In 2018, a South Korean team described the smallest known footprints attributed to a carnivorous dinosaur – likely a Microraptor. The footprints were discovered near the southern town of Jinju. Measuring no more than a centimeter, the imprints were fossilized after the animal passed through the muddy shores of a lake over 100 million years ago. Their discoverer, Kyung Soo Kim (Chinju National University of Education, Gyeongnam), noted that their spacing suggested the creature, a biped the size of a sparrow, could have reached an impressive speed of 40 km/h (about 10.5 m/s).

In a new study published Monday, October 21 in PNAS, he collaborated with an international team to determine how such a small animal could run so fast. The conclusion? It might not have actually reached such speeds; rather, its long stride could have been the result of using wingbeats to aid propulsion. This mode of locomotion could shed light on the origins of flapping flight in birds – even though Microraptors do not belong to the lineage of dinosaurs that gave rise to birds.

To reach this conclusion, the researchers used a tool borrowed from fluid mechanics: a version of the Froude number, which expresses the relationship between an animal's speed and the size of its limbs that are supposed to propel it, for a given gravitational force. In the case of the Korean footprints, the estimated Froude number was 238, nearly twice that of the cheetah, the fastest land animal. Meanwhile, the ostrich is measured at 54 and humans at 15, with small dinosaurs just slightly above us.

A Microraptor's bipedal movement at 10.5 m/s therefore seemed unrealistic from a biophysical point of view – it would have required leg muscle strength unheard of in the animal kingdom. However, Microraptors are also known to have feathered forelimbs. By introducing wings into the equation, "we found that the creator of the footprints could move at a much more plausible speed (for example, around 6 m/s) to produce the observed tracks, provided its running was assisted by its wings," explained Michael Pittman (Chinese University of Hong Kong).

Did it move by leaping, followed by a kind of gliding flight? "What is likely, if it wasn't trying to take off, is that flapping its wings gave it a quick boost allowing it to accelerate much faster than it could without wings, possibly to escape predators," said co-author Alexander Dececchi (Dakota State University, Madison). This behavior would have put it on an evolutionary take-off path. At a certain speed, the wings would generate enough lift to overcome gravity. "I calculated that a larger specimen – around 1 kilogram compared to our Microraptor's 25 grams – could take off by flapping its wings at this speed of 10 meters per second," said the researcher. For species weighing 10 to 15 kilograms, which are known to neither fly nor glide, flapping could provide an acceleration boost of around 10% to 25%.

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