Hallucigenia sparsa, one of most common creatures 500m years ago, had previously baffled scientists as an anatomical misfit without distinguishable head or rear

Nature has produced many oddities, but an ancient creature resembling a prickly sea worm is one of the few to have left scientists so baffled they were unable to distinguish its head from its the rear.

The organism, called hallucigenia sparsa, was once one of the world’s most common creatures, but its unearthly appearance has led it to be regarded as an evolutionary misfit - not least because this basic anatomical question has remained unresolved.

Now the discovery of a pair of simple eyes and a ring of needle-like teeth, has finally confirmed which way around the animal faced.

Martin Smith, who led the work at the University of Cambridge, said: “Finding the head is the main scientific result. There’s been lingering controversy about this.”

Facebook Twitter Pinterest Hallucigenia sparsa - the worm that didn’t appear to have a head

The hallucigenia, which was around 35mm long, lived in the oceans around 505m years ago during the Cambrian explosion when most major animal groups first appear in the fossil record.

Early descriptions in the 1970s suggested that it walked on its spikes with a row of waving tentacles on its back. However, scientists later concluded that they had the creature upside down and it is now clear that they also had front and back confused.

“At the tail of the animal there’s a large dark balloon-like orb, which was originally thought to be the head,” said Smith.

After examining dozens of specimens using a powerful electron microscope, however, the scientists identified a pair of basic eyes and teeth at the opposite end. They were also able to confirm what the orb-like structure represented.

“The simple answer is poo,” said Smith. “More technically, it’s a sort of soup of the inside of the animal that’s been squeezed out of the point of least resistance as it was squashed under layers of mud.”

In a commentary accompanying the paper, Xiaoya Ma, of Yunnan University, in China, welcomed the findigns saying they “once and for all clarify the anterior–posterior orientation of hallucigenia”.

The discoveries help shed light on the lifestyle of the primitive creature at a time when lifeforms on Earth were rapidly evolving towards the more complex versions that exist today. Hallucigenia would have had basic visual awareness of light and dark. “They’d probably be able to make out night versus day, but wouldn’t be able to see predators looming up at 400 paces,” said Smith.

Facebook Twitter Pinterest Hallucigenia sparsa fossil from the Burgess Shale. Photograph: Martin R Smith

The ring of spiny teeth, pointing backwards down the throat, would probably have been used to “shepherd” food such as plankton towards the stomach after it was sucked in at the mouth.

The discovery of teeth was a surprise because the animal’s closest modern relative, the velvet worm, is toothless suggesting that teeth appeared and then were lost again.

“We were not expecting at all to see the teeth smiling back at us under the microscope,” said Smith.

The findings also help place hallucigenia, once considered too strange to have any living relatives, more confidently on the evolutionary tree. Scientists now believe it belonged to a massive group of animals, called the ecdysozoans, which includes arthropods (insects and spiders), velvet worms and water bears (tardigrades).

“The early evolutionary history of this huge group is pretty much uncharted,” said Smith. “While we know that the animals in this group are united by the fact that they moult, we haven’t been able to find many physical characteristics that unite them.”

The latest work, published in Nature, will help scientists develop a clearer picture of what the ancient common ancestor to all these animals might have looked like.