Danielle Dufault via AP
This image provided by Danielle Dufault shows a rendering of a Hallucigenia sparsa worm which lived 508 million years ago. Up to 2 inches long, it had legs, and spiky spines on its back. 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. But Martin Smith of Cambridge University and a Canadian colleague reveal its head in the Thursday, June 25, 2015 issue of the journal Nature.

NEW YORK — 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.

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