Xistos de Burgess foi o termo informal que
Charles Walcott usou para se referir a unidade fossilífera, que mais tarde passou a ser aplicada mais amplamente para descrever o tipo de agrupamento de fósseis que é encontrado na pedreira de Walcott.
O sítio fossilífero pertence a
formação Stephen, que possui uma parte "fina" e outra "grossa", alguns pesquisadores consideram que a parte "fina" deva ser separada como formação dos Xistos de Burgess.
Ottoia, a soft-bodied worm, abundant in the Burgess Shale. (From Smith et al. 2015)
The
Burgess Shale is a
fossil-bearing deposit exposed in the
Canadian Rockies of
British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At
508 million years old (
middle Cambrian), it is one of the earliest fossil beds containing soft-part imprints.
The rock unit is a black
shale and crops out at a number of localities near the town of
Field in
Yoho National Park and the
Kicking Horse Pass. Another outcrop is in
Kootenay National Park 42 km to the south.
History and significance
The Burgess Shale was discovered by
palaeontologist Charles Walcott on 30 August 1909, towards the end of the season's fieldwork. He returned in 1910 with his sons, daughter, and wife, 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 him to return to the quarry almost every year until 1924. At that 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, and as a result, the fossils were regarded as little more than curiosities at the time. It was not 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 that 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 slightest resemblance to other known animals. Examples include
Opabinia, with five eyes and a snout like a vacuum cleaner hose and
Hallucigenia, which was originally reconstructed upside down, 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 both higher and lower than the original Walcott quarry. These localities continue to yield new organisms faster than they can be studied.
Stephen Jay 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 indicates that life forms at the time were much more disparate in body form than those that survive today, and that many of the unique lineages were evolutionary experiments that became extinct. Gould's interpretation of the diversity of Cambrian fauna relied heavily on
Simon Conway Morris's reinterpretation of Charles Walcott's original publications. However, Conway Morris strongly disagreed with Gould's conclusions, arguing that almost all the Cambrian fauna could be classified into modern day
phyla.
The Burgess Shale has attracted the interest of
paleoclimatologists who want to study and predict long-term future changes in Earth's climate. According to
Peter Ward and Donald Brownlee in the 2003 book
The Life and Death of Planet Earth, climatologists study the fossil records in the Burgess Shale to understand the climate of the
Cambrian explosion, and use it to predict what Earth's climate would look like 500 million years in the future when a warming and expanding Sun combined with declining
CO2 and oxygen levels eventually heat the Earth toward temperatures not seen since the
Archean Eon 3 billion years ago, before the first plants and animals appeared, and therefore understand how and when the last living things will die out. See also
Future of the Earth.
After the Burgess Shale site was registered as a
World Heritage Site in 1980, it was included in the Canadian Rocky Mountain Parks WHS designation in 1984.
In February 2014, the discovery was announced of another Burgess Shale outcrop in
Kootenay National Park to the south. In just 15 days of field collecting in 2013, 50 animal species were unearthed at the new site.
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