An Explosion of Life
(Inside Science) -- At the base of a ridge in the Canadian Rocky Mountains lie the types of fossils that make geologists giddy, the remnants of a flourishing undersea world from more than 500 million years ago. They are preserved today in a rock layer called the Burgess Shale near the town of Field, British Columbia. This year marks the 100th anniversary of the discovery of the shale and its astonishingly well-preserved fossils, revealed by Smithsonian Institution paleontologist Charles Walcott in late August 1909.
For those uninitiated into the world of ancient time and long-lost fossils, the shale appears to be nothing more than rocks layered and stacked into mountains. But the late paleontologist Stephen Jay Gould summed up the importance of this rock formation to science in his bestselling book "Wonderful Life: The Burgess Shale and the Nature of History." The shale, he said, contains "the world's most important animal fossils."
Most animal fossils consist of hard parts like shells, teeth and bones that are less likely to decay and rot away than soft tissue. But the Burgess fossils, despite hundreds of millions of years of geological processes crushing, twisting, and heating them, are extraordinarily well-preserved, with soft tissue such as eyeballs and guts transformed into rock. In the century since this discovery, only a few other high quality soft fossil sites have been found around the globe.
And it is not just the quality of the Burgess fossils that is considered remarkable. The extraordinary number of fossilized animals found within the relatively small volume of rock also sets the shale apart.
Earlier this month, paleontologists met in Banff, Alberta, Canada, to present their research regarding the "Cambrian explosion," the term for the period of time covering roughly 542 to 488 million years BP, or "before the present," when primitive life begat recognizable ancestors of today's complicated array of animals.
"The aim [was] to celebrate the one hundredth anniversary of the discovering of the Burgess Shale by Charles Walcott and to bring delegates from all across the world to talk about Cambrian fossils," said Jean-Bernard Caron, associate curator at the Royal Ontario Museum in Toronto and conference organizer.
The Burgess Shale sits high in the mountains amid glaciers, lakes, and pristine forests. "It felt almost like a religious experience to walk into the Walcott quarry and think, 'Wow, this is where it all started.'" said Allison Daley, a doctoral student in paleobiology at Uppsala University in Sweden.
The formation lies next to the bottom of what was a submarine cliff more than 505 million years ago. The area where the fossils rest was deep enough under water that waves or storms did not disturb the sediments that buried the animals. It is generally agreed that repeated flows of mud buried numerous groups of living or very recently dead animals over the course of about 100,000 years. The mud and low levels of oxygen within it stopped the decay process that would usually destroy the animals' vulnerable soft parts. The animals' structures have been maintained through the long history of intensive processes that solidified the sediments into rock and built the Rocky Mountains.
Just how that happened is not completely clear. Paleontologist Alex Page from the University of Cambridge in England recently developed a new theory, that the layers of clay surrounding many of the Burgess fossils did not form immediately after the animals were buried, but tens to hundreds of millions of years later, challenging the understanding of what exactly protected the soft parts of the animals.
Early Blueprints for Modern Animals
During the Cambrian period represented by the Burgess Shale, the evolutionary outburst of life established most of the primary categories, called phyla, of modern animals, which are organized around significant body features.
For pure weirdness, it's difficult to imagine a group of animals that could surpass the array found in the ancient sea sediments of the Burgess Shale. One animal looked like a thin worm surrounded by a pincushion of spikes and legs. Another had five eyes and a feature that looked a bit like an elephant's trunk with two sharp claws on the end.
"We recognize that even though these animals are very strange looking, they still do have [characteristics] that are similar to what we see in the modern world," said Daley. "They are primitive, and of course they have weird and unusual features."
In March, Daley published an article in the journal Science describing an animal called Hurdia. When Walcott first looked at the fossil, he thought the mouth parts were a jellyfish, the front legs were shrimp, the main body a sea cucumber, and a tear-drop shaped shell, another animal. "It's totally strange," said Daley, who described the last piece as three sides of a rectangular box, sticking out in front of the mouth and head.
While studying the tear drop structure, which she calls a carapace, Daley checked to see if it fit over the rest of the body, where it might have provided protection. "There are these little notches at the posterior side of the carapace where the eyestalks stick through," she said. "So if the carapace had slid forward that would mean the eyes were growing out of the posterior end." Daley thinks that the structure stuck out in front of Hurdia's head and was somehow related to the collection of food as a sort of digging tool or funnel.
Before scientists can understand the evolutionary relationships of these fossil animals -- to each other and to life today -- they must puzzle out the shape they took while living. Although the fossils of the Burgess Shale are well preserved, the squished and twisted features of these unfamiliar creatures conspire to make them difficult to interpret.
Caron explained that the process is like a Martian trying to draw a three dimensional picture of human anatomy based on only two dimensional views of human bodies. If the entire front side of a human fossil were facing the Martian directly, figuring out the original shape of nose would be difficult, because it would be squished. The problem might be solved by looking at other humans found in different orientations that could reveal more about the shape of the nose.
Understanding Continues to Evolve
There are certainly more details to decipher and more species to describe. The scientists note that analyzing the fossils is just the first step in the research, for to fully understand the environment in which they lived and the processes that preserved them so well, the entire Burgess Shale formation must be understood.
In the 20 years since Gould's book was published, research has flourished. New examinations and evidence have led to fresh interpretations. Innovative techniques and improved technologies contributed to this, but so has the discovery of other fossil beds from the same era located in other parts of the world -- especially the Chengjiang fossils in China.
"We have now a more complete picture of early animal evolution and the relation of the fossils of the Burgess Shale to it than we had twenty years ago," Page said. "[This] highlights the fact that the Burgess Shale is still at the forefront of science."