Hallucigenia Gets A Head (And A Whole Lot Weirder)

Hallucigenia is an iconic member of the Burgess Shale fauna. When its anatomy was announced, scientists quite literally laughed. It seemed an anatomical impossibility. It was only when it was examined with fresh eyes that it became apparent that initial reconstructions of the animal were topsy turvy. A row of tentacles along its back became legs and the spines which propped it up on the seabed became a means of defence against predators. Yet there remained a problem. We were still unable to tell from the fossils which were the head and tail ends of the animal.

A fossil of Hallucigenia from the Burgess Shale

Recent analysis, however, has allowed us to distinguish arses from apexes. 'Prior to our study there was still some uncertainty as to which end of the animal represented the head, and which the tail,' said Dr Martin Smith from the University of Cambridge. 'A large balloon-like orb at one end of the specimen was originally thought to be the head, but 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 as the animal was flattened during burial.'

Identifying the bulbous stain as demarcating the tail end, the researchers excavated the sediment surrounding the head and and subjected the newly prepared fossils to scans under an electron microscope. 'When we put the fossils in the electron microscope, we were initially hoping that we might find eyes, and were astonished when we also found the teeth smiling back at us!' said Jean Bernard Caron from the Royal Ontario Museum. These teeth formed a ring around the mouth and were most likely used to generate suction by flexing in and out of the mouth like a plunger. The animal's throat was also lined with needle-like teeth. These would have acted as a ratchet by catching on food items as they were drawn into the mouth by suction.

The teeth have helped pin down Hallucigenia's precise biological relationships. A 2014 study looking at its claws showed that it had strong links with the onychophorans or velvet worms to use their common name, alongside a wider relationships with the arthropod family. Overall Hallucigenia, the onychophorans and the arthropods, alongside a few other groups, comprise a clade of animals known as the ecdysozoans. These are all organisms which moult in order to grow. The teeth have helped identify Hallucigenia's place within the ecdysozoans.

An artist's impression of Hallucigenia based on the new fossil evidence

While it is not ancestral to all ecdysozoans, it is a certainly a primitive member of the group. 'These teeth resemble those we see in many early moulting animals, suggesting that a tooth-lined throat was present in a common ancestor,' said Caron. '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.'

'The early evolutionary history of this huge group is pretty much uncharted,' said Smith. 'While we know that the animals in this group are united by the fact that they moult, we haven't been able to find many physical characteristics that unite them.' What this study shows is that the first ecdysozoans were more anatomically complex than we gave them credit for; ring-like, plate-bearing worms with an armoured throat and a mouth surrounded by spines. Understanding the anatomy of the creatures involved will help us construct the precise sequence of events and divergences that comprise the Cambrian Explosion.