Hallucigenia fossil unravelled

510 million years ago, in a period of time called the cambrian the world was suddenly abuzz with diverse animal life, for the first time in history. Huge varieties of body plans started to appear, after millions of years of animals no more complicated than seaweed. This period of time is key to understanding early animal evolution, and one of the windows we have to this time period is through a fossil bed called the burgess shale. One of the most famous fossils discovered here is a strange alien-like creature called Hallucigenia, and it has always been something of a mystery to palaeontologists. This week, research from Cambridge University has flipped our understanding of the animal on its head, quite literally. Georgia Mills went to visit Dr Martin Smith, a palaeontologist from the department of earth sciences at Cambridge University to meet Hallucigenia and hear about its many secrets...

Martin - Hallucigenia was a sea creature that would have lived in reasonably deep waters and would have been surrounded by a vast range of diverse and weird creatures. So, there are things like Anomalocaris which looks like a cross between a lobster and can opener, Opabinia that looks like a shrimp that's swallowed a vacuum cleaner, there are sort of squid-like things, there are fish-like things, there are sponges, there are seaweeds. In stark contrast to the seas of 20 million years earlier where there was basically nothing much more complex than seaweeds.

Georgia - And I see you have a little model of Hallucigenia here. Is this it?

Martin - That's right. So, we've got a small model of Hallucigenia. This is a cast of one of the fossils that we've been working with. It's about a centimetre long or so. The fossil looks a bit like a hockey stick, but it's thin as a pin. It's really tiny. You can just see 7 pairs of long spines sticking out of its back. It's got long, slender tentacle-like legs dangling from the other side of its body.

Georgia - Looking at this, I think if I found this, I would think it was just a sort of scratch. It looks nothing like an animal. What do people make of this when they first discovered it?

Martin - When it was first discovered, I think it was sort of put to the back of the drawer as, "What on Earth is this?" It was formally described in the 1970s quite a bit later. At that point again, it was very hard to know what to make of it. So, we could see both of the spines in each pair, but one side of the legs were actually still buried in the mud. And so, the paleontologist describing the creature didn't recognise this and thought, "Okay, we've got pairs of spines, but then we've got one dangly thing associated with them." It must have been walking on the spines. The spines must've been its legs and who knows what this sort of dangly thing was. Maybe it was a tentacle passing foods to the head or something like that. People say, "Well, you can't walk on spines. They're stiff. They don't even bend." And so, someone built a robot to say that you could walk on stiff pointy spines. I think it would still get stuck in the mud somehow. But it's a really bizarre reconstruction of an animal. I think that's what led to its name, Hallucigenia. It's sort of dream-like and other worldly appearance. It was the 1990s when we realised that actually, Hallucigenia was the wrong way up.

Georgia - And now, what have you discovered?

Martin - When we went back to Hallucigenia, we knew which way up it was. We still didn't really know which end was the head and which end was the tail. Royal Ontario Museum crews have collected over the last 40 years dozens of new specimens. So, we figured it was time to redescribe the animal from the ground up. So, the first thing we did is we went looking at this big bulbous, orb-like structure that's at one end of the body. And we said, "Well, is this really the head? It looks a little bit weird. It's a bit inconsistent." We can actually show that it's got a different composition to the body tissue. So, it's not actually part of the animal at all. What we think it represents is decay fluids or gut contents that were squeezed out of the end of the organism as it was buried. So, that's not the head at all. So we thought, "Okay, well what's going on at the other end? Where is the actual head?" And so, my colleague, John Bernard, dug away at the rock that was covering that end of the organism and we saw that this is probably the head. It would be nice to prove it for sure. The couple of specimens we looked at the microscope, we could see something that looked a little bit like eyes. But because it's so tiny (it's way less than a millimetre across), we couldn't really see any detail. So, the next thing we did was we said, "We've got to put these in an electron microscope" and that lets us get down to thousands of a millimetre in resolution. And so, we put the fossils in the electron microscope and lo and behold, we saw not just a pair of eyes looking back at us, but underneath in there is this enormous sort of grin almost. There a little circle of teeth below the eyes. This was completely unexpected. We know that Hallucigenia's closest modern relatives are animals called the velvet worms. They live in rainforest. They've got very simple mouths - no teeth at all. And so, to find teeth not just around the mouth of Hallucigenia but also in the throat, we found in the throat of Hallucigenia, there's lots of little needle-like teeth. This was a complete surprise to us.

Georgia - Hallucigenia lived 500 million years ago. Why should we care which way around it goes?

Martin - Hallucigenia is really weird and it's really unexpected. The point is that it's telling us something that we'd never know just looking at the animals that we can see today. It's only through looking at fossils like Hallucigenia that we can understand how the modern groups of animals are related to one another and how they evolved in the first place, and how they became complex and how quickly. So really, Hallucigenia was a unique window into this really early and exciting period of evolution of life on Earth.

Georgia - So now we've got Hallucigenia - well first in '90s, it was put the right way up and now, you know which way around it goes. Is this it for Hallucigenia? Have we sold it or is there anything more to find out?

Martin - Well, I think it's fundamentally solved at least. We kind of know what it was doing, but I definitely wouldn't say there's nothing more we'll find out. Hallucigenia is full of surprises and almost each time we go back to the fossil, we find something new. So, who knows what's to be next. One thing that's quite exciting is a lot of Cambrian fossils we're finding have nervous tissue in. so who knows? Maybe we'll find new fossils which show Hallucigenia's brain and its nerve cord and that could tell us something really fundamentally new again about how it's related to modern groups and the origin of modern animal diversity.