This month marks the centenary of the discovery of the Burgess Shales in Canada, formed during the greatest expansion of life yet seen on earth, writes MARY MULVIHILL.
THEY DON’T LOOK like much, mostly a black smear on dark rock. So visitors to Toronto’s Royal Ontario Museum (Rom) usually ignore the three cabinets of small fossils and head for the dinosaurs.
Yet these Burgess shale fossils, from Canada’s Rocky Mountains, are arguably among the most important in the world. At 505 million years old, they come from that great “biodiversity big bang” when all the major groups of animals evolved in just a few tens of millions of years, what scientists call the Cambrian explosion.
Now a Unesco World Heritage Site, the Burgess shale fossils were discovered 100 years ago, in August 1909, and this week scientists gather there for a centenary conference. Among them, UCD graduate Lorna O’Brien, now a PhD student at Toronto studying the fossil ecosystems preserved in the shale.
Even O’Brien admits the fossils “don’t look impressive. But when you view them under water and in polarised light you get to see the really fine detail, and then they’re fantastic”.
ACCORDING TO Rom curator and Burgess shale expert, Dr Jean-Bernard Caron, almost all animal life can trace its origins to the Cambrian explosion. Before, there were simple single-celled animals, sponges and microscopic “small shelly fossils”, but the Cambrian gives us whole ecosystems, with the first predators, and locomotion, and a tremendous array of body plans. “And without the Burgess shale fossils, we’d know very little about the Cambrian explosion.”
Similar fossil collections have now been found in Greenland, China and elsewhere, greatly adding to our understanding of this crucial event. Significantly, most are soft-bodied marine organisms, not the hard-bodied animals usually preserved as fossils.
Why this is so is part of the puzzle, according to Caron, who identifies some of the possible causes: a quick burial, in very fine mud, and with little oxygen in the environment at that time.
The water chemistry was also probably important, likewise the absence of burrowing animals, and the fact that the remains were deposited at the bottom of a 100m underwater cliff, far from disturbing waves and storms, would also have helped preserve the fossils.
Though discovered in 1909 by a Smithsonian geologist, Charles Walcott, the fossils’ significance was not realised until the 1970s, when scientists began to piece more of the specimens together.
Consider, for example, Anomalocaris(odd shrimp), the largest known Cambrian animal. A predator that ate trilobites and could grow to nearly one metre long, it was initially described as three separate species: an "arm" was the original Anomalocaris; its mouth was later identified as a jellyfish; and its body as a sea cucumber. Only in the 1980s, when the first intact specimen was found, did the true picture emerge.
Working with Burgess shale fossils, it seems, is like trying to do millions of different jigsaws all jumbled together, and often with no picture on the box to guide you.
Today, after three decades collecting in the Rockies, Toronto’s museum boasts the world’s largest Burgess shale collection, with more than 150,000 specimens from several new localities within the area.
With more specimens and locations comes more information. "If you've only one specimen you've only one view, and perhaps just part of an animal," Caron told The Irish Times.
Caron's own research with the small snail-like Odontogriphus, reveals what can now be done. This "toothed riddle" was a puzzle for decades, but Caron and colleagues studied 187 specimens and found "compelling evidence" that the creature ate like a snail, with two rows of tiny teeth on a tongue-like structure (radula). Their findings, published in Nature(vol 442, July 13th, 2006), mean Odontogriphuscould be an early mollusc, that grazed the plant mats growing on rocks in the early Cambrian period.
For Caron it confirms that even such puzzling fossils have descendants among modern animals. And not, as the late Stephen J Gould suggested in Wonderful Life, his 1989 bestseller, that many of the Cambrian species disappeared, leaving no descendants.
Today, Rom projects on the fossils include research into how they formed, and the detailed ecosystems that existed at each point in time, comparing Canadian and Chinese collections, as well as genetic lineages and evolution.
O’Brien is studying the ecosystem that existed in one new location, called S7, and the attractively named “tulip animal”, a 25cm species not found anywhere else, but which lived there in “vast meadows”.
It is research that will also shed light on how we can be here, reading this, 505 million years later.
The Burgess shale fossils are in Yoho National Park, British Columbia, near Banff, and can be visited with a national parks guide.
A full-colour 108-page centenary booklet, Burgess Shale Primer, can be ordered from burgess-shale.info/contact.