Burgess_Shale

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Burgess Shale

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. 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.

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

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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 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.