Prof makes fossil breakthrough in overlooked section of Burgess Shale


What he discovered was a new species of the mysterious Stenothecoids

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The world-famous Burgess Shale fossil bed in Yoho National Park is continuing to reveal its mysteries to scientists more than a century after the site was first discovered.

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The shale’s two-dimensional fossil images have puzzled scientists for years as they have tried to determine what the 505-million-year-old creatures looked like in the real world.

However Paul Johnston, PhD and associate professor at Mount Royal University’s earth and environmental sciences department, used a unique technique to extract three-dimensional fossils from a mostly overlooked limestone deposit at the site.

“People tend not to bother with the limestone beds in the Burgess Shale because the best fossils tend to come from the shales,” said Johnston.

“I had to get down on my hands and knees because these fossils are small. And then I saw there were in fact some fossils, and with my hand lens I could see they had been replaced with silica during their long burial history.”

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As a student, Johnston had used hydrochloric acid to extract fossilized shells from younger limestone deposits in Australia. He realized he might be able to do the same thing with these older deposits, and made an application to Parks Canada to try it.

“And voila — I was able to get these things out as three-dimensional objects,” he said.

Mount Royal University associate professor Paul Johnston extracted fossils from limestone at the Burgess Shale site in Yoho National Park.
Mount Royal University associate professor Paul Johnston extracted fossils from limestone at the Burgess Shale site in Yoho National Park.

What he had discovered was a new species of the mysterious Stenothecoids. Johnston named his discovery Stenothecoides rasettii, in tribute to Italian physicist Franco Rasetti, who worked with Enrico Fermi on the problem of nuclear fission and had a side interest in paleontology.

“(Rasetti) came up to Yoho National Park and he found Stenothecoids in rock strata older — below — the Burgess Shale,” said Johnston, adding that he often relies on Rasetti’s previous work in his own research.

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With about 200 examples of the new Stenothecoid species in his possession, Johnston was able to make a breakthrough in the question that had puzzled scientists since Stenothecoids were first discovered: what exactly were they?

Previously, the best guess was that they were a type of mollusk, although some had suggested they might be brachiopods, while others wanted to create a new phylum to categorize them.

“I work on the evolutionary history of mollusks. Like other paleontologists, I couldn’t make them fit very well in the evolutionary tree of mollusks,” said Johnston.

However, one tiny hole that was common across his collection of fossilized shells revealed that the Stenothecoids were in fact more closely related to brachiopods, and were likely an early offshoot of that evolutionary branch.

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“We were able to resolve that because the preservation was quite good and we could see anatomical details that really told the story,” he said.

He teamed up with brachiopod expert Michael Streng at Uppsala University in Sweden. Together they published their findings in the journal Acta Palaeontologica Polonica.

Coincidentally, the paleontologist who first found Stenothecoids in Nevada in the 1800s was the same person who discovered the Burgess Shale.

Paleontologist Charles Walcott was administrator of the Smithsonian Institution for 20 years. He noted his discovery of Stenothecoids in 1884, and later documented the Burgess Shale in 1909.

Johnston’s discovery brings Walcott’s two discoveries together in a unique way across time and geography.

“Obviously, Walcott didn’t look at the limestone component because he was busy with the shales,” said Johnston.

Johnston’s fossils will now reside at the Royal Tyrrell Museum and at the Royal Ontario Museum in Toronto.

[email protected]

Twitter: @brodie_thomas

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