Authors: Davide Foffa, Research Fellow in Palaeobiology, University of Birmingham; Alfio Alessandro Chiarenza, Royal Society Newton International Fellow in Palaeontology, UCL, and Emma Dunne, Assistant Professor in Paleobiology, Friedrich-Alexander-Universität Erlangen-Nürnberg

pterosaurs

Illustration: Dmitry Bogdanov – dmitrchel@mail.ru, CC BY-SA 3.0 / Wikimedia Commons

Ever since the first fragments of pterosaur bone surfaced nearly 250 years ago, palaeontologists have puzzled over one question: how did these close cousins of land-bound dinosaurs take to the air and evolve powered flight? The first flying vertebrates seemed to appear on the geological stage fully formed, leaving almost no trace of their first tentative steps into the air.

Taken at face value, the fossil record implies that pterosaurs suddenly originated in the later part of the Triassic period (around 215 million years ago), close to the equator on the northern super-continent Pangaea. They then spread quickly between the Triassic and the Jurassic periods, about 10 million years later, in the wake of a mass extinction that was most likely caused by massive volcanic activity.

Most of the handful of Triassic specimens come from narrow seams of dark shale in Italy and Austria, with other fragments discovered in Greenland, Argentina and the southwestern US. These skeletons appear fully adapted for flight, with a hyper-elongated fourth finger supporting membrane-wings. Yet older rocks show no trace of intermediate gliders or other transitional forms that you might expect as evidence of pterosaurs’ evolution over time.

How did pterosaurs evolve powered flight?

There are two classic competing explanations for this. The literal reading says pterosaurs evolved elsewhere and did not reach those regions where most have been discovered until very late in the Triassic period, by which time they were already adept flyers. The sceptical reading notes that pterosaurs’ wafer-thin, hollow bones could easily vanish from the fossil record, dissolve, get crushed or simply be overlooked, creating this false gap.

Eudimorphodon ranzii fossil from Bergamo in 1973 is one of many pterosaur discoveries from southern Europe. Photo:
Wikimedia, CC BY-SA

For decades, the debate stalled as a result of too few fossils or too many missing rocks. This impasse began to change in 2020, when scientists identified the closest relatives of pterosaurs in a group of smallish upright reptiles called lagerpetids.

From comparing many anatomical traits across different species, the researchers established that pterosaurs and lagerpetids shared many similarities including their skulls, skeletons and inner ears. While this discovery did not bring any “missing link” to the table, it showed what the ancestor of pterosaurs would have looked like: a rat-to-dog-sized creature that lived on land and in trees.

This brought new evidence about when pterosaurs may have originated. Pterosaurs and lagerpetids like Scleromochlus, a small land-dwelling reptile, diverged at some point after the end-Permian mass extinction. It occurred some 250 million years ago, 35 million years before the first pterosaur appearance in the fossil record.

Scleromochlus is one of the lagerpetids, the closest known relatives to the pterosaurs. Illustration:
Gabriel Ugueto

Pterosaurs and their closest kin did not share the same habitats, however. Our new study, featuring new fossil maps, shows that soon after lagerpetids appeared (in southern Pangaea), they spread across wide areas, including harsh deserts, that many other groups were unable to get past. Lagerpetids lived both in these deserts and in humid floodplains.

They tolerated hotter, drier settings better than any early pterosaur, implying that they had evolved to cope with extreme temperatures. Pterosaurs, by contrast, were more restricted. Their earliest fossils cluster in the river and lake beds of the Chinle and Dockum basins (southwest US) and in moist coastal belts fringing the northern arm of the Tethys Sea, a huge area that occupied today’s Alps.

Scientists have inferred from analysing a combination of fossil distributions, rock features and climate simulations that pterosaurs lived in areas that were warm but not scorching. The rainfall would have been comparable to today’s tropical forests rather than inland deserts.

This suggests that the earliest flying dinosaurs may have lived in tree canopies, using foliage both for take-off and to protect themselves from predators and heat. As a result of this confined habitat, the distances that they flew may have been quite limited.

Changing climates

We were then able to add a fresh dimension to the story using a method called ecological niche modelling. This is routinely used in modern conservation to project where endangered animals and plants might live as the climate gets hotter. By applying this approach to later Triassic temperatures, rainfall and coastlines, we asked where early pterosaurs lived, regardless of whether they’ve shown up there in the fossil record.

Many celebrated fossil sites in Europe emerge as poor pterosaur habitat until very late in the Triassic period: they were simply too hot, too dry or otherwise inhospitable before the Carnian age, around 235 million years ago. The fact that no specimens have been discovered there that are more than about 215 million years old may be because the climate conditions were still unsuitable or simply because we don’t have the right type of rocks preserved of that age.

In contrast, parts of the south-western US, Morocco, India, Brazil, Tanzania and southern China seem to have offered welcoming environments several million years earlier than the age of our oldest discoveries. This rewrites the search map. If pterosaurs could have thrived in those regions much more than 215 million years ago, but we have not found them there, the problem may again lie not with biology but with geology: the right rocks have not been explored, or they preserve fragile fossils only under exceptional conditions.

Our study flags a dozen geological formations, from rivers with fine sediment deposits to lake beds, as potential prime targets for the next breakthrough discovery. They include the Timezgadiouine beds of Morocco, the Guanling Formation of south-west China and, in South America, several layers of rock from the Carnian age, such as the Santa Maria Formation, Chañares Formation and Ischigualasto Formation.

Pterosaurs were initially confined to tropical treetops near the equator. When global climates shifted and forested corridors opened, pterosaurs’ wings catapulted them into every corner of the planet and ultimately carried them through one of Earth’s greatest extinctions. What began as a tale of missing fossils has become a textbook example of how climate, ecology and evolutionary science have come together to illuminate a fragmentary history that has intrigued paleontologists for over two centuries. The Conversation

This article is republished from The Conversation under a Creative Commons license. Read the Original article.

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