Heads or Tails? Scientists Decipher Which End is Which on Ancient Worm

Image: A rendering of a Hallucigenia sparsa worm
A rendering of a Hallucigenia sparsa worm, which lived 508 million years ago. Scientists first described the creature in 1977, and for 15 years they thought those spines were legs. The question of which end was the front has lingered even longer. Danielle Dufault via AP

A bizarre-looking fossil worm that's been a puzzle for scientists has given up a secret: Researchers now know which end is which. The giveaway was finding evidence of eyes and teeth in the black traces of carbon the creature left behind in ancient rock.

The worm, called Hallucigenia sparsa (huh-loo-sih-JEE'-nee-uh SPAHR'-suh), lived 508 million years ago. It was up to 2 inches long. It had legs, and spiky spines sticking out of its back. Until the 1990s, scientists thought those spines were legs. So their view of the animal was upside-down. Some researchers also falsely identified a head at what turned out to be the tail end.

But scientists said on Wednesday a new analysis of fossils of Hallucigenia, so named for its fantastical appearance, has given them for the first time a complete understanding of this little sea oddball.

Hallucigenia is one of the species emblematic of the Cambrian Period, a pivotal juncture in the history of life on Earth when most major groups of animals first appeared and many unusual body designs came and went.

"It is nice to finally know rather fundamental things such as how many legs it has, and to know its head from its tail," University of Cambridge paleontologist Martin Smith said.

The actual head, the scientists have now learned after using sophisticated imaging techniques on the fossils, is at the end of a long and slender neck resembling a hose-like tube. Near the end of the head were two bean-shaped eyes.

"Below the eyes, like an almighty grin, sits a ring of teeth," Smith said.

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There was a small mouth cavity in front of these teeth to suck food into the throat. Once there, the food was gripped by tiny needle-like teeth lining the throat to speed it to the stomach.

"It would have been quiet a sight," said Royal Ontario Museum paleontologist Jean-Bernard Caron.

Researchers report their new results in a paper released Wednesday by the journal Nature.