Body fossils give us clues about what theropod dinosaurs could do and how they might have lived, but they don’t show us their actual behaviors while they were alive. On the other hand, trace fossils like footprints capture evidence of their real-life actions, which can help us improve our understanding of how these dinosaurs behaved.
Dromaeosauriformipes rarus refers to tiny, two-toed fossil footprints left by an unknown dinosaur species about 100 million years ago. These tracks were likely made by a raptor no larger than a modern sparrow. Paleontologists were surprised to discover evidence of the animal’s surprisingly large stride preserved in a rock slab found in South Korea.
A new study by UMD paleontologist Thomas R. Holtz Jr. demonstrates how theropod trackways can be used as indirect evidence of pre-avian aerial behavior, expanding the approaches available to study vertebrate flight origins. The paleontologist recreated the movements of a bird-sized raptor, shedding new light on the origin of flight.
A study led by researchers from the Chinese University of Hong Kong and Dakota State University found that Dromaeosauriformipes rarus, a small dinosaur related to birds, didn’t just run on land. Instead, it used its feathered arms to flap and achieve lift, allowing it to move faster than it could with just its legs.
This “flap running” movement is a mix between running and flying. It generates enough lift for short bursts, enabling the dinosaur to climb trees, but it’s not full flight. While microraptorines are relatives of Velociraptor and modern birds, it’s still unclear if D. rarus could fly for extended periods.
This research shows that flap running wasn’t exclusive to birds, providing new insights into the origins of flight and similar movements in prehistoric animals.
Study author and CUHK researcher Michael Pittman said, “We can now move past debate about whether pre-avian dinosaurs used their arms to help them move before flight evolved and start to uncover missing details like which species had these abilities and when and to what extent they were developed.”
To understand how Dromaeosauriformipes rarus moved millions of years ago, researchers must first rule out a few ideas. Since complete skeletons of small microraptorines are rare, Holtz used bones from 17 similar species to estimate the size and proportions of this tiny dinosaur that left its tracks in South Korea’s Jinju Formation during the Cretaceous Period.
One theory suggested that the dinosaur might have had very long legs, like a character from a Dr. Seuss book, but this seemed unlikely. Another idea was that it was simply very fast. After determining the height of the dinosaur’s hips, researchers applied a formula to calculate how fast it would need to run to create such a long stride.
Study author and Dakota State University researcher Alex Dececchi said, “Our findings suggest that D. rarus would have needed to run at about 10.5 meters per second [23.5 miles per hour] to create the trackway using solely hindlimb power. 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.”
Holtz said, “The footprints also suggest that the raptor was 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. The microraptorines that were capable of powered flight were still nowhere near as sophisticated—in terms of their flight apparatus—as modern birds. They would have been relatively clumsy.”
Studying the origin of flight can be challenging because trace fossils, which show how animals moved, are rare. Holtz noted that paleontologists almost missed the Dromaeosauriformipes rarus trackway because the footprints were so tiny and unremarkable. Now that researchers know what to look for, it could lead to more discoveries about flap running.
Holtz explained, “We have a clear idea of what these tracks look like, which may encourage others studying trackways in places like Bolivia, Madagascar, or Australia to check their photos or return to their sites to see if they have similar finds.”
He believes there’s no reason to think these trackways were only in East Asia during the early Cretaceous, and he hopes others will find more footprints.
Journal Reference:
- T. Alexander Dececchi, Kyung Soo Kim, Martin G. Lockley, and Michael Pittman. Theropod trackways as indirect evidence of pre-avian aerial behavior. PNAS. DOI: 10.1073/pnas.2413810121
