CELPIP Reading Part 3 Practice Test: The Burgess Shale Fossils of Yoho

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High on a ridge in Yoho National Park, in the Rocky Mountains of eastern British Columbia, lies one of the most important fossil sites ever discovered. The Burgess Shale, named after nearby Burgess Pass, was found in 1909 by Charles Walcott, an American palaeontologist who was then head of the Smithsonian Institution in Washington. Over several summers, Walcott and members of his family hauled tens of thousands of specimens down the mountain, and most of them were shipped back to the United States. The rocks they collected are roughly 508 million years old, dating from the Cambrian period, a time when nearly all life on Earth was still found in the sea.

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What makes the Burgess Shale extraordinary is not its age but its state of preservation. Most fossils record only hard parts such as shells, bones and teeth, because soft tissue usually decays long before it can turn to stone. Here, however, animals living near the foot of a towering underwater cliff appear to have been buried suddenly by flows of fine mud. With little oxygen in the surrounding water, the bodies escaped normal decay, and features as delicate as gills, guts and even muscles were preserved as thin, dark films in the rock. Such conditions are extremely rare, which is why the site is so valuable to scientists trying to understand how complex animals first appeared.

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Many of the creatures found in the shale look strange even to experts. The most common, Marrella, was a small, lace-like early arthropod. Opabinia had five eyes and a long, flexible nozzle with a grasping claw at its tip. Hallucigenia, a worm-like animal with spines along its body, was first reconstructed upside down and back to front, with its spines mistaken for legs. Anomalocaris, one of the largest predators of its time, was originally known only from separate pieces that scientists at first thought belonged to several different animals. When researchers at the University of Cambridge re-examined Walcott’s collection in the 1960s and 1970s, their work helped convince scientists that early animal life had been far more varied than once believed.

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Today the Burgess Shale is part of a UNESCO World Heritage Site, and collecting fossils there is strictly forbidden. Members of the public may visit the main fossil beds only on guided hikes led by Parks Canada or the Burgess Shale Geoscience Foundation, and the steep round trip can take most of a day. Research has not stopped, either. Scientists from the Royal Ontario Museum in Toronto, which holds the largest collection of Burgess Shale fossils in the world, have led many expeditions in the region, and in the 2010s they uncovered a rich new fossil site in nearby Kootenay National Park.