Some of the weirdest fossils on record come from the Cambrian period, which occurred half a billion years ago. But even with that in mind, the aptly named creature Hallucigenia is in a class all its own when it comes to pure, unmitigated eccentricity.
These wormlike animals ranged from about 10 to 50 millimeters in length and were decked out with an impressive set of dorsal spines, neck tentacles, and flexible claw-like legs. They were such crazy jumbles of oddities that paleontologists have reconstructed them in all kinds of different orientations, with no clear idea which side represented the animal's head or whether its characteristic spines were on its underbelly or its back.
But fear not, Hallucigenia fans, because new research suggests that the debate over this eccentric creature's body plan and evolutionary history is finally coming into sharper focus.
In a study published today in Nature, evolutionary biologist Martin Smith and paleontologist Jean-Bernard Caron definitively identify the head of the animal for the first time. On top of that, the team discovered that Hallucigenia possessed an idiosyncratic double set of teeth, which may, at last, clarify its relationship to its extant relatives.
"We've revealed these two sets of mouthparts," Smith, the study's lead author, told me over the phone. "We've got the teeth that go around the mouth, and then we've got sort of these needle-like teeth running around the throat."
Martin Smith summarizes his new study in this Nature video. Credit: YouTube/nature video
"What I think is going on is that the teeth around the mouth sort of operate a bit like a valve, essentially creating suction," he continued. "Each time it flips in and out, it's bringing in another suck of water, and whatever is within it, down the throat. The teeth in the throat are presumably to stop food slipping out each time the mouthpart flips outwards, so it's able to keep channeling things towards the gut."
Though this seems to clear up the mechanism behind Hallucigenia's feeding, there is still a lot of speculation about what the animal actually ate. "It's often depicted clinging on to sponges," Smith noted. "It's got these really long and flimsy legs that look a little bit too floppy to imagine it walking. Maybe it could, but you can sort of imagine it cuddling a sponge, and clinging on with its claws, sucking away bits and digesting it. That's an appealing reconstruction."
"We've also observed these three very narrow pairs of tentacles that come off the neck," he added. "It's hard to imagine what they were used for, but potentially they were involved in food processing or manipulating particles that they came across, and getting those towards the mouth, so there's a possibility of sort of scavenging or grazing with those as well."
When it comes to the function of the animal's distinctive spikes, however, the answer is somewhat less open to speculation. "I think they've got to be defensive," Smith told me. "You look at Hallucigenia and it's all spikes really, isn't it? There's lots of [predator] candidates preserved in the [Cambrian period] Burgess Shale, like Anomalocaris, which grew up to a meter long, and looks sort of like a lobster crossed with a can-opener. There's a huge range of things that were likely going around eating the less armored relatives of Hallucigenia."
Of course, given the sheer weirdness of the rest of Hallucigenia's body, it should come as no shock that its newly discovered feeding mechanisms and defensive spikes seem positively Lovecraftian. But Smith and Caron's new study has implications far beyond fleshing out this elusive creature's features, given that Hallucigenia was also an early progenitor of the superphylum Ecdysozoa.
This massive group of moulting animals includes tardigrades, velvet worms, crustaceans, insects, arachnids, nematodes, and a wide diversity of other creatures that moult—or, in other words, discard their exoskeletons as they mature. Despite the incredibly influential role ecdysozoans have played in the history of life on Earth, very little is known about the group's early evolutionary history. Hallucigenia may be key to unlocking these murky family roots.
"What's really interesting about Hallucigenia is that these new features we found at the mouthparts are giving us a new picture of what we think the ancestor of all moulting animals looked like," Smith said.
"This ring of teeth around the mouth and the teeth along the throat is something we also see in a group of ecdysozoan worms," he noted. "Finding a comparable mouthpart in Hallucigenia suggests that this morphology was ancestral to the whole of moulting animals, which made up the majority of animal diversity at the time, as well as today, in the oceans."
This entire story is made all the more fascinating considering how rare Cambrian fossils are in the first place—especially in the case of invertebrate animals like Hallucigenia. "Complete Hallucigenia fossils are incredibly rare," Smith said. "Conventional fossilization just won't preserve them at all, so it's really fortunate to have this direct window into the early Cambrian period."
It seems that slowly but surely, scientists like Smith and Caron are closing in on the mysteries surrounding this spectacularly unusual animal. Hallucigenia is named for its ability to make people doubt their own eyes, but as this new study demonstrates, this group of Cambrian weirdos was not only very real, but its extant relatives still dominate our planet to this day.