Two bird-sized dinosaurs could glide, but time in the skies was short-lived

Oct. 22 (UPI) -- Often, evolutionary biologists focus on success. The planet's tremendous biodiversity offers a treasure trove of success stories, but evolutionary history is also littered with short-lived experiments and forgotten failures.

Yi and Ambopteryx are two such failures. Roughly 160 million years ago, during the Late Jurassic, evolutionary tinkering allowed the pair of bird-like dinosaurs to take the skies across Asia -- kind of. Their achievement was short-lived.

New research, published Thursday in the journal iScience, suggest Yi and Ambopteryx were unable to actually fly. Despite their bat-like wings, analysis of the duo's skeletal and musculature makeup suggests the two dinosaurs species were only able to glide short distances -- and clumsily.

To more precisely characterize the physiology of Yi and Ambopteryx, researchers relied on an imaging technology called laser-stimulated fluorescence.

"It allows you to see soft tissue structures, skin, feather impressions and the like, that are not visible under white light," first author Alex Dececchi, an assistant professor of biology at Mount Marty University, told UPI in an email.

"It can help to bring out the fine details which are so important in reconstructing the life history of these creatures," said Dececchi, an assistant professor of biology at Mounty Marty University.

Having gathered information about the arrangement and distribution of tissue and mass across the bodies and wings of Yi and Ambopteryx, Dececchi and his research partners turned to a series of math models designed to decipher the physics of flight.

First, scientists ran a simple model that showed flight was at least in the realm of possibility for the two bird-like dinosaurs. The pair didn't have too much weight over their wings, for example.

"Then we used mathematical models, based on living flying organisms under the conditions that these dinosaurs would have experienced, to see things like: how fast they would need to go to generate enough lift to keep them in the air, how much power they would need fly by flapping, how big a turn radius they would need if they had to bank, etc.," Dececchi said.

"We also compared these values to both modern gliding animals, other fossil gliders and to other dinosaurs and early birds who are thought to have some flight capabilities to see their relative flight worthiness," he said.

The models showed that at high speeds, but not impossibly high speeds, the two dinosaurs could achieve a glide, which explains why the pair sported large membranous "wings."

"But the models also showed that these guys could not take off from the ground by flapping," Dececchi said. "As well, they could not generate enough power to sustain flapping flight if they did somehow take off. Thus, we can say that Yi and Ambopteryx could have used their wings to glide but not to flap fly."

The fossil record suggests Yi and Ambopteryx went extinct not long after they developed their gliding abilities. Researchers suspect the arrival of real flying birds helped drive the duo's decline.

"There were also pterosaurs flying at that time and some of them were hawk to eagle size, likely acting as a predator," Dececchi said. "So with predators that were better at flying on one side and the rise of competitors for food and resources who were better from the other, these creatures had nowhere to hide and went extinct."

Yi and Ambopteryx were effectively squeezed from the skies.

Of course, gliding itself is not an evolutionary dead end. To this day, a variety of species glide. But Dececchi suggests most successful gliders have been able to occupy small niches, and limit competition with birds or bats.

"Think of flying squirrels, colugos or sugar gliders and other gliding mammals, these animals are nocturnal to minimize the competition and predation from birds. These creatures are also simply better at it, which also helps," Dececchi said. "I think this group [Yi and Ambopteryx] simply didn't have the time to evolve a more efficient and effective gliding flight before birds came and pushed them out."

Researchers suggest their work is a reminder of the value in studying evolutionary failure. By analyzing the different ways dinosaurs tried and failed to take to the skies, researchers can begin to pinpoint the evolutionary solutions that helped dinosaurs and birds get airborne -- solutions that had long-term evolutionary impacts.

"I think this paper is a piece of the bigger puzzle of how over a 20 to 30 million year span the air space became a battlefield for which groups were going to seize what roles," Dececchi said. "And the outcome of those battles has shaped how our modern ecosystems look and function."