Researchers made a robotic dinosaur skeleton run on a treadmill. Here's what they learned - CNN

Gliding is a form of flight that was associated with both dinosaurs and birds, but there's mounting evidence that both would flap their wings for flight, too.

How did that evolve?

The study, published Thursday in the journal PLOS Computational Biology, suggests that winged two-legged dinosaurs may have learned to flap their wings as a side effect of running.

The researchers looked at Caudipteryx, a primitive dinosaur that didn't fly but had feathered "proto-wings" that were essentially like half-wings, weighed only about 11 pounds and could run up to 18 miles per hour.

They were about the size of peacocks, reaching 3 feet tall, and lived during the early Cretaceous Period, between 122 million and 130 million years ago. The name means "tail feather" because the end of its tail looked like a fan of feathers.

This 'enigmatic' feathered dinosaur didn't fly like modern birds

Researchers modeled and analyzed how running would effect Caudipteryx's proto-wings. If it was running between 5.5 and 13 miles per hour, vibrations would have caused the wings to flap, according to the study.

The researchers explored their idea further by building a robot of a life-size Caudipteryx skeleton and having it run on a treadmill. They were able to see the flapping motion of the wings at different speeds, as they had calculated.

A Caudipteryx robot built for testing passive flapping flight.

They also placed artificial wings on a young ostrich, a good modern stand-in for Caudipteryx, and saw the wings flap as it ran around. Further experiments with longer, larger wings showed that the running speed afforded a greater force of lift.

"Our work shows that the motion of flapping feathered wings was developed passively and naturally as the dinosaur ran on the ground," said Jing-Shan Zhao, study author and associate professor of mechanical engineering at Tsinghua University in China, in a statement. "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."

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