Exceptional Jurassic fossil lizard sheds light on early lizard evolution

A tiny Scottish fossil is having a big impact on our understanding of lizards. 

Measuring just six centimetres long, Bellairsia gracilis would have roamed around freshwater lagoons in what are now the Inner Hebrides over 166 million years ago.

An almost intact specimen uncovered six years ago provides a snapshot of a crucial time in the development of lizards, snakes, and limbless amphisbaenians (collectively known as squamates).

It has a mixture of features found in both ancient and modern squamates, showing that some of their abilities, including those which help them to feed, could have been possessed by the ancestors of the group as a whole.

When Bellairsia's characteristics are included in a family tree of the squamates, it helps to clarify early lizard relationships. Disagreement between molecular and fossil evidence had led to confusion over whether geckos or iguanas were the earliest evolving modern squamates, with the new study coming down firmly on the side of the geckos.

Dr Mateusz Tałanda, the lead author of a study describing the specimen, says, 'In palaeontology, you rarely have occasion to work with such complete, well-preserved specimens coming from a time about which we know so little. This little fossil, however, lets us see evolution in action.'

Dr Marc Jones, who is the Museum's curator of fossil reptiles and was not involved in the study, adds, 'This specimen of Bellairsia gracilis is articulated and nearly complete, which is really exceptional for the Middle Jurassic.'

'Articulated skeletons such as this add a lot more information to the anatomy of these species, which allows us to go back to isolated fossils found elsewhere and gain a deeper insight into what it represents.'

The study, which also used the Museum's Bellairsia specimens, was published in Nature. 

How did lizards and other squamates evolve?

With around 11,000 known species, squamates are the most diverse terrestrial vertebrates living today, but their evolution has been shrouded in mystery. Studies of their genetics suggested the group evolved in the Middle Triassic over 240 million years ago, but the earliest definite fossils only date to 170 million years ago.

'The overriding issue is that the fossil record is very patchy, particularly for small animals,' Marc explains. 'Until the late 1980s, reptile fossils were referred to lizards on the basis of very few anatomical characters, such as their small size and small conical teeth.'

'As a result, many fossils were grouped together as possible lizards, or lizard ancestors, even though they didn't really share many characteristics. A greater focus on detailed anatomical work, and more rigorous comparisons, has given us a more precise understanding of what a lizard actually is.'

In the past decade the discovery of new fossils has helped to shed some light on this issue. For example, a fossil reptile discovered in Germany, Megachirella wachtleri, was reinterpreted as a Middle Triassic squamate, and therefore an ancient cousin to modern snakes and lizards.

It is believed to have lived around 240 million years ago, shortly after the squamates diverged from a group of lizard-like animals known as the rhynchocephalians. Within the squamates there were a series of early species before the common ancestor of all living squamates appeared around 193 million years ago.

Known as the stem squamates, these first species are themselves difficult to identify. 

'Living lizards have a lot of very specialised reptile characteristics, such as their braincase, ankle and knee, as well as soft tissue structures,' Marc says. 'Sadly, these characters can be quite difficult to recognise in the fossil record because they are often damaged or not preserved.'

The recently uncovered Bellairsia specimen exhibits many of these modern characteristics, but also some ancestral ones. Detailed analysis of available evidence suggested that is actually one of these rare stem squamates. Its exquisite preservation provides new evidence that helps to firm up the evolution of the earliest lizards. 

What does the Skye Bellairsia specimen reveal?

The new specimen was uncovered in 2016 during an expedition to the Isle of Skye, Scotland, by co-author Dr Elsa Panciroli, who was new to the area at the time.

'It was one of the first fossils I found when I began working on Skye,' Elsa says. '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.'

The rocks the Bellairsia specimen was found in is part of the Kilmaluag Formation, which over recent years has provided a number of scientifically valuable specimens, including the world's largest Jurassic pterosaur.

'Kilmaluag has been really crucial to investigating how small reptiles and amphibians evolved,' Marc says. 'There are other Middle Jurassic sites in England which have a huge diversity of fossil vertebrates in them, but the material is often isolated, disarticulated and broken, which makes it hard to reconstruct.'

'The material at Kilmaluag is valuable scientifically because it is associated, so it helps us determine which bones came from which animal.'

After being examined, the researchers found that while the Bellairsia specimen was compressed, and missing the tips of its nose and tail, it is in otherwise remarkably good condition. 

Among the modern characteristics of squamates possessed by the specimen is cranial kinesis, which allows parts of the skull and jaw to move independently of each other, although this was in no way as developed as seen in some species of snake which can separate their jaw to swallow very large prey.  

Discovering more well-preserved specimens would help to refine these relationships further and give scientists a better idea of how one of the most diverse groups on Earth evolved.

'There is still a massive gap in the squamate fossil record of the Triassic, and many of the fossils from the Cretaceous already look quite modern,' Marc says. 'Having examples from all over the world from a wide variety of environments would clarify the uncertainty surrounding their evolution.'