Evolution of Flight: Before Birds, There Were Small, Bat-Like Dinosaurs

Evolution is an experiment.

The massive wandering albatross has the longest wingspan of any living bird — a feathered giant that swoops down to the ocean with dazzlingly white wings tipped in inky black. This bird, like all birds, is descended from theropod dinosaurs. However, a study released Wednesday demonstrates that not all theropods evolved to become feathered, soaring birds like the albatross. Some theropods were more of an evolutionary experiment.

Enter the Ambopteryx longibrachium. Paleontologists describe this newly discovered species of scansoriopterygid dinosaur in Nature as a 200-gram, bat-like creature who lived 163 million years ago in present-day China. Its membrane wings, supported by a long, pointed wrist bone, make this species unique among theropod dinosaurs, and its existence shows that feathered wings were not the only way to go airborne during the period of dinosaur-bird transition.

These sort of wings have been seen before in non-dinosaur flying vertebrates, like bats and pterosaurs, who evolved the ability to fly independently from birds. But this type of wing hasn’t been absolutely confirmed in theropod dinosaurs before. First author Min Wang, Ph.D., a researcher at the Chinese Academy of Sciences, tells Inverse that this means the Ambopteryx represents a split between its clade and the avian-lineage of theropods.

Fossilized evidence of a 163-million-year old non-avian theropod has been found in China. 

Min Wang, Chinese Academy of Sciences 

It’s obvious now that the latter group’s experimentation with feathered-wing flight was the more successful route to sky-domination and survival. The two groups, Wang explains, “underwent very different paths to become volant.” Avian-theropods went on to live through the asteroid collision that struck Chicxulub, Mexico — and the Ambopteryx, well, didn’t.

“We think this animal could have some extended gliding ability,” Wang says. “But flapping flight as in birds is less likely.”

But its existence also contributes to an image make-over for the scansoriopterygids, which Wang and his co-authors describe as “one of the most bizarre clades of non-avian theropods.” The Ambopteryx identified here is the most complete specimen of this extinct family ever found — and as of now, that group only physically exists in the form of five well-preserved fossils. In 2015, scientists discovered another species within the family labeled Yi qi. This specimen, which possessed an unusual elongated third finger, was the first clue that these dinosaurs weren’t just little tree-climbers, as was suspected. Instead, they could fly — sort of.

Artists representation of how these dinosaurs glided. 

Min Wang

The Ambopteryx fossil specimen clearly shows the bony elements and epidermal structures that indicate it had membranous wings. In turn, Wang says its existence provides strong evidence that Yi qi could also fly. With its avian-like body plan — elongated forelimbs and a short tail — and wings, this Cretaceous-period dinosaur represents a sort of creature scientists didn’t know could exist.

Feathered wings gave rise to the success of modern birds. But for a time, air space was shared by incredibly small dinosaurs who had no time for plumes.

Powered flight evolved independently in vertebrates in the pterosaurs, birds, and bats, each of which has a different configuration of the bony elements and epidermal structures that form the wings. Whereas the early fossil records of pterosaurs and bats are sparse, mounting evidence (primarily from China) of feathered non-avian dinosaurs and stemward avians that derive primarily from the Middle-Upper Jurassic and Lower Cretaceous periods has enabled the slow piecing together of the origins of avian flight. These fossils demonstrate that, close to the origin of flight, dinosaurs closely related to birds were experimenting with a diversity of wing structure. One of the most surprising of these is that of the scansoriopterygid (Theropoda, Maniraptora) Yi qi, which has membranous wings—a flight apparatus that was previously unknown among theropods but that is used by both the pterosaur and bat lineages. This observation was not universally accepted7 . Here we describe a newly identified scansoriopterygid—which we name Ambopteryx longibrachium, gen. et sp. nov.—from the Upper Jurassic period. This specimen provides support for the widespread existence of membranous wings and the styliform element in the Scansoriopterygidae, as well as evidence for the diet of this enigmatic theropod clade. Our analyses show that marked changes in wing architecture evolved near the split between the Scansoriopterygidae and the avian lineage, as the two clades traveled along very different paths to becoming volant. The membranous wings supported by elongate forelimbs that are present in scansoriopterygids probably represent a short-lived experimentation with volant behavior, and feathered wings were ultimately favored during the later evolution of Paraves.
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