Using robotic and animal models, researchers have shown that some dinosaurs were already flapping their rudimentary wings as a side effect of running, prior to evolving the ability to fly. The finding offers a unique perspective on the origins of flight, but experts say more evidence is needed.
New research published today in PLOS Computational Biology is pointing to a previously under-appreciated factor that may have led to the origin of flight in avian dinosaurs.
A team led by Jing-Shan Zhao from Tsinghua University in Beijing used some fancy math, a robot, and a juvenile ostrich to experimentally demonstrate that some feathered dinosaurs were already flapping their proto-wings prior to being able to fly. This flapping motion was passive—a side effect of running on the ground. But as the new study contends, this inadvertent movement during running may have “trained” certain dinosaurs to flap their wings in a way that eventually led to actual flight once their wings were robust enough to support flying.
The origin of avian flight has flummoxed evolutionary biologists since the discovery of Archaeopteryx, a winged Jurassic dinosaur. Scientists don’t fully know, for example, which dinosaurs were the precursor species to avian fliers, whether or not gliding flight or flapping flight came first, or which physical characteristics emerged that made flying possible. The new study is interesting in that it’s presenting a possible gateway to this capacity—the passive flapping of proto-wings during running. It’s an intriguing possibility, but owing to the complex, multi-faceted nature of flight, it’s likely an insufficient answer to this longstanding question.
For the new study, the Tsinghua University researchers considered a dinosaur known to paleontologists: Caudipteryx. This animal is considered the most basal, or most primitive, non-flying dinosaur to be equipped with feathered proto-wings. Caudipteryx was an 11-pound (5 kilogram) dinosaur not capable of flight, but it could run around 26 feet per second (8 meters/second).
By using a mathematical approach known as modal effective mass theory, the researchers were able to predict the mechanical effects of running on various parts of Caudipteryx’s body. Numerical models suggested the passive flapping motion at speeds between 8.2 and 19 feet per second (2.5 to 5.8 m/s). Not content to rely on numbers alone, the researchers built a life-sized robot of Caudipteryx capable of running at different speeds. They also fitted a young ostrich—an actual, living dinosaur—with a set of artificial proto-wings. In both cases, running movements triggered a passive flapping motion, affirming the modal effective mass calculations.
So by using both mathematical and real-world models, the researchers were able to demonstrate a motion that, albeit superficially, resembles the flapping of bird wings.
“Our work shows that the motion of flapping feathered wings was developed passively and naturally as the dinosaur ran on the ground,” said Zhao in a press release. “Although this flapping motion could not lift the dinosaur into the air at that time, the motion of flapping wings may have developed earlier than gliding.”
Importantly, the researchers admitted that the aerodynamic forces created by this flapping motion are not known, and likely cannot be compared to the forces actually required for flapping flight.
Dennis Voeten, a paleontologist at Palacký University in the Czech Republic who was not involved in the study, said the authors presented an “elegant demonstration” of the passive flapping motion, but in terms of how this may have influenced the actual development of flapping flight in dinosaurs, Voeten believes “more research is certainly needed.”
A major concern expressed by Voeten is how the robot failed to take the actual shoulder dynamics and musculature of Caudipteryx into account. Instead, the researchers replaced these critically important anatomical structures with elastic springs. This made it “impossible to visualise any skeletal behaviour that would have accommodated such motions during life,” wrote Voeten to Gizmodo in an email. Voeten is “convinced” that the forces exerted by running can influence the movement of free limbs, but “this effect for explaining the origins of dinosaurian flight remains hypothetical,” he said.
Voeten also took minor issue with the use of Caudipteryx in the study.
“Although Caudipteryx is morphologically among the most primitive members of the dinosaur group characterised by broadly bird-like feathers, it lived in a period when dinosaur flight was already well established,” he said. “Dinosaur flight may have evolved more than one time but it is highly unlikely that Caudipteryx itself was an ancestor to any flying dinosaur.”
Paleontologist Michael Pittman from the University of Hong Kong said the new paper presented an “interesting hypothesis” that’s worth exploring in more detail.
“Our work with Laser-Stimulated Fluorescence (LSF) has revealed otherwise invisible body outline data of oviraptorosaurs, including Caudipteryx, which will help to refine the models used in this study as well as of other functional models of theropod dinosaurs,” Pittman, who wasn’t involved in the new study, wrote in an email to Gizmodo. “These LSF data would be especially useful in future analyses of the lift and thrust of Caudipteryx’s feathered wings during the suggested passive flapping process.’’
And indeed, this just happens to be the next area of focus for the Tsinghua University team, who will seek to better understand the aerodynamic forces exerted by passive flapping. But until more is known, the new study—as interesting as its methods and conclusions are—contributes very little to our understanding of the origins of avian flight.