Researchers Solve Mystery of ‘Dinky’ Dinosaur’s Unusually Long Stride

The tiny, two-toed fossil footprints made by an unidentified dinosaur species some 100 million years ago and preserved in a slab of rock in South Korea are thought to have been made by a raptor no bigger than a modern sparrow. What stumped paleontologists was the animal’s enormous stride; the unknown raptor’s footprints had larger gaps between them than expected for such a diminutive dinosaur.

“This guy is dinky—one of the smallest dinosaurs that we have fossils of,” said University of Maryland paleontologist Thomas R. Holtz Jr. “These tracks were a puzzle because their footprints are so tiny, but they're so far apart.”

Holtz, a principal lecturer in the College of Computer, Mathematical, and Natural Sciences’ Department of Geology who studies carnivorous dinosaurs, co-authored new research in the journal Proceedings of the National Academy of Sciences (PNAS) that solves this mystery of the tracks known as Dromaeosauriformipes rarus. Led by researchers at the Chinese University of Hong Kong (CUHK) and Dakota State University, the study revealed that the animal—a species of microraptorine related to the ancestors of birds—didn’t merely run on land. It flapped its feathered arms to achieve lift, which allowed it to travel faster than if it had relied solely on the strength of its legs.

Known as “flap running,” this form of movement falls somewhere between running and flying. It generates enough aerodynamic force to lift an animal off the ground in bursts—enabling them to run up a tree, for example—but falls short of full-powered flight. Though microraptorines like the one that left the tracks are the cousins of Velociraptor as well as modern birds, it’s unknown whether they could fly for longer periods of time.

Despite the lingering questions, this new research in PNAS confirms that prehistoric flap running was not limited to birds, helping to shed light on the origin of flight—or at least flight-adjacent movements.

“We can now move past debate about whether pre-avian dinosaurs used their arms to help them move before flight evolved,” said study author and CUHK researcher Michael Pittman, “and start to uncover missing details like which species had these abilities and when and to what extent they were developed."

To retrace the D. rarus footsteps of millions of years later, researchers had to rule out a few other theories first. Because fully intact skeletons of tiny microraptorines are hard to come by, Holtz used the bones of 17 comparable species to reconstruct the proportions of the particular animal that left its mark on South Korea’s Jinju Formation during the Cretaceous Period.

One theory was that the dinosaur could have had long, stilt-like legs like a Dr. Seuss character, Holtz said, but that seemed unlikely. The other theory was that this animal was just extremely fast. Once researchers determined the animal’s hip height, they used a formula to calculate the speed it would have needed to run to achieve such a long stride.

“Our findings suggest that D. rarus would have needed to run at about 10.5 meters per second [23.5 mph] to create the trackway using solely hind-limb power,” said study author and Dakota State University researcher Alex Dececchi. “The relative speed shown by our tracks is higher than any living running animal, including ostriches and cheetahs.”

Because this also seemed highly improbable, the researchers proposed “that the trackway was produced at lower speeds, with the dinosaur elongating its stride length using the aerodynamic force generated by flapping its feathered arms,” Dececchi said.

Holtz said the footprints also suggest that the raptor was in the midst of taking off or landing.

“It’s kind of like when a plane is coming down and bounces a little bit on the runway before slowing down,” Holtz said. “The microraptorines that were capable of powered flight were still nowhere near as sophisticated—in terms of their flight apparatus—as a modern bird. They would have been relatively clumsy.”

The origin of flight can be tricky to study because of the rarity of “trace” fossils that capture a living animal’s movements. Holtz said that paleontologists nearly overlooked this D. rarus trackway because the footprints were so small and unassuming.

“We now have a search image of what this type of track looks like, which could inspire people looking at trackways in Bolivia or Madagascar or Australia to look back at their photos or return to their field sites to see if they might have some, too,” Holtz said. “There’s no reason to suspect these trackways were only in East Asia during the early Cretaceous, so we’re hoping that people will look at their footprint slabs and find something else.”