Pterosaurs may have dominated the skies for 150 million years, but how they gained their wings has been a mystery.
Now, recently unearthed fossils may help fill a gap in the flying predators’ family tree, according to a study published in Nature today.
The skulls and skeletons are of a group of ancient reptiles — lagerpetids — that trotted around the supercontinent Pangea between 237 and 210 million years ago.
These two-legged, wingless creatures share anatomical features with pterosaurs, which were the first vertebrates to evolve flapping flight.
But study lead author Martín Ezcurra, a palaeontologist at the Argentinian Museum of Natural History, didn’t set out to beef up the pterosaurs’ evolutionary history.
“My main interest was the evolution of dinosaurs,” he says.
So Dr Ezcurra and his team were investigating lagerpetids as a potential part of the dinosaurs’ evolutionary tree.
But what they discovered may help fill a 28-million-year gap in the evolution of flying reptiles instead.
Precious and rare fossils
Pterosaur lineage has been a particularly tricky puzzle to piece together because fossils from that period are so rare.
Unlike the gigantic flying beasts of the Late Cretaceous, which had wingspans of up to 11 metres, the earliest pterosaurs were miniature in comparison: less than a metre from wingtip to wingtip.
Lagerpetids, which means “rabbit-reptiles”, were also typically pretty small — only a metre or so long.
And since both pterosaurs and lagerpetids had hollow bones, the odds of them being fossilised in the first place — and remain relatively undisturbed for the best part of a quarter billion years — was even less likely.
This means only a handful of lagerpetid remains have ever been identified.
The first lagerpetid fossils were collected decades ago, and in recent years, more skulls and “arms” have been found.
To get a good look at the precious specimens without damaging them, Dr Ezcurra and his colleagues in Europe and the Americas scanned them using high-resolution CT.
These 3D reconstructions revealed the tiniest of features and those otherwise hidden from view.
They showed that pterosaurs and lagerpetids shared anatomical characteristics, such as the shape of the inner ear, and brain features related to enhanced agility.
Closer to dinosaurs or pterosaurs?
While lagerpetids do have some dinosaur-like features, the researchers calculated that the rate of evolution to progress from lagerpetid to early dinosaur would have been unnaturally speedy.
“If we considered the original position of lagerpetids as closer to dinosaurs, we needed very quick evolutionary rates to get the dinosaur body plan,” Dr Ezcurra said.
“But when we see lagerpetids as the closest relatives of pterosaurs, these evolutionary rates are lower and actually not different to other main groups of reptiles.
“So that shows that lagerpetids fill, not completely, but in a large degree, this anatomical gap between pterosaurs and other [earlier] reptiles.”
If lagerpetids were indeed precursors to flying reptiles, Dr Ezcurra and his team have shortened the gap in pterosaur lineage from 28 million years to 18 million years.
And lagerpetids fit the pterosaur-ancestor profile, writes University of California Berkeley palaetontologist Kevin Padian in an accompanying News and Views article: “The proportions of their slender limbs, as well as the shape of their back, are fully consistent with a bird-like body plan.”
Adele Pentland, a palaeontology PhD student at Swinburne University who studies Australian pterosaurs, said the study’s conclusions “made perfect sense”.
“For the adaptations that would have been needed in order to fly, it makes more sense that they would have been there before flight itself evolved.”
So … where are the wings?
What’s missing from lagerpetids, though, are wings — or even the beginnings of wings.
“Lagerpetids lack almost all of the typical features of pterosaurs that are related to flight,” Dr Ezcurra said.
For instance, pterosaurs had an extremely long fourth “finger” on which their wing membrane is attached, and their main arm bone, the humerus, had a pronounced “crest” on which flying muscles were probably attached.
Lagerpetids didn’t show any signs of a developing “wingfinger” or a large humeral crest.
“It would be very interesting to find, at some point, a more intermediate form between lagerpetids and pterosaurs that has a narrower evolution of these traits related to flight,” Dr Ezcurra said.
Ms Pentland said any work in the area of pterosaur evolution is “of great importance and significance”.
And while we may never find that perfect in-between fossil, further discoveries will only add to the story, she said.
“And whether it changes if we find additional fossils, only time will tell.”