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The rare fossils of Yoho National Park

by Timber Press on October 9, 2017

in Natural History

The Walcott Quarry—a fossil-rich, 505-millionyear- old slice of ocean floor—is located about midway across this ridgeline in Yoho National Park. Photo by the author.

Not all geologic treasures can be easily seen from the air. Canada’s Yoho National Park, west of Banff on the west side of the Canadian Rockies, doesn’t really stand out in the midst of surrounding mountains. You’ll have to get much closer to see Yoho’s most famous features: rare fossils representing some of the earliest complex life-forms on Earth.

Most fossils are formed from mineralized shells, bones, teeth, and other hard parts of longdead organisms. But in a few uncommon cases, fossils are found that preserve everything, including seldom-seen soft body parts such as eyes, gills, and even stomachs containing last meals. One of the most famous localities for these extraordinary fossils is the Burgess Shale, on a ridge in Yoho National Park. The fossils found here date back 505 million years, only 65 million years aft er the planet’s first complex life-forms evolved.

“Stone bugs” found in Yoho National Park are actually 500-million-yearold trilobites, one of the earliest known arthropods. Photo by the author.

Around 570 million years ago, the first multicellular life developed in shallow seas. This momentous event was soon followed by the Cambrian Explosion—the rapid evolution of many types of life-forms. Within a few million years, virtually all major phyla (a taxonomical classification of organisms) appeared on the scene, including the early ancestors of vertebrates. The fossils found in the Burgess Shale provide an exquisitely detailed snapshot of this pivotal time in evolution.

You might fly over Yoho National Park en route to Calgary, Alberta. People are allowed to visit the site on foot, but you must go with a certified guide through Parks Canada or the Burgess Shale Geoscience Foundation. Photo by Jesse Allen, NASA.

Fossilization requires something of a perfect storm. To become a fossil, an organism must avoid predation, scavengers, and decomposition and come to rest in an ideal environment for preservation, most oft en through rapid burial of the carcass. This layer must then undergo lithification into rock in a way that preserves the delicate structure of the fossil. The rock layer must avoid metamorphosis, compression, and distortion for millions of years. In a lucky happenstance of nature, the Burgess Shale fossils cleared all those hurdles to become some of the most celebrated fossils in all of paleontology.

Cambrian-era soft -bodied fossils are found at a few sites around the world, but the most famous quarries are located in Yoho. When the Canadian Pacific Railway was being built through Kicking Horse Valley, workers would occasionally find “stone bugs,” which were actually fossilized trilobites, a type of marine arthropod, now extinct. In 1886, one of these fossils made its way to a scientist with the Geological Survey of Canada, who combed the ridges above the valley until he found the source: a trove of trilobite fossils on the side of Mount Stephen, near the town of Field, British Columbia.

The Walcott Quarry is not very big, but it’s the prolific source of some of the most important fossils in the paleontological record. Photo by the author.

Across the river, the Walcott Quarry, discovered in 1909, would become the most famous source for Burgess Shale fossils—over 100,000 specimens have been removed from the site and placed in the Smithsonian Institution in Washington, DC, and the Royal Ontario Museum in Toronto. This famous fossil quarry is only about ten feet high and less than a city block long, capped on either end by metamorphosed rocks that contain no fossils. But despite the quarry’s small size and over a century of rigorous excavation, many fossils still remain, waiting to be discovered.

Mary Caperton Morton is a freelance science and travel writer. A regular contributor to EARTH magazine, where her favorite beat is the Travels in Geology column, Mary also inspires people to “See More of the World” with her blog Travels with the Blonde Coyote.

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