The Burgess Shale is an area with many fossils found in the Canadian Rockies of British Columbia, Canada. It is known for the well-preserved soft body parts of its fossils. At 508 million years old, it is one of the oldest fossil sites that show soft-part imprints.

The rock layer is black shale and can be seen at several places near the town of Field in Yoho National Park and the Kicking Horse Pass. Another area with this rock is in Kootenay National Park, 42 kilometers to the south.

History and significance

The Burgess Shale was discovered by scientist Charles Walcott on August 30, 1909, near the end of the season's fieldwork. In 1910, he returned with his family to set up a dig site on the sides of Fossil Ridge. Walcott recognized the importance of the soft-bodied fossils and the many new species he found. He visited the site almost every year until 1924, collecting over 65,000 fossils by the time he was 74. He continued studying the fossils until his death in 1927. At the time, Walcott tried to classify all the fossils into known groups, which made scientists view the fossils as unusual but not fully understood. It was not until 1962 that scientist Alberto Simonetta studied the fossils again, showing that Walcott had not fully understood their importance. This study also revealed that the fossils did not fit neatly into modern groups.

Walcott's classifications of most fossils are no longer accepted, but they were supported by scientists at the time. He changed his view only in a few cases where others disagreed. Later studies benefited from new tools and knowledge that Walcott did not have.

In the 1970s, the Geological Survey of Canada, with the help of scientist Harry Blackmore Whittington, restarted digging at the Walcott Quarry. A new site, the Raymond Quarry, was also established higher up on Fossil Ridge. Whittington, along with researchers Derek Briggs and Simon Conway Morris from the University of Cambridge, studied the fossils again. They found that the ancient life forms were much more varied and strange than Walcott had realized. Many animals had unusual body parts, such as Opabinia, which had five eyes and a snout shaped like a hose, and Hallucigenia, which was originally thought to walk on spines that were symmetrical on both sides.

After the Burgess Shale was recognized as a World Heritage Site in 1980, collecting fossils became more difficult due to legal protections. The Royal Ontario Museum continued to collect fossils, and its scientist, Desmond Collins, found other fossil sites above and below the original Walcott Quarry. These areas keep producing new species faster than scientists can study them.

In 1989, scientist Stephen Jay Gould wrote a book called Wonderful Life, which brought public attention to the Burgess Shale. Gould argued that the fossils showed life forms in the Cambrian period were very different from modern animals, and many of these forms later disappeared. His ideas were based on Simon Conway Morris's reanalysis of Walcott's work. However, Conway Morris disagreed, saying most Cambrian fossils could be placed into modern groups.

Scientists who study Earth's climate, called paleoclimatologists, use the Burgess Shale to understand past climate changes and predict future ones. According to scientists Peter Ward and Donald Brownlee, the fossils help researchers study the climate during the Cambrian period. This information can help predict Earth's future climate as the Sun warms and carbon dioxide levels drop. These studies also help scientists understand when Earth's last living organisms might die out.

The Burgess Shale was added to the Canadian Rocky Mountain Parks World Heritage Site in 1984. In 2012, a new Burgess Shale site was discovered in Kootenay National Park. In 2013, scientists found 50 animal species in just 15 days of digging there.

The Burgess Shale is famous for its rare preservation of soft tissues and its complete record of Cambrian marine ecosystems. In 2022, the International Union of Geological Sciences (IUGS) added the "Burgess Shale Cambrian Paleontological Record" to its list of 100 important global geological sites. These sites are places with significant geological features that help scientists study Earth's history.

Geology

The fossil-rich layers of the Burgess Shale are part of the Stephen Formation, a group of dark, slightly limestone-like mudstones that are about 508 million years old. These layers were deposited at the base of a cliff approximately 160 meters tall, below the area disturbed by wave action during storms. This cliff was made of limestone reefs from the Cathedral Formation, which likely formed just before the Burgess Shale was deposited. While the exact process of how these layers formed is not fully understood, the most common explanation suggests that a part of the Cathedral Formation reef became separated from the rest, slid downward, and moved several kilometers away from the reef. Later, movement along faults at the base of the formation caused the layers to break apart around 509 million years ago. This process would have created a steep cliff, with the bottom protected from pressure changes because the limestone of the Cathedral Formation is hard to compress. This protection explains why fossils found farther from the Cathedral Formation are harder to study—pressure from tectonic forces created vertical cracks in the rock, causing it to split perpendicular to the fossils. The Walcott quarry is famous for its well-preserved fossils because it is very close to the Stephen Formation; the quarry has been dug to the edge of the Cambrian cliff.

Early scientists believed the Burgess Shale was deposited in conditions with little to no oxygen. However, recent research shows that oxygen was present in the sediment. This oxygen helped preserve soft parts of ancient organisms by slowing decay and creating chemical conditions that supported fossilization. It also limited the number of burrowing organisms, as burrows and trackways are rare in these layers and usually found only in limited areas. Another idea is that brine seeps contributed to preservation; for more details, see discussions on Burgess Shale-type preservation.

Sometimes, the Burgess Shale is considered a separate unit called the Burgess Shale Formation, rather than just a part of the Stephen Formation.

Taphonomy and diagenesis

The organisms in the Burgess Shale are believed to have been buried quickly by water currents carrying sediment, which prevented them from decaying. These currents might have carried the organisms far before they were buried. The fossils are preserved as extremely thin layers of carbon, less than one micrometre thick (a thousandth of a millimetre).

There are many other similar Cambrian fossil sites, and these types of fossil assemblages are much more common during the Cambrian period than in any other time. This is mainly because burrowing by organisms was limited during the Cambrian. As burrowing became more common over time, environments that could preserve soft parts of organisms became rarer. The fossil record of animals before the Cambrian period is limited and unclear, as seen in the Ediacaran biota.

Biota

The living things found in the Burgess Shale are similar to those in other middle Cambrian rock layers. Even though only about 14% of the organisms in the Burgess Shale have hard parts, similar numbers of these organisms are found in other Cambrian areas. This shows that the soft-bodied organisms are not unusual; many of them are also found in other fossil sites from different times and places.

The Burgess Shale contains a wide variety of life forms. Free-swimming organisms are rare, while most are bottom-dwelling. These bottom-dwellers either move around or are permanently attached to the sea floor. About two-thirds of the organisms ate from the muddy sea floor, and nearly a third filtered tiny particles from the water. Less than 10% were predators or scavengers. However, because these larger animals had more mass, the total amount of living material was about the same for each type of feeding behavior.

Many organisms in the Burgess Shale are early ancestors of today's animal groups. Some are also from modern groups that exist today.

Working with the Burgess Shale

The fossils found in the Burgess Shale are covered in a dark layer of carbon on black rocks, which makes it hard to take clear pictures of them. However, special techniques such as using a type of microscope that uses electrons, mapping the elements present, and a drawing method that helps show details can improve the quality of the images.

After images are taken, scientists must consider how the fossils changed over time due to decay and the process of fossil formation before they can accurately reconstruct the body structure. By studying the different features of the fossils, researchers can determine which group of organisms the fossils belong to.