The first Hallucigenia fossil was discovered more than 100 years ago, and scientists could not figure out its shape. One of the most bizarre-looking fossils, Hallucigenia was a worm-like creature with legs, spikes and a head difficult to distinguish from its tail.

Researchers have now confirmed after laborious study as to which side of Hallucigenia was the head -- and found its circular "grinning" mouth lined with teeth.

The animal had been considered an "evolutionary misfit" as it was not clear how it was related to modern animal groups. The creature was less than two-cm-long and thinner than a hair.