A prehistoric worm-like creature thought to be an early common ancestor of insects and spiders had strange needle shaped teeth and beady little eyes.

The bizarre organism, called Hallucigenia sparsa, is so strange that it has baffled palaeontologists for more than 100 years since it was first discovered.

New research, however, has finally revealed the creature’s head for the first time and helped researchers reconstruct how it would have looked.

Hallucigenia sparsa lived in the oceans around 505 million years ago. Fossils of the creature were first discovered 100 years ago, but new research has finally revealed what the animal would have looked like, as shown in the reconstruction above. It had a pair of simple eyes and a mouth filled with rings of teeth

Once one of the world’s most numerous creatures, Hallucingenia was found to have a pair of simple eyes and a mouth filled with plates and ring of sharp teeth, giving it a strange grinning appearance.

Scientists believe it will have walked on long flexible legs while its back bristled with pairs of spines running down its length.

AN EXPLOSION OF LIFE ON EARTH The Cambrian explosion, or Cambrian radiation, was the relatively rapid appearance, around 530 million years ago, of most major animal phyla, as demonstrated in the fossil record. Before about 580 million years ago, most organisms were simple, composed of individual cells occasionally organized into colonies. Over the following 70 or 80 million years, the rate of evolution accelerated by an order of magnitude (as defined in terms of the extinction and origination rate of species) and the diversity of life began to resemble that of today. It ended with the Cambrian-Ordovician extinction event, approximately 488 million years ago. Advertisement

The researchers say the animal’s mouth is similar to those seen in nematode worms but also indicates Hallucingenia was a common ancestor of athropods – a taxonomic group that includes insects, arachnids and crustaceans like prawns.

Dr Martin Smith, a palaeontologist at the University of Cambridge who led the work, said: ‘Hallucingenia is named for its dream like appearance and scientists have found it difficult to know what to do with this unusual spiny worm.

‘It had pairs of stiff spines running along one side of the organism and long flexible appendages on the other side.

‘When it was described in the 1970s it was interpreted as warlking along these stiff spines as if it was walking along pairs of stilts.

‘It is quite a weird reconstruction and a very bizarre animal and not something we have any modern analogue of today.

‘But we know now it walked the other way up. It looks a bit like a caterpillar with long flimsy extensible legs and pairs of conical spines running along its back.

‘It looks a bit like a twig with loads of sticks coming out if it.’

Hallucigenia grew to be only around one centimetre long but was once one of the most numerous creatures on the planet. Above is one of the fossils studied by researchers, in a piece of Burgess shale. The head can be seen on the right, but scientists had for a long time thought this was the animal's tail

Hallucigenia grew to be around 1cm long and is thought to have lived in the oceans around 505 million years ago during the Cambrian explosion.

This is when there was a sudden burst of evolution and animal life emerged in large numbers in many bizarre forms.

Dr Smith said Hallucigenia is one of the most bizarre creatures to have been found from the period.

Fossils of the creature suggested it had a large bulbous head and a long tail, but Dr Smith and his colleagues, whose work is published in the journal Nature, examined the fossils in a microscope.

They found what appeared to be the bulbous head was in fact staining caused by material from the creature’s gut being squeezed out as it was fossilised.

This reconstruction of Hallucigenia sparsa was only possible after scientists discovered the creature's head by examining fossils under a microscope that revealed its two simple eyes and strange grinning mouth

The bulbous section on the left of this Hallucigenia fossil was what had widely been assumed to be the animals head, but in fact turns out to be a stain caused by material squeezed from the creature's gut while it was fossilised. The researchers found the animal's head was at the opposite end

It suggested scientists had been examining the fossil the wrong way around.

When his team looked at the other end of the creature they discovered a pair of simple eyes and teeth.

Dr Smith said: ‘When we put it in the microscope to where the head might be we saw not only the eyes but this almighty grin, this large semi-circular smile.’

They say the findings may now help to shed light on the primitive creatures at the time and unravel how some of the earliest arthropods evolved.

With its basic eyes, Hallucigenia would probably have been able to detect light and dark, but would not have been able to focus or see fine details.

The ring of spiny teeth, which point backwards down the throat, were also probably used to pass food such as plankton towards the stomach.