Colour reconstruction of Hallucigenia sparsa, a very weird, very ancient animal. (Danielle Dufault)

Hallucigenia is aptly named: With long spines down its back, seven pairs of clawed legs, and six tentacles gracing its neck, the 500 million year old ocean-floor dweller would have looked like something from a very weird dream. But decades after its discovery, researchers have finally put together the jigsaw puzzle of strange features to show what Hallucigenia might have looked like. And because of where it sits on the tree of life, it provides some major clues as to how the common ancestor of all arthropods -- everything from shrimps to spiders to butterflies -- might have looked.

Several months ago, researchers announced that they'd finally found enough evidence to place Hallucigenia, a worm with legs and spikes that lived during the so-called Cambrian Explosion, on the family tree of life. The strange new critter's legs showed features quite similar to those in jaws of modern velvet worms. In a new study published Wednesday in Nature, the same researchers report that they've gotten the first good look at Hallucigenia's face, providing them with even more clues.

[Hallucigenia: An ancient ‘misfit’ finally finds a family]

As an ancient relative of the velvet worm, Hallucigenia is a very early member of the group Ecdysozoa, which includes all molting animals. Because that group is so diverse — it includes everything from tiny water bears to delicious lobsters — you might expect their common ancestor to be very simple, allowing for each species to evolve strikingly different new features.

But Hallucigenia, though quite old, is also very complex: Once researchers figured out where its head was, they found it had more advanced features than they'd realized.

"When it was formally described originally, it was actually upside down, with the spikes being mistaken for legs and the legs being mistaken for tentacles on the back," lead study author Martin Smith of the University of Cambridge told The Post. "Even once we got the right way up, there was a lot of uncertainty about which end was the head and which was the tail."

[Ancient filter-feeding giants hint at the evolution of today’s insects]

Some researchers thought a bloated orb at one end must be the head, but Smith and his colleagues showed that the composition was all wrong for a head -- that bloat was actually some kind of remnant of the animal's decomposition. So it was time to take a closer look at the other end.

[Ancient brain fossils hint at the first evolution of heads]

"We were very optimistic, we thought we might see a pair of eyes," Smith said. "But we didn't just see eyes. We saw a cheeky little grin, which was a complete surprise."


Hallucigenia sparsa from the Burgess Shale. The fossil is 15 mm long. (Jean-Bernard Caron)

Hallucigenia has a very special smile, indeed. The worm's mouth had a ring of teeth, but it also had a tooth-lined throat. Smith believes the ring of teeth at the mouth would be used to provide suction, while the needle-like teeth inside the throat would catch prey in place to keep it from sliding back out mid-slurp.

Velvet worms don't have teeth like these. But other creatures from the Cambrian period -- like penis worms -- had them too. The researchers suggest that this could mean the ultimate ancestor of all Ecdysozoa had traits like this, and that the ancestors of modern arthropods had to shed them before evolving their own distinct traits.

[Now there’s a guide to penis worm teeth]

"These teeth resemble those we see in many early moulting animals, suggesting that a tooth-lined throat was present in a common ancestor," study author Jean-Bernard Caron, Curator of Invertebrate Palaeontology at the Royal Ontario Museum, said in a statement. "So where previously there was little reason to think that arthropod mouths had much in common with the mouths of animals such as penis worms, Hallucigenia tells us that arthropods and velvet worms did ancestrally have round-the-mouth plates and down-the-throat teeth - they just lost or simplified them later."

Now, Smith said, he'll be looking more closely at fossils that match the proposed features of a common ancestor: A tiny worm with a ring of fine teeth, and a throat studded with more of them. If his group's findings are correct, they may be able to track down the ugly little worm we can thank for shrimp cocktail.

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