When paleontologists first laid eyes on the half-a-billion-year-old fossils of the worm Hallucigenia found in western Canada's famous Burgess Shale, they really didn't know what they were looking at — clearly. Originally, they thought the worm's head was its tail and that the spines along its back were its legs.

Since the discovery of this unearthly-looking worm in the 1970s, scientists have sorted out some of its anatomy, but uncertainty over which end is which remained. Now, in a paper published in the journal Nature, scientists have settled the question of heads-or-tails for the worm once and for all. Upon closer inspection of the fossils, they found teeth on one end.

"When we put the fossils in the electron microscope, we were initially hoping that we might find eyes, and were astonished when we also found the teeth smiling back at us!" said Jean-Bernard Caron, Curator of Invertebrate Palaeontology at the Royal Ontario Museum and associate professor in the Departments of Earth Sciences and Ecology & Evolutionary Biology at the University of Toronto, in a statement

The worm's "smile" is not as charming as Caron makes it sound — it is composed of needle-like teeth that line the creature's throat. However, they are informative for evolutionary biologists.

"These teeth resemble those we see in many early-moulting animals, suggesting that a tooth-lined throat was present in a common ancestor," continued Caron. 

Hallucigenia is an ancestor of a group of organisms called "velvet worms." Before this work, scientists thought that neither velvet worms nor their ancestors had teeth.

"But Hallucigenia tells us that actually, velvet worm ancestors had them, and living forms just lost their teeth over time," Caron said.

As bizarre as this 14-legged ancient worm is, it still isn't the odd one out among molting animals. The group is incredibly diverse and has few traits that unite them other than the fact that they all molt. As a result, reconstructing the group's early evolutionary history based on fossils has been particularly challenging. According to Smith, this history remains "pretty much uncharted."

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