B. C.’ S BURGESS SHALE UNDER THE MICROSCOPE
B. C.’ s Burgess Shale deposit reveals links between fossils’ ‘ rasping tongue’ and today’s octopus, snail and soft- bodied animals
A Canadian scientist probing 500- million- year- old rocks from B. C.’ s famous Burgess Shale fossil site has solved an enduring mystery about the origins of mollusks, the hugely successful group of species that ranges from the tiniest slugs to the giant squid.
University of Toronto paleontologist Martin Smith has examined hundreds of Burgess Shale specimens collected by researchers over the past century, which are held now at the Royal Ontario Museum.
Smith has identified and reconstructed the earliest known examples of a mollusk’s radula — the conveyorbelt-like rows of teeth that evolved over eons into the feeding mechanisms used today by the octopus, garden snail and thousands of other members of the wildly diverse family of slimy, soft- bodied animals.
His research, published this week in the journal Proceedings of the Royal Society B, resolves a long- standing question about whether two marine organisms known from the B. C. fossil bed — Odontogriphus omalus and Wiwaxia corrugate — were ancestral mollusks or the forerunners of today’s earthworms.
Smith’s findings, based on electron microscope examinations of the fossils and 3- D reconstructions of their teeth using modelling clay, showed clear links between the two extinct species found petrified in the Rocky Mountains and the vast multitude of mollusk species living on Earth today.
“Of all the fossils we currently know, Odontogriphus and Wiwaxia are probably the closest to the ancestral mollusk — all living mollusks are probably descended from an animal that looked very similar to these fossils,” Smith told Postmedia News. “There would probably have been a host of rather similar- looking species around at the time, but because different species can’t interbreed, only one can have been ancestral to the modern mollusks. Odontogriphus or Wiwaxia might have been this ancestral species.”
Key to unravelling the puzzle was Smith’s reconstruction of each animal’s radula or “rasping tongue,” which showed how the extinct species’ mouth parts matched — in several telltale ways — those of all modern mollusks, with the exception of filter- feeding bivalves such as clams and mussels.
“I put the fossils in the [ electron] microscope, and the mouth parts just leaped out,” Smith, a PhD candidate in the U of T’s department of ecology and evolutionary biology, said in a summary of the study. “You could see details you’d never guess were there if you just had a normal microscope.”
With the fossils “squashed completely flat” since the specimens were alive a half- billion years ago, Smith described how he surrounded himself with enlarged images of the creatures’ mouths “and lumps of Plasticine, and spent weeks trying to come up with a model that made sense of the fossils.”
He determined that the Cambrianera animals would have used their rows of teeth to gather algae and other tiny bits of food from the ocean floor, just as many species of marine mollusks do today.
“When I set out, I just hoped to be a bit closer to knowing what these mysterious fossils were,” said Smith. “Now, with this picture of the earliest radula, we are one step closer to understanding where the mollusks came from and how they became so successful today.”
The study shedding light on mollusk origins isn’t Smith’s first breakthrough discovery from the Burgess Shale. He was among a group of Canadian researchers who published an article two years ago in the journal Nature describing a five- centimetrelong, fossilized creature from B. C. — Nectocaris pteryx — that represented the first known animal to employ jet propulsion to shoot itself through the water, like modern- day squids, cuttlefish and octopuses.
Protected within Yoho National Park as part of a UNESCO World Heritage Site, the Burgess Shale fossil bed — an exquisitely preserved record of the era when primitive life first exploded in diversity and complexity — was one of the most important discoveries ever in the study of evolution.
Workers on the Canadian Pacific Railway in the 1880s initially spotted the “stone bugs” that later drew U. S. paleontologist Charles Doolittle Walcott’s attention to the region in 1909, and he soon located the fossil motherlode near Field.
Scientists have determined that the fossils were formed after the sudden burial of an entire seabed ecosystem in a catastrophic underwater landslide more than 500 million years ago.
It’s believed an avalanche of mudrich slurry killed tens of thousands of marine animals representing hundreds of different species, then sealed them instantly — and enduringly — in a deepsea tomb that eventually hardened and was uplifted to the Earth’s surface during the rise of the Rocky Mountains.