Evolution of lizards at the era of dinosaurs

“This little fossil lets us see the evolution in action. In palaeontology you rarely have an occasion to work with such complete, well-preserved fossils,” Dr Mateusz Tałanda, the Faculty of Biology of the University of Warsaw, said. The discovery of Bellairsia gracilis in Scotland has become a source of information about the early evolution of lizards at the era of dinosaurs.

The tiny, skeleton from the Isle of Skye, called Bellairsia gracilis, is only 6 cm long and dates back from the Middle Jurassic, 166 million years ago. It is the most complete fossil lizard of this age anywhere in the world.

Bellairsia has a mixture of ancestral and modern features in its skeleton, providing evidence of what the ancestor of today’s squamates (the animal group that includes lizards and snakes) might have looked like.

“This little fossil lets us see the evolution in action. In palaeontology you rarely have an occasion to work with such complete, well-preserved fossils coming from the time about which we know so little,” Dr Mateusz Tałanda of the Faculty of Biology of the University of Warsaw, said.

3D scanning of the Bellairsia gracilis skeleton

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 led by Dr Tałanda used X-ray computed tomography (CT) which, like medical CT, allows for non-invasive 3D imaging, revealing the entire fossil, which is still mostly hidden by surrounding rock. Whereas medical scanners work at the millimetre scale, the Oxford University CT scanner provided data of the full skeleton, showing details down to a few tens of micrometres. Parts of the skeleton were then imaged in even greater detail, including the skull, hindlimbs and pelvis, at the European Synchrotron (ESRF, Grenoble, France). The intensity of the synchrotron beam permits a resolution of 4 µm, revealing details of the smallest bones in the skeleton.

“Bellairsia has some modern lizard features, like traits related to cranial kinesis – that is the movement of the skull bones in relation to one another. This is an important functional feature of many living squamates,” Dr Tałanda explained.

The fossil was found in 2016 by a team led by the University of Oxford and National Museums Scotland. It is one of several new fossil discoveries from the island, including early amphibians and mammals, which are revealing evolution of important animal groups that persist to the present day.

“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 had found something important, but we had no idea until later that almost the whole skeleton was in there,” Dr Elsa Panciroli from the Oxford University Museum of Natural History and the National Museums Scotland, who discovered the fossil, said.

An evolutionary tree of the squamates

Squamates are the living group that includes lizards and snakes, and 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.

Although we know the earliest origins of squamates 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.

“It is wonderful to have a complete specimen of this tantalising little lizard, and to see where it fits in the evolutionary tree. Through fossils like Bellairsia we are gaining a better understanding of early lizard anatomy. Angus Bellairs, the lizard embryologist after which Bellairsia was originally named, would have been delighted,” Professor Susan Evans of the University College London, who first described and named Bellairsia from a few jaw and skull bones from Oxfordshire 25 years ago, said.

Analysing the new fossil alongside living and extinct fossil squamates confirms 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, and that the enigmatic fossil Oculudentavis, previously suggested to be a dinosaur, is also stem squamate.