About 100 million years ago, a sparrow-sized microraptorian (a long-lost cousin of modern birds) sped across the landscape of modern-day South Korea, leaving behind a mysterious set of footprints with surprisingly long strides. Now, paleontologists suggest that the species used a unique type of aerial motion: flap-running.
Published in a study in the journal PNAS in October, the findings may improve our understanding of the origins of flight.
“It had only two toes on each foot, and that’s what told us it was a raptor dinosaur,” says Thomas R. Holtz, a carnivorous dinosaur specialist at the University of Maryland and a co-author of the study. “As all Jurassic Park fans know, raptors walked on two toes, and their killing claw was lifted off the ground and didn’t make a toe print when they were walking. But what was curious about the trackway was the distance between the footfalls.”
Strange Microraptor Strides
Normally, scientists calculate the speed of a fossil animal using the animal’s hip height and the distance between the animal’s footprints. In the case of the microraptorian fossil, however, the speed calculated with these parameters was phenomenally — and unrealistically — fast.
“If we scaled it up to the size of a larger animal, it would run much faster than a cheetah or a pronghorn,” Holtz laughs. “If, in fact, it was an ordinary run.”
The research team considered other possibilities: Maybe there were tracks missing. Maybe the raptor had stilt-like legs akin to a Dr. Seuss character, or perhaps there was some form of aerial locomotion going on.
The latter possibility seemed like the most likely, specifically because small-bodied raptors are known to paleontologists for their long arm, leg, and tail feathers — delicate features preserved in ancient lake deposits in China.
“The team’s consensus now is that we’re seeing an animal that’s suspended in the air between these footprints,” Holtz explains. “And so, technically speaking, it’s not a true run, and that’s the reason why the math wasn’t working out properly.”
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A Leftover From Flight, or a Precursor?
It’s not a true run, it’s a “flap-run,” which is exactly what it sounds like: The microraptorian might have ran while flapping its feathered arms to achieve some lift, which would have given it greater speed.
“Ever since we discovered that Microraptor and its cousins had wings, researchers have suspected that they had some aerial locomotion, including possibly powered flight,” Holtz says.
In fact, the study authors speculated that the tracks in question could have preserved the moment a microraptorian landed or accelerated to take off.
The question of flight, however, led to a contentious disagreement within the paleontology community: Did microraptorians and birds inherit aerial locomotion from a flying common ancestor, or did they develop their aerial locomotion independently?
Even if the former one day proves to be true, it doesn’t necessarily negate the flap-running theory; the species in the study may have evolved away from flight while retaining some of its features, like an ostrich.
In the case of the latter, however, the microraptorian could have had some of the mechanics that evolved into flight in its descendants. This could give us clues about the “basic equipment” that the ancestors of birds may have also had, Holtz says.
The Origin of Flight
Since flight doesn’t leave behind traces the way running does, its evolutionary origin is exceedingly difficult to trace. Bone fossils almost never capture an animal in motion. And trace fossils tend to focus on more terrestrial forms of motion. This makes the South Korean fossil all the more special, even though the paleontologists’ conclusions are just a theory.
“Trackways are the remains of living things, rather than dead things,” Holtz says. “They were made when the animal’s heart was beating, and its lungs were breathing, and so forth. So they tell us a story that the bones themselves won’t tell.”
Holtz also stresses that evolution doesn’t have a long-term goal beyond survival. In other words, feathers didn’t exist so that an animal could become a bird. At first they existed for other reasons, and various animals used them in different ways, whether for flap-running, flying, or some other function.
This means that, at the very least, the fossil “might be documenting how animals were using feathers in a way that we don’t see anymore,” he says.
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