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The Burgess Shale: Evolution's Big BangExplore more photos from the story |
The fossil-hunting expedition began with a lung-busting hike, accompanied by an incessant ring-ding-ting-clank-clank-ring-ting-ding-clank. The soundtrack came courtesy of an anti-bear bell attached to the backpack of the group's leader, Jean-Bernard Caron, a curator of invertebrate paleontology at Toronto's Royal Ontario Museum. After four hours of hiking up switchbacks through an evergreen forest deep in the Canadian Rockies, Caron suddenly took off like a mountain goat. As the others caught their breath, he zipped his way across loose and jagged rock up the final ascent. Eventually the team reunited at the top of the cliff and collapsed, surveying the view over the Burgess Shale.
"Yay! Fossils! We're really here!" exclaimed Allison Daley, a graduate student from Sweden's Uppsala University. She bit into an Oreo cookie and fanned her face in nervous excitement.
The Burgess Shale is Mecca for paleontologists. Charles Doolittle Walcott, the fourth Secretary of the Smithsonian Institution, discovered this rich fossil bed a century ago, in the summer of 1909, and named it for nearby Mount Burgess. At the end of his first field season here, Walcott wrote in a letter to a colleague that he had "found some very interesting things." Talk about understatement. The Burgess fossils tell nothing less than the story of the Cambrian explosion—evolution's Big Bang—when relatively simple organisms rapidly diversified into the sorts of animals that live today. The exquisitely preserved Burgess specimens (most likely entombed by underwater mudslides) include the remnants of soft-bodied organisms, which are rare in the fossil record. The animals inhabited the ocean floor 505 million years ago, near the end of the Cambrian Period.
"Most of the phyla we know today we can see already in the Cambrian and the animals of the Burgess Shale," said Caron. But the fossils still look very strange. "They certainly show evidence of evolution," Caron said. "The animals that you see there don't look like the ones we have today."
After recovering from the climb, the crew began to explore the most famous Burgess Shale site, a tennis-court- size rock exposure now called Walcott Quarry, where Walcott first found fossils. Over nine field seasons he collected 65,000 specimens, and the site has since been picked over by innumerable expeditions; finding any more fossils of interest would test both patience and passion. Even so, the team dispersed over the scorching hill, pulling out one slab of green-gray shale after another, checking both sides for a faintly textured squiggle or shadow. Since fossils are somewhat easier to see when the rocks are wet, the researchers occasionally licked the slabs.
"It's like playing the slots," said Jason Loxton, a graduate student at Dalhousie University in Halifax, Nova Scotia. "Pulling and pulling, and pulling and pulling and pulling. You've gotta get one eventually. Then maybe someone beside you wins. And then you just have to keep going."
Walcott identified his specimens as members of extinct or modern groups, such as the arthropods (shrimp, crabs, insects and the like) or annelids (segmented worms). In 1989, paleontologist Stephen Jay Gould popularized Burgess' "weird wonders" in his bestselling book Wonderful Life. But he argued that Walcott had erroneously shoehorned the fossils into existing taxonomic groups. He suggested that the curious "problematica" fossils that had long defied scientific identification—such as Hallucigenia, an inch-long creature with two rows of spines on its back—deserved their own taxonomic groupings.
Lately, paleontologists have begun re-examining the classifications yet again, largely in response to the discovery of Burgess-type fossils in Australia, China, Greenland, Russia, Spain and the United States. With more specimens, scientists are better able to see similarities among animals, and so they are shifting their emphasis from unique to shared characteristics.
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