New lizard fossil pushes lineage back 5 million years

A 250 million-year-old lizard-like fossil has been discovered in Devon. Scientists say it could be the earliest known member of the reptile group called lepidosaurs - pushing back their origins by around five million years. The find may offer new insights into reptile evolution. The research was led by Mike Denton at the University of Bristol, and I've been speaking with him…

Mike - This was a very important time in Earth’s history. There had just been an enormous mass extinction event a little beyond 250 million years ago that cleared out huge numbers of species and only about 5% survived. So during this time interval, which is called the Triassic, from 250 to 200 million years ago, life was recovering and evolving really fast. A lot of the modern groups originated at that time: the mammals, lizards it seems, based on the new evidence we’re presenting, and the dinosaurs, and all sorts of new groups were emerging.

Chris - What did life on land look like at that point then? What did these reptiles evolve from? What was their ancestor that would have come upstream of this specimen you’ve discovered?

Mike - Around the time of this beast, although it’s very small and would sit neatly on the palm of your hand, we’ve given it the rather large name of Agriodontosaurus, referring to its teeth. It would have been a pretty small cousin at the time, feeding on cockroaches and other insects in the undergrowth, probably pretty terrified of being eaten by some dinosaur ancestors. There were some quite active two-legged animals close to dinosaurs, but not quite. There were also the ancestors of mammals, which were already warm-blooded and pretty nifty hunters. So this earliest lizard-like creature would have lived in a bit of fear of all these other creatures of the day, but it was clearly doing something quite important because it then eventually led to such a huge and successful group.

Chris - What tells you it is a lizard then, and not one of those other erstwhile dinosaur progenitors?

Mike - It’s still four-legged. One thing lizards rarely do is go up on their hind limbs. It’s got a particular kind of skull, which has very definite features, particularly in the region behind the eye socket, and the nature of the teeth. This is a close relative, in fact, of a very unusual living lizard-like animal called the tuatara, which lives in New Zealand. The big mystery we had to resolve was what we would find with this very earliest representative of the whole lizard-like group. Would it be like the modern lizards and snakes, which have very mobile skulls and can throw their jaws open quite wide to capture their prey, which in many cases are quite big, juicy insects? Or would it be like the living tuatara, with a much more solid, less mobile kind of skull?

Chris - Where did this one turn up?

Mike - It was found on the beach in Devon, and one of our co-authors, Rob Coram, actually collected it a number of years ago. When he dug it up, he thought, my goodness, what’s this? Because normally in these red rocks, you don’t find complete skeletons, just isolated teeth and bits and pieces. He poked about with a needle and then realised that trying to clean it that way wouldn’t work. So, thank goodness, he got in touch, and then we discussed what to do. For this kind of fossil, which is concealed in the rock, we benefit enormously from CT scanning, which is effectively X-ray scanning. In the end, we had to take it to the synchrotron. This is one of the big physics machines. We scanned it in Grenoble and in Diamond at Oxford to give us the enormously high detail we needed, because it’s a tiny animal. The skull is about the size of your thumbnail. In order to see the teeth and all the tiny little bones which we needed to see in the skull, we had to get the highest magnification of X-rays that we possibly could.

Chris - Have you actually extracted it from the rock now, or are you going, in this publication, entirely from those synchrotron and CT images?

Mike - We wouldn’t dare try to get it out of the rock because we’d probably destroy it. The glory of X-ray CT imaging, particularly on the synchrotron, is you get so much detail. We were able to process this in the computer to clean it up in the sense of getting rid of little blobs and fragments of rock that were confusing the scan, because it depends on the density of the material. In the end, we were able to show from the scan a really beautiful three-dimensional skull, pretty complete, slightly squished. But when you look at the pictures, you can see these enormous choppers. So although it’s a tiny animal, it’s got just a limited number, about 15 really pretty big teeth, and it’s not a juvenile. This is a little adult, and it was probably feeding on cockroaches and other insects on the forest floor that were as big as its head or bigger. It would be like us biting a rabbit and eating it whole.

Chris - What does this tell us? How does this advance knowledge? And what’s next on your wishlist, apart from corroborating it, obviously, finding more examples, but what next do we need to do to firm this up?

Mike - What this tells us is the age of the origin of the whole group of modern lizards and the tuatara, 12,000 species, is pushed back about 10 million years earlier than had been thought. That has consequences for redating many of the other key evolutionary branching points, particularly in the early phases of evolution of the group. Secondly, it tells us that this ancestral form, currently the oldest member of the group, had a solid or firm skull without the kind of additional joints that we see in the modern lizard or snake skull, where they can flip the snout up a little and move the jaw joint back in special ways. It’s got these particular fixed teeth in the jaws. They’re fused to the bone rather than being somewhat loosely attached, as we see in many modern lizards. There’s also a structure behind the eyes, running back from the cheek area, called the lower temporal bar. That is missing, unexpectedly, in this early form because it had been predicted that this lower temporal bar was a primitive feature that was later lost in all the lizards. So it’s realigning things and giving us a lot of information on which we can reconstruct aspects of functional evolution of these early lizard-like animals.