Two new studies have uncovered details about how pterosaurs – the winged cousins of dinosaurs – evolved to become deadly masters of the sky.
One reveals that pterosaurs became twice as good at flying throughout their existence, while the other performs a spot of fossil dentistry to discover how their diet changed as they evolved.
Both lines of research have important implications not only for the evolutionary history of these reptilian aviators, but also for their role in the wider ecosystem.
“Pterosaur” is the proper name for “pterodactyl”, a word that for many may conjure a Jurassic Park-inspired image of a bizarre-looking lizard with leathery wings and murderous inclinations. But the fossil record preserves more than 200 species of pterosaurs of all shapes and sizes, many of which would have lived at the same time and occupied divided habitats, as modern birds do.
“Pterosaurs were a diverse group of winged lizards, with some the size of sparrows and others with the wingspan of a light aircraft,” says evolutionary biologist Chris Venditti from the University of Reading, lead author of the first study, published in Nature.
These weird wonders on wings evolved in the Triassic period, diverging from dinosaur ancestors around 245 million years ago. They went on to become the first vertebrates to evolve active flight – tens of millions of years before birds and bats –and dominated the skies until their untimely demise 65 million years ago.
“Despite their eventual prowess in the air being well-known, the question of whether pterosaurs got better at flying and whether this gave them an advantage over their ancestors has puzzled scientists for decades,” Venditti says.
The early ancestors of pterosaurs were small, probably bipedal, and well-adapted to life on the ground – so how did they develop their ultralight skeletons, strong limbs and specially-adapted wings, and thus make that crucial evolutionary transition towards flight?
Frustratingly, the fossil record from the first 25 million years of pterosaur evolution is empty. Pterosaur fossils in general are incredibly scarce, as their hollow, paper-thin bones have long since vanished to dust, but the lack of evidence from this vital transitionary period is particularly maddening for scientists.
Venditti and colleagues set out to fill in the gaps. By combining fossil records with statistical methods and biophysical models, they were able to track the gradual evolution of pterosaur flight.
Specifically, they monitored flight efficiency by measuring the wingspan and body size of fossils over at different stages of evolution, then applied a new model of flight based on today’s living birds.
The team discovered that over 150 million years pterosaurs evolved from inefficient flyers who could only travel short distances to capable aviators able to soar for long periods of time. They achieved this by adapting their body shape and size to use 50% less energy while in the air – even though many species increased in weight, with some ultimately weighing more than 300 kilograms.
This detailed study also showed that the evolution took place in incremental steps over millions of years, rather than rapid bursts of advancement as previously suggested.
“Our new method has allowed us to study long-term evolution in a completely new way and answer this question at last by comparing the creatures at different stages of their evolutionary sequence over many millions of years,” Venditti explains.
Not every pterosaur was equally skilled in the air, however. The study found that one group, the azhdarchoids, did not improve their flight efficiency over their evolutionary history.
Co-author Joanna Baker, also from the University of Reading, explains: “This is unique evidence that although these animals were competent fliers, they probably spent much of their time on the ground. Highly efficient flight probably didn’t offer them much of an advantage, and our finding that they had smaller wings for their body size is in line with fossil evidence for their reduced reliance on flight.”
The large size of azhdarchoids likely offered them a survival advantage on the ground; one species, the monstrous Quetzalcoatlus, was as tall as a giraffe.
Since hundreds of species of pterosaurs existed around the world in a range of terrestrial, coastal and marine ecosystems, scientists must employ many different approaches to build a complete picture of how these reptiles lived.
Dietary studies are one method, though they are inherently difficult: there are no modern descendants of pterosaurs to provide a basis for models, and fossilised stomach contents are few and far between, limited to just small number of specimens.
But now, as described in a paper in Nature Communications, a UK team led by Jordan Bestwick from the University of Leicester has used fossilised teeth to investigate pterosaur diets.
Just as teeth leaves marks on food, food can also leave marks on teeth. As an animal bites, pierces and chews different types of food, tiny indications of wear form on tooth crowns. These can be preserved in the fossil record, a fact exploited by Bestwick and colleagues who analysed the microscopic food patterns left on the teeth of pterosaur species across 17 different genera.
The patterns were compared to living animals like crocodiles, monitor lizards and bats, revealing that pterosaurs evolved to eat a wide range of foodstuffs, with diets varying hugely across species.
The study demonstrates that while some pterosaurs specialised in a certain type of prey, others were more generalist. It also suggests a long-term trend in diet: the researchers propose that while ancestral pterosaurs ate invertebrates, later pterosaurs evolved to become meat- and fish-eaters.
Such a shift, the team says, might reflect growing competition between birds and pterosaurs.
In their paper, they write that pterosaurs “were important components of Mesozoic ecosystems, and reconstructing pterosaur diets is vital for understanding their origins, their roles within Mesozoic food webs and the impact of other flying vertebrates (i.e. birds) on their evolution”.
Both of these new studies – the first advancing our knowledge of the evolution of flight, and the second exploring the ecological niche of this diverse creature – use intriguing methodologies that may act as blueprints for further, nuanced studies of evolutionary change in ancient ecosystems.
Lauren Fuge is a science journalist at The Royal Institution of Australia.
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