A new fossil from Devon reveals what the oldest members of the lizard group looked like, and there are some surprises, according to a research team from the University of Bristol. The team teamed up with the ESRF to shed light on this exceptional 242-million-year-old fossil. The study is published in Nature.
Today, lizards and their relatives, such as snakes, together with the unique tuatara from New Zealand, are the most successful group of land vertebrates, with over 12,000 species – more than birds and more than mammals. But what is it about lizards, snakes and the tuatara, called collectively the Lepidosauria, that has made them so successful?
Surprising insights into lizard origins
It was always expected that the first lepidosaurs would have had some of the lizard characters such as a partially hinged skull, an open lower temporal bar, and abundant teeth on the roof of the mouth (palate). These are all features of modern lizards and snakes that enable them to manipulate large prey by opening their mouths super-wide (skull hinge) and use teeth on the palate to grasp wriggling small prey animals.
The lower temporal bar is essentially the cheek bone, a bony rod that runs between the cheek and the jaw hinge and is absent in lizards and snakes today. Snakes and many lizards have all these features, as well as some additional flexibility of the skull. Only the tuatara has a complete lower temporal bar, giving it an archaic look reminiscent of some of the earliest and ancestral reptiles; and it also has some large palatal teeth.
“The new fossil shows almost none of what we expected,” said Dan Marke, Master student in Palaeobiology at Bristol. “It has no teeth on the palate, and no sign of any hinging. It does, though, have the open temporal bar, so one out of three. Not only this, but it possesses some spectacularly large teeth compared to its closest relatives.”
Unlocking fossil secrets with synchrotron X-rays
“In modern palaeontological studies we often X-ray scan the fossils,” added Dr David Whiteside, from the University of Bristol. “But the exceptional resolution and quality of scans from synchrotron X-ray sources show us all the fine details and save any risk of damage.”
To study this tiny fossil from the Middle Triassic, the team came to the European Synchrotron (ESRF), to the new beamline BM18 dedicated to high sensitivity phase-contrast tomography in large and complex samples, where they teamed up with Vincent Fernandez, paleontologist at the ESRF. “This fossil is a good example of what makes a beamline like ESRF’s BM18 shine: the specimen itself is only a few centimetres in size, embedded in a large piece of rock, and it requires high-energy phase-contrast imaging — the flagship technique of BM18.” says Fernandez.
“When you look at the fossil, the whole skeleton sits in the palm of your hand,” explained Michael Benton, Professor of Vertebrate Paleontology in the School of Earth Sciences at the University of Bristol. “But after the scans and the hard work of our students cleaning up the scan data, we can see the most amazing detail. The new beast has relatively large triangular-shaped teeth and probably used these to pierce and shear the hard cuticles of its insect prey, pretty much as the tuatara does today.”
“The new animal is unlike anything yet discovered and has made us all think again about the evolution of the lizard, snakes and the tuatara,” said Dan Marke. “We had to give it a name to distinguish it from everything else, and we chose Agriodontosaurus helsbypetrae, quite a mouthful, meaning ‘fierce toothed lizard from the Helsby rock”, after the Helsby Sandstone Formation in which it was discovered.
Read more on ESRF website
