In addition to learning more about Oesia, its habitat, and filter-feeding habits, the fossils also help scientists confirm that the worms were part of a group called hemichordates, which in turn are part of an even bigger group – deuterostomes, a branch of which is taken up by vertebrates such as humans.
"Oesia fossils are pretty enigmatic. They are very rare, and until now we could not prove which group they belonged to," said Simon Conway Morris, of the University of Cambridge. "Now we know that they were primitive hemichordates –- perhaps the most primitive of all."
Identifying the animal in this way could lead to a greater understanding of our ancestry.
"Hemichordates are central to our understanding of how deuterostomes evolved," added the study's lead author Karma Nanglu, of the University of Toronto. "Through them, we can get clues about the anatomy and lifestyle of the last common ancestor that we all share, and this adds further evidence to the hypothesis that the ancestor was a filter-feeder like Oesia."
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The researchers say that the at some point in fossil history the worms left their tubes and instead took to living underneath the sea floor. They suggest that as more predators evolved over time, digging underground must have become a more appealing life strategy. Today's acorn worms did just that, the scientists note, putting their filter-feeding ways behind them and dining instead on nutrients in the sediment.
As Conway Morris explained, the creatures knew they needed better protection. "One way of doing this," he said, "was to abandon life filter feeding in a tube, and instead to dig into the sediment and eat mud. Once there, they found a new niche and were able to make a perfectly good life for themselves."