An exquisitely preserved Archaeopteryx fossil has delivered fresh insights into how the earliest birds first took flight 150m years ago.
The fossil is the first Archaeopteryx in which scientists have been able to identify specialised wing feathers that would have made flight possible. These tertial feathers on the upper arm bone create a smooth aerodynamic line from wing to body and are not seen in feathered flightless dinosaurs that existed alongside the first birds, suggesting that this was a crucial evolutionary change required for lift-off.
Dr Jingmai O’Connor, an associate curator of fossil reptiles at the Field Museum in Chicago, who led the analysis, said: “Archaeopteryx isn’t the first dinosaur to have feathers, or the first dinosaur to have ‘wings’. But we think it’s the earliest known dinosaur that was able to use its feathers to fly.
“These feathers are missing in feathered dinosaurs that are closely related to birds but aren’t quite birds. Their wing feathers stop at the elbow. That tells us that these non-avian dinosaurs couldn’t fly, but Archaeopteryx could.”
The first Archaeopteryx fossil was found 160 years ago in a German quarry and its visible feathers immediately made it a candidate for the earliest known bird. Unlike modern birds, the Archaeopteryx genus also has some dinosaur-like features, including jaws with sharp teeth, a long bony tail and hyperextensible second toes, sometimes known as the “killing claw”. The Chicago specimen was in private ownership before being acquired by the Field Museum in 2022.
A key debate has been the adaptations that were required for Archaeopteryx to take flight. Scientists have previously observed that, unlike feathered dinosaurs, Archaeopteryx has asymmetric feathers, with one side of the central shaft wider than the other. This is also seen in modern birds and is essential for generating thrust during flight. The latest observations appear to identify a second crucial evolutionary adaptation for flight.
“Compared to most living birds, Archaeopteryx has a very long upper arm bone,” said O’Connor. “And if you’re trying to fly, having a long upper arm bone can create a gap between the long primary and secondary feathers of the wing and the rest of your body. If air passes through that gap, that disrupts the lift you’re generating, and you can’t fly.”
“It’s important that this is the first time these feathers have been seen,” said Dr John Nudds, a senior lecturer in palaeontology at the University of Manchester, who was not involved in the research. “These new feathers seen in this beautifully preserved specimen – as well as the asymmetric feathers – confirms it could fly.”
The fossil’s tiny, hollow bones and tissues are almost the same colour as the extremely hard surrounding limestone. The Field Museum team CT scanned and illuminated the specimen with UV light to delineate the fossil’s boundaries before carefully chipping away the rock with sub-millimetre precision over the course of more than a year, to reveal a more complete picture.
“Our specimen is the first Archaeopteryx that was preserved and prepared in such a way that we can see its long tertial feathers,” said O’Connor.
The analysis,published in Nature, also highlights bones in the roof of the mouth that appear to be a step towards a feature called cranial kinesis – a feature in modern birds that lets the beak move independently from the braincase. Small, tightly packed scales preserved in the pads of the feet also bolster the view that Archaeopteryx spent a lot of its time walking on the ground and might even have been able to climb trees.