgess shale. The precise formation mechanism is not known for certain, but the most widely accepted hypothesis suggests that the edge of the Cathedral formation reef became detached from the rest of the reef, slumping and being transported some distance – perhaps kilometers – away from the reef edge. This would have left a steep cliff, the bottom of which would be protected from both general oceanic circulation – producing anoxic conditions – and later, because the limestone of the Cathedral formation is difficult to compress, from tectonic decompression. This latter protection explains why fossils preserved further from the Cathedral formation are impossible to work with – tectonic squeezing of the beds has produced a vertical cleavage which fractures the rocks, so they split perpendicular to the fossils. The Walcott quarry produced such spectacular fossils because it was so close the the Stephen formation – indeed the quarry has now been excavated to the very edge of the Cambrian cliff.

The anoxic setting not only protected the newly dead organisms from decay, but it also created chemical conditions allowing the preservation of the soft parts of the organisms. Further, it reduced the abundance of burrowing organisms – burrows and trackways "are" found in beds containing soft-bodied organisms, but they are rare and generally of limited vertical extent.

Taphonomy and diagenesis

"Please expand this section"cite journal
url = http://links.jstor.org/sici?sici=0094–8373(199022)16%3A3%3C272%3AOPONOA%3E2.0.CO%3B2-%23
author = Butterfield, N.J.
journal = Paleobiology
volume = 16
issue = 3
pages = 272–286
year = 1990
accessdate = 2008-06-22
title=Organic Preservation of Non-Mineralizing Organisms and the Taphonomy of the Burgess Shale] cite journal
doi = 10.1666/0094-8373(2002)028<0155:LGATIO>2.0.CO;2
author = Butterfield, N.J.
journal = Paleobiology
volume = 28
issue = 1
pages = 155–171
year = 2002
title = "Leanchoilia" guts and the interpretation of three-dimensional structures in Burgess Shale-type fossils] cite journal
url = http://www.sciencemag.org/cgi/content/abstract/281/5380/1173
doi = 10.1126/science.281.5380.1173
pmid = 9712577
author = Orr, Patrick J.; Briggs, Derek E. G.; Kearns, Stuart L.
journal = Science
volume = 281
issue = 5380
pages = 1173
year = 1998
publisher = AAAS
title = Cambrian Burgess Shale Animals Replicated in Clay Minerals
accessdate = 2008-06-22] cite journal
doi = 10.2110/palo.2003.P05-070R
author = CARON, JEAN-BERNARD; JACKSON, DONALD A.
journal = PALAIOS
volume = 21
issue = 5
pages = 451–465
year = 2006
publisher = Society for Sedimentary Geology
title = Taphonomy Of The Greater Phyllopod Bed Community, Burgess Shale] cite journal
url = http://linkinghub.elsevier.com/retrieve/pii/S003101820400584X
doi = 10.1016/j.palaeo.2004.07.034
author = Gaines, R.R.; Kennedy, M.J.; Droser, M.L.
journal = Palaeogeography, Palaeoclimatology, Palaeoecology
volume = 220
issue = 1–2
pages = 193–205
year = 2005
publisher = Elsevier
title = A new hypothesis for organic preservation of Burgess Shale taxa in the middle Cambrian Wheeler Formation, House Range, Utah
accessdate = 2008-06-22] cite journal
url = http://www.blackwell-synergy.com/doi/abs/10.1111/j.1475-4983.2007.00656.x
doi = 10.1111/j.1475-4983.2007.00656.x
author = Butterfield, N.J.; Balthasar, U.W.E.; Wilson, L.A.
journal = Palaeontology
volume = 50
issue = 3
pages = 537–543
year = 2007
publisher = Blackwell Synergy
title = Fossil Diagenesis In The Burgess Shale
accessdate = 2008-06-22]

There are many other comparable Cambrian lagerstatten; indeed such assemblages are far more common in the Cambrian than in any other period. This is mainly due to the limited extent of burrowing activity; as such bioturbation became more prevalent throughout the Cambrian, environments capable of preserving organisms' soft parts became much rarer.

Biota

The biota of the Burgess Shale appears to be typical of Middle Cambrian deposits. Although the hard-part bearing organisms make up as little as 14% of the community, these same organisms are found in similar proportions in other Cambrian localities. This means that there is no reason to assume that the organisms without hard parts are exceptional in any way; indeed, many appear in other Lagerstatten of different age and locations.

The biota consists of a range of organisms. Free-swimming (wict|nectonic) organisms are relatively rare, with the majority of organisms being bottom dwelling (benthic) - either moving about (vagrant) or permanently attached to the sea floor (sessile). About two-thirds of the Burgess Shale organisms lived by feeding on the organic content in the muddy sea floor, while almost a third filtered out fine particles from the water column. Under 10% of organisms were predators or scavengers, although since these organisms were larger, the biomass was split equally between each of the filter feeding, deposit feeding, predatory and scavenging organisms.

ee also

* Body form
* Invertebrate paleontology
* History of invertebrate paleozoology
* List of fossil sites "(with link directory)"
* List of notable fossils

References

Further reading

*Gould and Conway Morris debating the significance of the Burgess Shale: citation|title= [http://www.stephenjaygould.org/library/naturalhistory_cambrian.html Simon Conway Morris and Stephen Jay Gould, "Showdown on the Burgess Shale," Natural History magazine, 107 (10): 48-55]

*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
*Derek E. G. Briggs, Douglas H. Erwin, & Frederick J. Collier, "The Fossils of the Burgess Shale", Smithsonian, 1994. ISBN1-56098-364-7

ources

* [http://www.burgess-shale.bc.ca/ The Burgess Shale Geoscience Foundation] — official website
* [http://www.washington.edu/burkemuseum/bshale/index.html The Burgess Shale — Evolution's Big Bang] — Burke Museum of Natural History and Culture web pages resourcing an exhibition devoted to the Burgess Shale
* [http://tabla.geo.ucalgary.ca/%7Emacrae/Burgess_Shale Burgess Shale Fossils]
* [http://www.bbc.co.uk/radio4/history/inourtime/inourtime_20050217.shtml The Cambrian Explosion] — BBC Radio 4 broadcast, "In Our Time", 17 February 2005, hosted by Melvyn Bragg (includes links to resource pages)
* [http://paleodb.org/cgi-bin/bridge.pl?action=displayCollectionDetails&collection_no=215 Paleobiology Database The Burgess Shale (skeletonized fauna), Stephen Fm., British Columbia, Canada: St Davids, British Columbia]
* [http://paleodb.org/cgi-bin/bridge.pl?action=displayCollectionDetails&collection_no=6854 Paleobiology Database Hanburia gloriosa, Phyllopod Bed, Burgess Shale, Canada — Whittington 1998: St Davids — Merioneth, British Columbia ]
* [http://www.nmnh.si.edu/paleo/shale/pamsci.htm Smithsonian Museum]
* [http://www.nmnh.si.edu/paleo/shale/pfoslidx.htm Species index from the Smithsonian Institution]