Burgess Shale, Yoho National Park

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A Middle Cambrian deposit of about 508 million years old on Fossil Ridge between Wapta Mountain and Mount Field in Yoho National Park, British Columbia, and the most consequential fossil locality ever found for understanding early animals. Charles Doolittle Walcott, then Secretary of the Smithsonian Institution, located the productive beds in 1909 and quarried them over the following years, shipping tens of thousands of specimens to Washington. The animals were buried by sediment flows off the edge of a carbonate platform and preserved as flattened carbon films that retain gills, guts, eyes and limbs. Walcott assigned most of them to existing groups. A re-study begun at Cambridge in 1971 by Harry Whittington with Derek Briggs and Simon Conway Morris dismantled that reading, showing that animals such as Opabinia and Hallucigenia fitted no living phylum comfortably, and the argument over what that means — a lost disparity of body plans, or stem lineages of groups that survive — became one of the defining palaeontological debates of the late twentieth century. The site was inscribed as a World Heritage Site in 1980 and later incorporated into the Canadian Rocky Mountain Parks inscription.