In the Canadian Rockies of British Columbia, Canada, there can be found a field of shale. It's 508 million years old, from the time before the dinosaurs, and contains fossils of some of the strangest creatures we have ever seen.
In fact, the Burgess Shale is believed to be the most significant fossil discovery in the world: the preservation of the organisms, both hard shells and soft tissue, is nothing short of extraordinary -- and it's one of the earliest bed containing soft-tissue fossils, as well as one of the most diverse: at least 184 species and 135 genera discovered in the rock have been described to date.
One of the most peculiar and iconic is a creature called Hallucigenia sparsa.
Hallucigenia sparsa is a type of worm, and much like earthworms, it is hard to know its head from its tail and its top from its bottom. In fact, when it was first described as a genus, it was reconstructed both back-to-front and upside-down.
This is not, however, because it is as featureless as the earthworm. The tube-shaped Hallucigenia -- typically measuring 10-50 millimetres in length -- had spikes extending from its back; in describing the animal in the 1970s, paleontologist Simon Conway Morris took the spikes for legs and the legs for tentacles.
As for the creature's head, no features could be positively identified at either end that would confirm its location.
Researchers from the University of Cambridge, the Royal Ontario Museum and the University of Toronto have now found this evidence: a pair of small, simple eyes and a ring of needle-like teeth lining Hallucigenia's throat.
The team was using electron microscopy to examine Hallucigenia specimens from the Royal Ontario Museum and the Smithsonian Institution in order to figure out head from tail once and for all. They started by examining a bulb found in the fossils that previous researchers had thought to be the animal's head.
"Prior to our study there was still some uncertainty as to which end of the animal represented the head, and which the tail," said lead author on the paper published today in the journal Nature, Dr. Martin Smith, a postdoctoral researcher in Cambridge's Department of Earth Sciences.
"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."
This discovery led the team to tentatively identify the bulb end as the animal's tail, sending them to examine the other end -- a floppy appendage that would often end up buried in the mud as mudslides killed the animal.
"This let us get the new images of the head," said Dr. Jean-Bernard Caron, curator of invertebrate palaeontology at the Royal Ontario Museum and associate professor in the Departments of Earth Sciences and Ecology & Evolutionary Biology at the University of Toronto.
"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!"
The teeth, the researchers said, probably helped generate suction, flexing in and out to suck its food into its mouth. Additionally, they would have helped to keep food from slipping out of Hallucigenia's mouth.
Hallucigenia is a precursor to velvet worms -- a relationship first proposed by Canadian paleontologist Desmond Collins of the University of Toronto in 2002 -- which in turn are closely related to arthoropods (invertebrates with exoskeletons, segmented bodies and jointed limbs) and tardigrades (aquatic segmented micro-animals, also known as waterbears). These animals all belong to a group called ecdysozoans, animals that moult.
"The early evolutionary history of this huge group is pretty much uncharted," Smith said. "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."
Although velvet worms do not have teeth, they do have claws on their limbs -- as do Hallucigenia on their own seven pairs of legs. Researchers at the University of Cambridge used this similarity to definitively link the two animals in 2014. As for Hallucigenia's teeth, they indicate that velvet worms -- or their ancestors -- used to have teeth.
"It turns out that the ancestors of moulting animals were much more anatomically advanced than we ever could have imagined: ring-like, plate-bearing worms with an armoured throat and a mouth surrounded by spines," said Caron. "We previously thought that neither velvet worms nor their ancestors had teeth. But Hallucigenia tells us that actually, velvet worm ancestors had them, and living forms just lost their teeth over time."