Scottish fossil reveals clues about the earliest pterosaurs

The identity of an enigmatic fossil has been revealed over a century after it was first discovered.

Since being described in 1907, Scleromochlus has variously been considered as an early pterosaur, a dinosaur, or an ancestor of ancient crocodiles. Confirming its exact identity has been difficult due to its preservation as impressions preserved in rock, known as a natural mould, rather than as the bones themselves.

However, advances in scanning technology have now allowed scientists to examine its fossils in unprecedented detail, clearing up one of palaeontology's historic mysteries in the process.

They have revealed it was a small, fast-running reptile that may have been able to run on two legs in certain circumstances.

Professor Paul Barrett, who co-authored a new paper documenting these discoveries, says, 'The identity of Scleromochlus has been the topic of much debate for years, and this work helps to settle this debate.'

'We've managed to get new information from previously unnoticed bones buried within the blocks as well as putting together a 3D model based on virtually filling-in the natural moulds, which gives us a better idea of what this animal would have looked like in life.'

'Scleromochlus was probably an agile, ground-dwelling animal which, while not a direct pterosaur ancestor, gives us some indication of what that ancestor might have looked like.'

Dr Davide Foffa, the lead author of the paper published in Nature, adds, 'It's exciting to be able to resolve a debate that’s been going on for over a century, but it is far more amazing to be able to see and understand an animal which lived 230 million years ago and its relationship with the first and largest animals ever to have flown.'

'This is another discovery which highlights Scotland's important place in the global fossil record, and also the importance of museum collections that preserve such specimens, allowing us to use new techniques and technologies to continue to learn from them long after their discovery.'

How did pterosaurs evolve?

Pterosaurs emerged over 220 million years ago in the Late Triassic. Unlike other ancient reptile groups, whose gradual evolution can be tracked through a number of intermediate stages, the earliest-known pterosaurs were already highly specialised by the time they first show up in the fossil record.

'The earliest pterosaur fossils that we know are already distinctly pterosaur-like and show the suite of specialised features that make this group distinctive,' Paul explains.

'We don't have any reptile fossils that show an intermediate stage of development, so we have very little to guide us on what the initial stages of transformation from a crocodile or lizard-like ancestor into a pterosaur were like.'

Until recently, the closest relatives of the pterosaurs remained elusive. In 2020, this changed when a group of dinosaur-like animals known as lagerpetids were found to be the 'sister group' of pterosaurs on the basis of shared similarities such as a lack of bony plates between the teeth.

The new investigation of Scleromochlus suggests it is the most primitive lagerpetid found to date, which is allowing researchers to make inferences as to the appearance of the earliest lagerpetids and pterosaurs.

'As the most primitive member of the lagerpetid group, Scleromochlus helps to narrow down what the common ancestor of pterosaurs and lagerpetids may have looked like,' Paul says. 'Their lack of adaptations for flight suggests the pterosaur body plan must have evolved in the 18 million years between when these animals lived and when flying reptiles begin to appear in the fossil record.'

'Scleromochlus is also the first lagerpetid known from Europe, showing that these animals were living in areas outside of the Americas and Madagascar; the only places where their fossils had been found previously.'

Uncovering the secrets of Scleromochlus

To uncover how Scleromochlus may have looked, researchers have previously relied on making casts of the fossils to reveal how the bones would have appeared before they dissolved away. However, this has led to confusion over the years as different casts highlight different aspects of the fossil.

'The historic techniques used to make casts often miss minute details in fossils and – as Scleromochlus is quite small – this can lead to important aspects of its anatomy being missed,' Paul says. 'This makes it hard to assess its relationships with other species.'

The new study is the first time that CT scans, which make use of x-rays taken at different angles, have been used on these fossils.

'Using the scans, we were able to produce virtual casts of these animals in unprecedented detail and produce the most detailed 3D impressions of what its bones looked like,' Paul says. 'This has allowed us to account for the deformation of the fossil and learn a lot more about the anatomy of this animal than we previously knew.'

'It's also revealed additional bones preserved inside the rock that we have seen for the very first time.'

The detailed models of Scleromochlus have allowed researchers to rule out suggestions about how it might have lived. Hopping, for instance, had previously been suggested as a form of locomotion but its pelvis is now known to be too small and weak to support this.

Other aspects of its biology remain uncertain. While Scleromochlus was not adapted for climbing, the researchers could not discount it as a possibility. It is also uncertain how it may have looked.

'We don't see any armour in these animals, which removes a feature which could have suggested Scleromochlus was a more crocodile-like animal,' Paul says. 'There are also impressions that might represent integument, but they're not well enough preserved to demonstrate whether the skin was scaly or downy, if it represents skin at all.'

The researchers are continuing to work on other specimens from the Lossiemouth Sandstone Formation where Scleromochlus was discovered in the hope it will reveal more about these unusual Late Triassic animals, and how the major reptile groups of the Mesozoic evolved.