For decades, scientists have been trying to piece together the anatomy of a tiny prehistoric worm so weird-looking that they named it Hallucigenia.

The stiff spikes on the long-extinct critter's back were long thought to be legs, while seven pairs of spindly limbs were mistaken for tentacles.

On Wednesday a research duo said the creature had not only been reconstructed upside down, but also back to front -- they found a pair of eyes and a toothy mouth in what was long thought to be its backside.

"Prior to our study, a large balloon-like orb at one end of the specimen had been interpreted as an amorphous head," said Martin Smith of the University of Cambridge, who co-authored a study published in the journal Nature.

"We can now demonstrate that this actually wasn't part of the body at all but a dark stain representing decay fluids or gut contents that oozed out of the anus as the animal was compressed during burial," he told AFP by email.

Smith and colleague Jean-Bernard Caron of the University of Toronto, used an electron microscope to analyse dozens of Hallucigenia fossils in museum collections, and uncovered "astounding new detail" of the worm that lived on the sea floor some 505-515 million years ago.

After identifying the worm's derriere, they decided to take a closer look at the other end -- removing the sediment covering several of the fossilised heads.

"When we put the fossils in the electron microscope, we were initially hoping that we might find eyes," said Smith.

"We were astonished when we found not just a pair of eyes, but also a cheeky grin -- a set of teeth smiling back at us!"

- Evolutionary clue -

The team's analysis showed that Hallucigenia's mouth was surrounded by a ring of spiny teeth, probably used to suck up food, while the throat was lined with a row of needle-like teeth which possibly prevented its lunch slipping back out.

At a mere one to five centimetres (0.4 to two inches) long, the armoured worm lived during a period of Earth's history called the Cambrian Explosion, when most major animal groups emerged.

First identified in the 1970s, Hallucigenia's closest living relative is the toothless velvet worm, which in turn belongs to a vast family known as ecdysozoa -- animals like many insects and worms, lobsters and spiders which shed their exoskeleton.

Hallucigenia's newly discovered choppers has led Smith and Caron to conclude that the ancestor of ecdysozoa must also have had a toothy mouth and throat.

"If so, this is very exciting as it helps us to constrain the timing of the origin of the group," said Smith.

https://youtu.be/Gny9SxByOw8 It seemed to indicate that all ecdysozoan sub-groups split off "within a geologically brief 20-million-per period of rapid evolution, providing evidence for a rapid 'Cambrian Explosion'," he said.

A reconstruction of Hallucigenia's walking gait can be seen here: https://youtu.be/Gny9SxByOw8
Source: AFP