By Rachel K Wentz ( | First Posted: Jun 25, 2015 01:25 PM EDT

(Photo : Wikimedia)

For years, scientists have been baffled by a small fossilized creature that lived over 500 million years ago. But a new find has finally brought their investigations to a head - literally.

The creature is called Hallucigenia. It lived among the Cambrian seas and, for the last century, has provoked quiet contemplation among paleontologists, for the fossils of Hallucigenia were always missing their heads.

Scientists had a pretty clear understanding of Hallucigenia's body. This 2-cm creature was hair-thin and sported pairs of enormous spines down its back, with stick-like clawed appendages from its sides. The only thing missing was its head.

Dr. Martin Smith from the University of Cambridge summed it up nicely. "It looks completely surreal. It is like something from another world."

Dr. Smith was referring to the new complete version of Hallucigenia, which was finally revealed when its remains were found tucked within the famed Burgess Shale in Canada, a site that has produced a plethora of fossils from the Cambrian period.

But the head wasn't the only conundrum centered on Hallucigenia. The creature has challenged scientists from the start.

"It's had a pretty checkered history," said Smith, who was joined in the analysis of Hallucigenia by Dr. Jean-Bernard Caron, from the University of Toronto, Canada. Their paper appears this month in the journal Nature.

"When it was first formally described, it was the wrong way up. It was only recently that we found on which side were its feet and which side was its back. Even then there has still been a lot of ambiguity as to which end was the head and which end was the tail."

But the fossil from the Burgess Shale finally clarified the situation. Not only were the scientists able to decipher Hallucigenia's head, but they were also able to glimpse the inside of its mouth, which contained a ring of teeth, with another set running down the length of its throat. This strange apparatus most likely enabled Hallucigenia to suck up its food, then move it down into its stomach.

The scientists, however, were most struck by the creature's small face.

"When we put it into the electron microscope, we were delighted to see not just a tiny pair of eyes looking back at us, but also beneath them a really cheeky semi-circular smile" Smith says.

"It was as if the fossil was grinning at us at the secrets it had been hiding."

The discovery also settles another mystery surrounding a previous Hallucigenia fossil; a dark blob adjacent to the fossil, mistakenly believed to be its head.

"What our study shows is that it has a different composition from the animal. And rather than representing part of its body, it actually represents decay fluid - the contents of its guts - squeezed out as the animal was buried and fossilized."

The discovery of Hallucigenia's head, as well as its strange body, are typical of the Burgess Shale, which has produced fossils from an array of bizarre creatures that characterize the Cambrian explosion - a time when life on Earth exploded in a vast array of body plans. Among such exotic animals, Hallucigenia is right at home.

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