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On left: the acorn shaped structure at the top is the proboscis, followed by a rectangular section called the collar, followed by a long trunk. The bright bulbous structure at the terminal end is an attachment structure used to anchor the worm in its tube. Right: an illustration showing Oesia inside their tube shaped dwellings.

Jean-Bernard Caron/Marianne Collins

A curious-looking worm-like creature that built cages for itself at the bottom of the sea over half a billion years ago is shedding new light on a poorly understood lineage of animals that lived in parallel with our own distant ancestors.

The find is the latest result to come out of a rich cache of fossils discovered in the Canadian Rockies in 2012 by paleontologists at the Royal Ontario Museum in Toronto. Dubbed "Marble Canyon" after a nearby hiking trail, the site has yielded dozens of species and thousands of individual specimens from the mid-Cambrian period, dating to roughly 508 million years ago – well before animal life migrated from the oceans onto land. Because of the abundance of fossils and their exquisite state of preservation, the site is proving to be one of the most vivid snapshots ever seen of that early period in life's history.

In this latest instance, the big discovery is not a new species but an unexpected merger of what were previously thought to be two unrelated species.

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The first is the wormy critter called Oesia. It has been known to paleontologists for more than a century, but a lack of good specimens has kept researchers from agreeing on how to classify it. The second is Margaretia, long thought to be a species of ancient seaweed.

Now, researchers working with the Marble Canyon fossils say that Margaretia is not a plant but a fibrous, perforated tube that Oesia constructed and inhabited in its undersea environment. The findings are backed up by many new specimens from Marble Canyon that clearly reveal Oesia occupying its tubular shelter.

"It's probably the third- or fourth-most abundant species there," said Karma Nanglu, a doctoral student at the University of Toronto and lead author of a report on the new findings, published Thursday in the open access journal BMC Biology. "They were clearly a really important ecological component of this community of animals," Mr. Nanglu said.

The tubes, combined with the gill-like structures preserved in the Oesia fossils, suggest the creature was a filter feeder that stayed in the safety of its do-it-yourself abode while it lived off nutrients suspended in the water flowing through. Oesia was also equipped with a pincer-shaped back end, which may have allowed it to anchor itself at various points within the tube.

Mr. Nanglu and his co-authors show that Oesia is likely a form of hemichordate, a type of animal still represented today by acorn worms and other species. Together with echinoderms (starfish and sea urchins), hemichordates separated during the early Cambrian from a group known as chordates, which today include humans and all other vertebrates.

By discovering how Oesia lived, in particular its dependence on filter feeding, Mr. Nanglu said the new fossils would help scientists in their efforts to learn more about the last common ancestor of all three branches.

"It's certainly a really interesting fossil," said Christopher Lowe, an evolutionary biologist at Stanford University who was not involved in the find. "It definitely helps give us a much deeper insight into what that ancestor may have been doing."

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Dr. Lowe has studied the genome of hemichordates to better understand how ancient groups of genes have been put to use in different ways by the various branches of the animal family tree. By comparing hemichordates with chordates, he hopes to shed light on the origins of some of the most basic elements of the human body plan, including the brain and nervous system.

He added that the emerging picture of Oesia at Marble Canyon suggests that genetic changes in support of mobility and a more active lifestyle were key ways in which chordates diverged from their ancient relatives during that period.

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