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Scottish fossil sheds light on origins of lizards


Scientists led by the University of Warsaw, London and Oxford have found modern features in an almost complete fossil lizard from the time of dinosaurs, thanks to X-ray tomography studies at the ESRF. Their research is published in Nature.

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The precious fossil is a tiny skeleton from the Isle of Skye, called Bellairsia gracilis, is only 6 cm long and dates from the Middle Jurassic, 166 million years ago. It is the most complete fossil lizard of this age anywhere in the world. First author Mateusz Tałanda of the University of Warsaw says: “This little fossil lets us see evolution in action. In palaeontology you rarely have occasion to work with such complete, well-preserved fossils coming from a time about which we know so little.”

Although it is known that the earliest origins of squamates (the animal group that includes lizards and snakes) lie 240 million years ago in the Triassic, a lack of fossils from the Triassic and Jurassic has made their early evolution and anatomy difficult to trace.

Bellairsia artist Reconstruction_by Elsa Panciroli.png

Artistic reconstruction of the fossil squamate, Bellairsia gracilis, basking on a dinosaur footprint. Artwork by Dr Elsa Panciroli.

The researchers found that Bellairsia has a mixture of ancestral and modern features in its skeleton, providing evidence of what the ancestor of today’s squamates might have looked like. Tałanda adds: “Bellairsia has some modern lizard features, like traits related to cranial kinesis – that’s the movement of the skull bones in relation to one another, including jaws opening and closing. This is an important functional feature of many living squamates.”

The exceptional new fossil comprises a near-complete skeleton in life-like articulation, missing only the snout and tail. To study the specimen, the team used X-ray computed tomography (CT) which allows for non-destructive 3D imaging, revealing the entire fossil, which is still mostly hidden by surrounding rock. First, they used a laboratory CT system at Oxford University to image the skull skeleton, but limitations of the instruments provided insufficient details on some important part such as the skull, or the smallest bones of the limbs.

They overcame the issue by scanning these parts at the ESRF. Using propagation phase-contrast X-ray micro-computed tomography at beamline ID19, they were able to reach a resolution of 4 µm, revealing important anatomical features in great detail. “After the first scans at Oxford, we realised that we needed a more powerful tool to examine specific, crucial parts of the skeleton and the ESRF was the best place to carry out these studies. Thanks to the highly intense beam, we gained an order of magnitude in resolution and in this way we could complete the dataset,”  explains Vincent Fernández, scientist at the ESRF and co-author of the publication.

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The actual fossil of Bellairsia gracilis, a fossil squamate from Middle Jurassic-aged rocks from the Isle of Skye, Scotland. Fossil held in the collections at National Museums Scotland. Photo by Dr Elsa Panciroli.

The fossil was found in 2016 by a team led by the University of Oxford and National Museums Scotland. Co-author Elsa Panciroli, of Oxford University Museum of Natural History and National Museums Scotland, who discovered the fossil, says: “It was one of the first fossils I found when I began working on Skye. The little black skull was poking out from the pale limestone, but it was so small I was lucky to spot it. Looking closer I saw the tiny teeth, and realised I’d found something important, but we had no idea until later that almost the whole skeleton was in there.”

Squamates have more than 10,000 species today, making them one of the most speciose living vertebrate animal groups. They include animals as diverse as snakes, chameleons and geckos, found around the world. The group is characterised by numerous specialised features of the skull and rest of the skeleton.

Analysing the new fossil alongside living and extinct fossil squamates confirms that Bellairsia belongs to the stem of the squamate family tree, meaning that it split from other lizards just before the origin of modern groups. The research also supports the finding that geckos are a very early branching lineage.

Synchrotron tomography of a stem lizard elucidates early squamate anatomy, M. Tałanda et al., Nature (2022);

Top image: Digital image of the fossil of Bellairsia gracilis inside the rock, as revealed using microCT scan data. Digital render by Matthew Humpage/NorthernRogue.