Preserved worm burrows in
Canadian rock were invisible to the naked eye. In this image, a large,
lined, horizontal burrow is almost 0.8 inches (20 mm) wide.
Credit: University of Saskatchewan
About a half billion years ago, an ancient sea covered what is now the
northernmost stretches of Canada. Its seafloor was long thought to be a
dead zone, devoid of the oxygen needed to support life.
But as it turns out, minuscule worms lived quite happily in these ocean
sediments — they even created their own "superhighway" of tunnels by
burrowing through the soil.
Traces of these fossilized tunnels were found in rocks collected
decades ago from Canada's Mackenzie Mountains in the Northwest
Territories. But scientists more recently found the tiny tunnels only
after reanalyzing them, they reported in a new study.
Their discovery sheds light on the region's ocean ecosystems during the Cambrian era
(543 million to 490 million years ago), suggesting that these
environments may have harbored more oxygen — and more life — than
expected, according to the study. [Cambrian Creatures Gallery: Photos of Primitive Sea Life]
The tunnels that the worms left behind in the weathered rock weren't
visible to the naked eye, and were detected purely by chance, lead study
author Brian Pratt, a professor of geological sciences with the College
of Arts and Sciences at the University of Saskatchewan in Canada, told
Live Science in an email.
Pratt and his co-author Julien Kimmig, a collections manager of
invertebrate paleontology with the Biodiversity Institute and Natural
History Museum at the University of Kansas, found the tunnels while
collaborating on another study published in 2018. (They described a "poop picnic" enjoyed by Cambrian worms at the same site where the worm tunnels were found, Pratt said.)
Pratt and Kimmig were preparing samples for the 2018 study — sawing and
grinding the rocks — when they discovered something that they hadn't
seen before.
"I noticed some slight variation in shading," Pratt said. He dampened
the smooth surface of a rock sample with alcohol, scanned it on a
flatbed scanner and enhanced the image brightness and contrast.
Suddenly, "a riot of burrows appeared," he said. Some areas were
crisscrossed by just a few tunnels, but other portions of rock were
"completely churned" by the worms' activities, he said.
The preserved tunnel shapes were exceptionally well-defined and had not
collapsed, hinting that the sediment around them was firm and not
"soupy," the study authors wrote.
Researcher Brian Pratt found
evidence of worm activity from 500 million years ago preserved in a
fossil-rich site in British Columbia, Canada.
Credit: University of Saskatchewan
In width, the tunnels measured from 0.02 to 0.6 inches (0.5 to 15
millimeters), made by worms ranging from about a millimeter in length to
finger-size, according to the study. Most of the burrows were tiny, dug
out by worms that scoured the ocean sediment looking for organic matter
to eat. The rare, larger tunnels likely housed predatory filter-feeders
— "animals that projected a feeding apparatus up into the water column
to catch organic particles and tiny animals," Pratt said.
Also contained in the rock were the preserved bodies of worms — not the
worms that dug the tunnels — and "large poops" containing shreds of
body tissues that likely belonged to other worms that were eaten,
according to Pratt.
Together, this evidence presents a fascinating snapshot of an ancient
seafloor habitat, in an ocean ecosystem that was far richer — in oxygen
and species — than expected, the scientists reported.
The findings were published online in the March issue of the journal Geology.
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