Shape of Hallucigenia discovered: Study

In a first, the head of an ancient worm Hallucigenia, a species that baffled scientists for decades because of its complex shape, has been found after painstaking research.

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.

"I think revisiting those worms and looking back in detail about how their mouths are organised could tell us exciting new things about how the moulting animals as a whole have evolved," co-author Martin Smith, a researcher of paleontology and evolution at the University of Cambridge was quoted as saying in Livescience.

Hallucigenia were tiny marine worms -- commonly just 15 mm in length -- that lived in the Cambrian period, when complex, multicellular life was starting to populate the Earth.

The team studied hundreds of fossils of Hallucigenia sparsa, which lived 508 million years ago.

"We saw not just a pair of eyes looking back at us, but underneath was this marvellous grin," Smith said.

The study was outlined in the journal Nature.