Scottish fossil sheds light on early lizard anatomy

A new fossil specimen of a long-extinct lizard, discovered on the Isle of Skye, has given paleontologists insight into the earliest origins of this reptile group. The tiny vertebrate, Bellairsia gracilis, is only 6 cm long and dates from the Middle Jurassic, around 166 million years ago. It was found in 2016 by a team led by scientists from the University of Oxford and National Museums Scotland, and is one of several new fossil discoveries from the island that cast light on the early evolution of vertebrate groups, including amphibians, reptiles and mammals. 

Details of the exceptional new fossil, which comprises the most complete skeleton of a fossil lizard anywhere in the world, have been published in the journal Nature. The research represents a joint project between scientists from the universities of Warsaw, Oxford and UCL, and it reports on the mixture of ancestral and modern features present in this early representative of the group known as the squamata – the largest group of living reptiles that includes lizards, chameleons and snakes.  

“This little fossil lets us see evolution in action. In paleontology you rarely have the opportunity to work with such complete, well-preserved fossils coming from a time about which we know so little. 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. This is an important functional feature of many living squamates,” explained study first author Dr. Mateusz Tałanda.

“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 realized I’d found something important, but we had no idea until later that almost the whole skeleton was in there,” said study co-author Dr. Elsa Panciroli.

As a group, the squamates show numerous specialized features of the skull and skeleton today, but fossil evidence of the origins of these features is rare. Although we know the earliest representatives were present some 240 million years ago, the lack of fossil evidence from the Triassic and Jurassic Periods has made it difficult to map out the evolution of their anatomy. Comparing the new fossil to other examples of living and extinct squamates confirms that Bellairsia belongs to the “stem” of the squamate family tree. This means that it split from other lizards just before the origin of modern groups. 

The researchers used X-ray computed tomography (CT) to study the details of the Bellairsia fossil, in much the same way that medical CT methods produce 3D images of a human body without needing to enter within. The researchers were able to obtain images of the entire fossil without removing it from the rock in which it formed all those millions of years ago. Whereas medical scanners work at the millimeter scale, the Oxford University CT scanner revealed details of the little lizard’s skeletal anatomy down to a few tens of micrometers.

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 micrometers, revealing details of the very smallest bones in the skeleton.

“Fossils like this Bellairsia specimen have huge value in filling gaps in our understanding of evolution and the history of life on Earth,” said study co-author Professor Roger Benson. “It used to be almost impossible to study such tiny fossils like this, but this study shows the power of new techniques, including CT scanning, to image these non-destructively and in great detail.”

Study co-author Professor Susan Evans (UCL) first described and named Bellairsia from a few jaw and skull bones from Oxfordshire 25 years ago. “It is wonderful to have a complete specimen of this tantalizing little lizard, and to see where it fits in the evolutionary tree,” said Professor Evans. “Through fossils like Bellairsia we are gaining a better understanding of early lizard anatomy. Angus Bellairs, the lizard embryologist after [whom] Bellairsia was originally named, would have been delighted.”

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By Alison Bosman, Earth.com Staff Writer