Early pterosaur fossils are hard to find – and we might now know why

Early pterosaurs loved temperate and humid habitats – and it might explain why today their fossils are hard to find.

While these flying reptiles became a major part of life during the Mesozoic era, between 252 and 66 million years ago, surprisingly little is known about how they first evolved. This is because when pterosaurs first appear in the fossil record they were already able to fly.

To understand their origins, palaeontologists have turned to the pterosaurs’ closest relatives, the lagerpetids. These non-flying reptiles are thought to have looked similar to pterosaurs before they developed wings, and so it is hoped they can offer insights about pterosaur evolution.

By combining information about pterosaur and lagerpetid fossils with climate data from more than 200 million years ago, a team of scientists have looked at where these animals lived during the Triassic. They found that while lagerpetids lived in a wide variety of hot and dry environments, pterosaurs preferred a limited selection of wet and cool coastal habitats.

Dr Davide Foffa, a University of Birmingham scientist who co-led the research alongside with colleagues Dr Emma Dunne and Dr Alessandro Chiarenza, says that it helps to explain why early pterosaurs have only been found at a few sites so far.

“Some have argued that it’s purely a consequence of the fossil record, and that the fragile bones of the earliest pterosaurs just haven’t survived,” Davide explains. “Others argue that our understanding of where pterosaurs come from is inaccurate.”

“With this paper, we’ve provided a possible explanation – many well-sampled fossil localities simply had the wrong climate for Triassic pterosaurs. It’s a small step forward in understanding the pterosaurs, but there’s still a long way to go.”

The findings of the study were published in the journal Nature Ecology & Evolution.

Where did pterosaurs and lagerpetids come from?

At the beginning of the Triassic, all the world’s major landmasses were gathered together into a supercontinent known as Pangaea.

In its southwestern corner, in what is now South America, the first lagerpetids appear around 240 million years ago. They then rapidly spread to many regions around the world including what is now the UK.

But despite being their closest relatives, the first known fossils of pterosaurs don’t appear until around 20 million years later. Even more confusingly, the earliest pterosaur fossils don’t come from South America, but from rocks now found in the Alps.

Professor Paul Barrett, who co-authored the research, says that pterosaurs and lagerpetids appear to have led fairly separate lives.

“While these animals are close relatives of a similar age, there are very few examples of their fossils overlapping,” Paul explains. “This is even the case in environments that would seem to be suitable for both pterosaurs and lagerpetids.”

“Even when lagerpetids are found alongside gliding reptiles, which have a similar build to pterosaurs, there doesn’t seem to be an overlap. As the environment itself doesn’t seem to be an issue, we wondered if their climatic preferences might help to explain this.”

Using the present to understand the past

To investigate, the team turned to an approach known as biogeography. These methods were originally developed to explain how habitat and climate affect where modern animals live but have more recently been applied to long-extinct species.

“My colleagues Emma Dunne and Alessandro Chiarenza have worked on applying biogeographic and climate modelling techniques to extinct animals such as the dinosaurs,” Davide says.

“It’s one of a number of ways that advances in other sciences are helping to increase our understanding of the past, whether that’s engineering methods which help to explain how dinosaurs fed or advanced medical scanning to peer inside fossils.”

The team found that while the generalist lagerpetids seem to have spread widely and consistently across the world, the more specialised pterosaurs had a more limited dispersal at first.

“It's surprising that pterosaurs are so limited in their distribution, because they have one big advantage over the lagerpetids: their ability to fly,” Paul explains. “It would be expected that pterosaurs could fly in or out of an environment relatively easily, so there must be something else going on.”

“It’s possible that they don’t have the ability to disperse over long distances at this point, or that there are aspects of their biology which mean they can’t tolerate warm climates.”

Whatever was initially holding them back, pterosaurs seem to have overcome their climate barriers by around 215 million years ago. From this point in time, they suddenly spread around the world achieving a global distribution until they were wiped out alongside the dinosaurs 66 million years ago.

Searching for pterosaur fossils

Explaining how the biology of the pterosaurs changed to allow them to conquer the Mesozoic skies is difficult with so few fossils from the time to study. However, the team’s research could help researchers to find more early pterosaur remains.

By figuring out the climate and environment in which the pterosaur-bearing Triassic rocks were laid down, the team can predict where else these animals might have lived.

“Our findings suggest that pterosaurs could live over a much larger range than their fossils alone suggest,” Paul says. “This isn’t that much of a surprise, as the fossil record represents just a fraction of the animals alive at the time.”

“By getting a more realistic idea of their range, however, we can target areas of the world where those fossils might exist but haven’t yet been found. It’s a predictive tool that could help to fill in some of the gaps in our knowledge, and reveal new information about sites where palaeontologists haven’t yet looked for these animals.”

The researchers also suspect that many early fossils may have historically been missed, and that it would be worth going back over material already excavated.

“As the bones of pterosaurs, lagerpetids and other small animals are small and fragile, their remains might have been overlooked when researchers have been looking for large fossils,” Davide explains

“Small species like this are the most diverse part of any ecosystem, so finding their remains will allow us to get a better picture of life during the Triassic and beyond.”