Burgess Shale: Charles Walcott's Cambrian Fossil Discovery

The Burgess Shale is a deposit of exceptionally preserved fossils in the Canadian Rockies of British Columbia, dating to about 508 million years ago. Discovered by the American palaeontologist Charles Doolittle Walcott in 1909, it preserves not only shells but also the soft bodies of worms, arthropods and other animals in remarkable detail. Its fossils have shaped scientific debate about the Cambrian Explosion and the early evolution of animals for more than a century.

Background: Walcott and the Cambrian

Charles Doolittle Walcott (1850–1927) was one of the leading geologists in the United States. Largely self-taught, he rose to become director of the United States Geological Survey and, from 1907, Secretary of the Smithsonian Institution in Washington, D.C.

Walcott specialised in Cambrian rocks and fossils, especially trilobites. Each summer he travelled with his family to the Canadian Rockies to study Cambrian rock sequences, which are well exposed in the steep mountains around the town of Field, along the Canadian Pacific Railway. Fossil trilobites had already been found there on Mount Stephen in the 1880s.

Discovery in 1909

At the end of August 1909, near the end of the field season, Walcott and his party were working on a ridge between Wapta Mountain and Mount Field, later called Fossil Ridge. There they found slabs of shale containing unusual soft-bodied fossils, including arthropods preserved as dark, shiny films.

Walcott’s diary records the finds, and he sketched several animals. The site lay near the Burgess Pass, named in the 1880s after Alexander Burgess, then Canada’s deputy minister of the interior, and Walcott adopted the name Burgess Shale for the rock.

He returned in 1910 with his family and located the layer that produced the best fossils, a band of rock about two metres thick that he called the Phyllopod bed. He opened what is now called the Walcott Quarry and came back in several later seasons, with a final visit in 1924. In all, he and his team collected around 65,000 specimens, now held mainly by the Smithsonian’s National Museum of Natural History.

Collecting was hard work. The quarry lies high on a steep mountainside, well above the tree line, and the team split the shale by hand, packed the slabs and carried them down by horse before they were shipped by rail to Washington. Walcott continued to describe Burgess Shale animals alongside his administrative duties until shortly before his death in 1927, but much of the collection remained unstudied for decades.

How the fossils formed

The Burgess Shale animals lived on a sea floor near the edge of the ancient continent of Laurentia, the core of North America, which then lay close to the equator. Nearby stood a huge underwater cliff of limestone reef rock, the Cathedral Escarpment, perhaps more than 100 metres high.

Mud flows periodically swept down the slopes at the foot of this cliff, burying animals quickly in fine sediment. Low oxygen in the water and the chemistry of the mud slowed decay, so that soft tissues were preserved as thin carbon-rich films. Later mountain building lifted the rocks thousands of metres above sea level without destroying the fossils.

The fossils

The Burgess Shale has yielded well over 100 genera of animals, most of them without hard shells. They include sponges, worms, arthropods, comb jellies, early chordates and many animals that are hard to classify.

Marrella splendens, a small arthropod with long sweeping spines on its head, is the most common animal in the Walcott Quarry. Olenoides, a trilobite, is preserved with its legs and antennae intact.

Opabinia regalis had five eyes and a long flexible nozzle ending in a grasping claw. When Harry Whittington presented his reconstruction at a scientific meeting in the early 1970s, the image reportedly drew laughter from the audience because it looked so strange.

Hallucigenia sparsa, named by Simon Conway Morris in 1977, was first reconstructed walking on its spines with tentacles on its back. In 1991 Lars Ramsköld and Hou Xianguang, using related fossils from China, showed that it had been drawn upside down: the “tentacles” were legs and the spines were on its back.

Anomalocaris was a large predator whose body parts had been described as three different animals before Whittington and Derek Briggs reconstructed it in 1985, as explained in the article on Anomalocaris.

Wiwaxia was a slug-like animal covered with scales and spines, possibly related to molluscs. Pikaia gracilens, a small swimming animal with a notochord, is one of the earliest known chordates and a distant relative of vertebrates; see the article on Pikaia.

A Cambrian ecosystem

Because it preserves soft-bodied animals, the Burgess Shale allows scientists to reconstruct a whole ancient community rather than just its shelled members. In a typical sample, arthropods are the most abundant and diverse animals, followed by sponges, worms and other groups. Shelled animals such as trilobites and brachiopods, which dominate ordinary Cambrian fossil sites, make up only a small part of the total.

The animals filled many roles. Sponges and some arthropods filtered food from the water. Worms such as the priapulid Ottoia burrowed in the mud, and fossil Ottoia have been found with the remains of other animals in their guts, showing that they ate prey, including small shelly animals. Arthropods such as Sidneyia, named after Walcott’s son Sidney, also preserve gut contents.

Large swimming predators such as Anomalocaris and other radiodonts hunted above the sea floor, while scavengers and grazers fed on the bottom. The result was a complex food web with several levels of predators, broadly similar in structure to marine ecosystems today.

Studying how common each species was, and how communities changed from one rock layer to the next, has helped researchers understand how Cambrian ecosystems worked and how quickly they could change.

Restudy, debate and new finds

The Cambridge restudy and Wonderful Life

Walcott described many Burgess animals in a series of papers from 1911 onwards. Busy with his duties at the Smithsonian, he generally placed them in existing groups, such as crustaceans, annelid worms or jellyfish.

In 1966 and 1967 the Geological Survey of Canada reopened the quarries, and Harry Whittington of the University of Cambridge began a detailed restudy. With his students Derek Briggs and Simon Conway Morris, he showed that many animals did not fit Walcott’s classification. Some appeared to represent entirely unknown body plans.

The American palaeontologist Stephen Jay Gould brought these discoveries to a wide audience in his 1989 book Wonderful Life. He argued that the Burgess Shale contained many groups that later died out, and that the survival of the groups we know today owed much to chance, or “contingency”. Conway Morris disagreed in his 1998 book The Crucible of Creation, emphasising instead that evolution often arrives at similar solutions.

Since then, most researchers have concluded that the strange Burgess animals are best understood as early branches of living groups, such as arthropods, molluscs and chordates, rather than as completely separate phyla. Even so, Gould’s book made the Burgess Shale one of the best-known fossil sites in the world.

New sites and protection

From 1975 the Royal Ontario Museum in Toronto, led by Desmond Collins, began a long programme of expeditions. Its teams found many new fossil localities in the area and built a large collection. In 2012 a team led by Jean-Bernard Caron found a rich new site at Marble Canyon in Kootenay National Park, about 40 kilometres south-east of the Walcott Quarry. Announced in 2014, it has yielded many new species.

The Burgess Shale was inscribed on the UNESCO World Heritage List in 1980. In 1984 it became part of the larger Canadian Rocky Mountain Parks World Heritage Site. Access to the fossil beds is restricted, and visitors can reach them only on guided hikes.

Myths and facts

Myth: Walcott found the site when his wife’s horse kicked over a fossil slab. This dramatic story was told for decades, but Walcott’s own diary gives no support to it. Gould discussed the legend in Wonderful Life and concluded that it was probably a later embellishment.

Fact: Chengjiang is older. The Chengjiang biota of China, discovered in 1984, is about 10 million years older and preserves many similar animals; see the article on the Chengjiang biota.

Timeline

• c. 508 million years ago: Animals living near the Cathedral Escarpment are buried by mud flows.
• 1886: Fossil trilobites are reported from Mount Stephen near Field, British Columbia.
• 1909, late August: Charles Doolittle Walcott discovers soft-bodied fossils on Fossil Ridge.
• 1910: Walcott locates the Phyllopod bed and opens the Walcott Quarry.
• 1911: Walcott begins publishing descriptions of the Burgess Shale animals.
• 1966–1967: The Geological Survey of Canada reopens the quarries; Harry Whittington leads a restudy.
• 1980: The Burgess Shale is inscribed as a UNESCO World Heritage Site.
• 1989: Stephen Jay Gould publishes Wonderful Life.
• 2014: Discovery of the Marble Canyon site in Kootenay National Park is announced.

Why it matters

The Burgess Shale was the first fossil site to reveal the full variety of a Cambrian animal community, soft-bodied creatures and all. It changed how scientists picture the early evolution of animals, sparked a lasting debate about chance and pattern in evolution, and remains one of the most important places on Earth for studying the origins of the animal world.