Nature

In 1909, paleontologist Charles Doolittle Walcott discovered a trove of fossils in the Canadian Rockies. Among them was a particularly puzzling piece—a many-tentacled sea creature, dating back around 508 million years, that didn't seem to have a clear front or back, or top or bottom.

Nature

The Smithsonian National Museum of Natural History offers this description:

"When discovered, it was thought that the animal walked on spiny stilts with tentacles along its back that served as feeding aids. It was impossible to determine which end was the head and which the tail."

Decades later, in 1977, paleontologist Simon Conway Morris described the creature and called it Hallucigenia for its "bizarre and dream-like appearance," (and also, presumably, because it was the seventies).

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Since then, scientists have struggled to make sense of how the animal functioned, and what it looked like. National Geographic explains:

"Seven pairs of long spines stuck out in one direction, while seven pairs of tentacles stuck out in the other, with what looked like little mouths on their tips. There was nothing distinct at one end, and a non-descript blob ballooning from the other…Conway-Morris guessed that the creature used its spines like stilts for walking along the ocean floor, while the tentacles plucked food out of the overlying water. Then, in 1991, a set of related Chinese fossils showed that Conway-Morris’ reconstruction was upside-down. The 'tentacles' were legs. The 'mouths' on their tips were claws. And the 'stilts' were spines that stuck out of the creature’s back, probably for defence."

So it took from 1901 to 1991 to figure out how the animal might have walked—but scientists still weren't clear on which end contained the animal's head. University of Cambridge's Dr. Martin Smith points out that for a long time, scientists assumed that an amorphous, fossilized blob could have been its head:

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But he and fellow researcher Toronto's  Jean-Bernard Caron don't think that blob represents the animal's head. Instead, they explain in a paper published in Nature this week, they think it shows its excreted guts. Or, in Smith's words, "rather than representing part of its body, it actually represents decay fluid—the contents of its guts—squeezed out as the animal was buried and fossilised."

Smith described his findings in a video:

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That means the creature's head is on the other side. New fossils show a slim, grinning head with many teeth. Per the BBC:

"Inside the creature's mouth, the researchers found a ring of teeth and then another set of teeth running from its throat down towards its stomach… The scientists believed Hallucigenia used its odd mouth arrangement to suck up food, and then move it down to its gut."

Which then, we guess, was removed from its insides and placed beside it to fossilize for hundreds of millions of years. When Hallucigenia was alive, it probably looked something like this:

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Pretty cute, until you remember it has teeth lining the inside of its tiny, worm-like body.

Danielle Wiener-Bronner is a news reporter.