Scientists Finally Decide Which Bit of This Weird Animal is the Head

In 1977, British palaeontologist Simon Conway-Morris discovered the fossil of a truly weird animal, which he named Hallucigenia because of its “bizarre and dream-like quality”. He wasn’t kidding. The creature was so strange that it took fourteen years for scientists to work out which way up it stood. And now, nearly four decades after its original discovery, we finally know—clearly and conclusively—which end is the head.

Hallucigenia lived around 508 million years ago, and it was first unearthed in a famous Canadian fossil field called the Burgess Shale. When Conway-Morris set his eyes upon it, he saw a long tubular body, between 1 and 5 centimetres long. Seven pairs of long spines stuck out in one direction, while seven pairs of tentacles stuck out in the other, with what looked like little mouths on their tips. There was nothing distinct at one end, and a non-descript blob ballooning from the other. Where was the head? Or the back?

Conway-Morris guessed that the creature used its spines like stilts for walking along the ocean floor, while the tentacles plucked food out of the overlying water. Then, in 1991, a set of related Chinese fossils showed that Conway-Morris’ reconstruction was upside-down. The “tentacles” were legs. The “mouths” on their tips were claws. And the “stilts” were spines that stuck out of the creature’s back, probably for defence.

When I wrote about Hallucigenia last year, I said, “Even now, no one’s entirely sure about the head.” Some thought that the blobby balloon was rather head-like, but others said that it represented bodily fluids, which had leaked out the animal’s backside after its death.

Now Martin Smith and Jean-Bernard Caron have finally solved that mystery. They looked at new well-preserved specimens, and also re-analysed the existing ones with the latest microscopes. “It was about time we took some modern techniques to see what secrets it had,” says Smith.

Their analysis revealed that the head is the end opposite the blobby balloon. For a start, it has eyes. They appear as a pair of dark spots, which “really pop out under an electron microscope,” says Smith. These spots probably represent the visual pigments that were part of the animal’s original eyes. Since they appear as solid masses rather than spotty grids, they imply that Hallucigenia had simple eyes rather than the compound ones of insects.

The end with the eyes also had a mouth—again, clearly the head. There was a hole with a ring of spines around it, and further back, a set of needle-like teeth in the throat. That sounds like a fearsome set-up but there’s nothing predatory about Hallucigenia. Instead, Smith thinks that it used the spines to make its mouth rigid so it could suck up its food, while the throat teeth helped to ratchet its mouthfuls towards its stomach. (In some specimens, Smith and Caron could even make our the creature’s entire gut.)

Behind the head, Hallucigenia had 10 pairs of limbs. The first three were slender, short, and clawless. Who knows what they were for? They could have been sensory organs, like antennae. They might have been covered in feathery growths for filtering plankton from the water. They might even have acted as tentacles. “You can imagine them cuddling a sponge,” says Smith.

The last seven pairs of limbs are clearly legs, though, and “in many specimens, it was clear that the body ends with a pair of legs,” says Smith. The legs had no joints, so Hallucigenia probably controlled them by changing the pressure of the fluid within, much like starfish and sea urchins do today. Alternatively, the large spines on the back might have acted as anchor points for large muscles, which powered into the legs. After all, there were seven pairs of spines and seven pairs of legs. Perhaps this extinct weirdo was a walking suspension bridge.

These discoveries are relevant to more than just Hallucigenia. The animal was most closely related today’s velvet worms—glue-spitting, colourful, multi-legged creatures. That would make them part of the ecdysozoans—a super-successful group of animals that includes insects, spiders and scorpions, crustaceans, centipedes and millipedes, nematode worms, velvet worms, and lesser known groups like the invincible tardigrades and the unfortunately named penis worms.

The mouths of these creatures are extremely variable. The velvet worms and arthropods basically have simple holes with unarmed throats. The penis worms, however, have mouths that are surrounded by spines and throats that are covered with teeth—which are uncannily close to what Smith and Caron saw in Hallucigenia. This suggests that the ancestor of the ecdysozoans also had similarly complex mouthparts, which were then lost in some lineages like the velvet worms.

Hallucigenia might have more to say about the ancestry of this super-group, but for now, Smith is content about finally knowing what it looks like. “I think it’s pretty unambiguous what we’re looking at,” he says. “I don’t think we’re going to turn it over again or back to front.”

Reference: Smith & Caron. 2015. Hallucigenia’s head and the pharyngeal armature of early ecdysozoans. Nature http://dx.doi.org/10.1038/nature14573