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Bat-winged dinosaurs were clumsy fliers

It is only known that two species of dinosaurs had wings made of stretched skin, such as bats. But unlike bats, these dinos were only able to glide in a limited way between trees, a new anatomical analysis suggests. Glider-wing gliders turned out to be a stalemate in the path of flight evolution, the researchers say.

“They’re a failed experiment,” says Alexander Dececchi, a paleontologist at Mount Marty University in Sioux Falls, S.D.

Leaves with feathered wings, instead of membranous wings, begin to appear in the fossil record only a few million years after bat-winged dinosaurs. Those feathered fliers may surpass gliders in their evolutionary niche, Dececchi and colleagues suggest Oct. 22 in iScience.

Yi qi and Ambopteryx longibrachium were crow-sized dinosaurs that lived about 160 million years ago (SN: 29/04/15). They were distant cousins, both belonging to a strange group of dinosaurs known as scansoriopterygids. However, unlike other scansoriopterygids, these two species wore large ones with membranes, the thin skin extending between the elongated arm bones.

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Scansoriopterygids were a branch of theropod dinosaurs, the same group that includes giants such as the Tyrannosaurus rex, as well as the ancestors of birds. Thus, recent discoveries of two bat-winged theropod dinosaurs have shaken long-standing ideas about the evolution of flight in birds. Scientists had thought that this path, although a bit of a circuit, focused on variations of a single basic, bird-like body plan.

But it wasn’t clear if Yi and Ambopteryx were really skilled at flying, like being able to launch from the ground or flap their wings. To assess the flight ability of dinosaurs, Dececchi and colleagues used laser-stimulated fluorescence images, which can capture details of soft tissues such as membranes or cartilage in fossils, to reanalyze the anatomy of Yi and Ambopteryx. The team made new estimates of the weight, shape of the wings, and the wingspan of the reptiles, and then simulated how those characteristics could translate into flapping, gliding, or launching.

New analyzes confirm that these two dinosaurs had wing structures very different from those of feather flyers, demonstrating that these two flight strategies evolved independently of each other. Yi and Ambopteryx, for example, had elongated bones of the forelimbs between the membranous wings, as well as a special wrist bone to help support the membrane. The skeletons of bird wings, on the other hand, consist of elongated metacarpals, similar to finger bones.

This evolutionary branching was ultimately a failure, says Dececchi. The team found that Yi and Ambopteryx were only able to plan short distances from one tree to another. Their mass-to-body-to-wings ratio was a little too high for them to hold high to plan longer distances.

As for active flight, dinosaurs lacked other attributes possessed by powerful fliers, whether they were feathered ones like birds or membrane wings like pterosaurs and bats. For example, the orientation of dinosaur wings suggests that they were not designed to beat in rotation. And the chest of the dinos shows no evidence of places where the key muscles of flight were made in modern birds (SN: 3/13/18).

Just a few million years after Yi and Ambopteryx, about 150 million years ago, the ancient bird Archeopteryx appears in the fossil record. Although Archeopteryx wasn’t a particularly large aviator either, Dececchi says, he was a stronger glider and could possibly flutter his wings a bit. Subsequent variations on his body theme continued to improve the flying ability of the dinosaurs, he said.

That bat-winged dinosaurs were not large aviators is no surprise, says paleontologist Jingmai O & # 39; Connor of the Chinese Academy of Sciences in Beijing, who and his colleagues described Ambopteryx in a study conducted in Nature in 2019. The & # 39; Connor points out that no fossil record appears in the fossil record after the Jurassic period, so it already seemed likely that they were not a successful group of dinosaurs. Still, she says, it's important to do this kind of careful, quantitative analysis to assess flight capability, even though "this group's fossil record is really bad." With so few fossils, many poorly preserved, these calculations will be based on "very poor data."

Connor says what makes animals stand out is "experimenting with different flight modes," he adds. It is clear that birds were not the only flying dinosaurs and these fossils reveal that the flight itself, whether gliding or motorized, evolved several times between them.

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