Physics meets paleontology: The hotly debated mechanics of pterosaur flight

Size and stickiness

Contrary to what one might expect, it tends to be more difficult to take off from the water at very small sizes than somewhat larger sizes.

 

Dr. Habib described how the force something has to overcome in order to escape a liquid is significantly impacted by the total contact surface area of any creature trying to lift itself from the surface. "If you’re small, you have a lot of surface relative to your volume and weight. So the surface tension is quite large relative to your mass and to your power output," he told Ars.

 

To put it another way, he said, "everything feels stickier when you’re small. A small pterosaur is living in a stickier world, and it’s got to break that tendency to stick to the surface."

 

Using a number of analyses, the team found evidence that this little pterosaur was a "quad-launcher": It used all four limbs—both front and hind—to propel itself off the water. This type of launch can be seen in today’s mallard ducks, which use both their webbed feet and wings to push out of water. Dr. Habib described how many other aquatic birds today are limited to the size of water they land in, as many need a larger runway to lift back out of it. Not so with mallards, which have earned the nickname "puddle-jumpers" for their capacity to get out of even the smallest bodies of water.

Capability vs. action

Little aurorazhdarchid pterosaurs appear to have been similar. But again, just because they could do something doesn’t mean they did.

 

"There’s enormous selective pressure for water-launch capacity for the animals that habitually feed in or around the water," Dr. Habib said. "One might expect that pterosaurs that were feeding in and around the water would have water-launch capacity, but it’s not a guarantee."

 

He stressed that the evidence suggests that webbed feet were an important part of this little pterosaur’s ability to push off from the water.

 

Dr. Manafzadeh was impressed with the paper’s transparency. “I think these authors really went above and beyond and just showed you exactly what numbers they were using and exactly what equations they were plugging them into," she said.

 

Pterosaur limb movement has been "hotly debated," she said with a laugh. She explained that previous papers have proposed extreme movement capabilities within pterosaur shoulders—“they require the animal’s shoulder to move a lot”—but in this paper, the authors’ models are based on a conservative range of possible motion. In other words, she said, the models show that even if it used a limited range of shoulder motion, this pterosaur could still launch from the water.

Wing mechanics

The preserved wing membrane, as seen through LSF, also offered rare evidence of what Dr. Habib describes as a kind of fiber, called "actinofibrils," running through the wing. Dr. Pittman described the fiber as “a very important support structure that we find in the wings and foot webbing of pterosaurs.”

 

The presence of actinofibrils offered important insight into an aspect of flying that anything—from birds to bats, airplanes, and pterosaurs—risks at certain speeds: aeroelastic flutter. This phenomenon refers to the vibrations a wing will undergo at high angles of attack or at high speeds. A very limited amount of flutter, Dr. Habib said, is OK. But when it reaches the point of extreme vibration—aeroelastic flutter—it’s almost like a flag flapping wildly in the wind. It’s unsafe.

 

“What we’ve been able to deduce from multiple wings that have soft-tissues preserved in the fossil record is that they’re doing two primary things in order to keep the wing from fluttering,” Dr. Habib said. “One of them is active control.” He explained that pterosaur wings contain a lot of muscle, and that muscle can contract to prevent the wing from vibrating.

 

“But,” he continued, “they also have passive stiffening of the wing.” That stiffening is done through a “highly keratinized” wing surface, but also from the presence of actinofibrils. “When the wing starts to automatically curve and starts to stretch, and it’s going to flutter, those actinofibrils will go taut.”

 

Despite all of these new details, it’s difficult to know how all the features were used by the pterosaurs. Dr. Manafzadeh feels that the paper could have gone even further in that direction.

 

“I think they make a pretty convincing case for the capabilities,” Dr. Manafzadeh said, “but it’s less clear whether the animal actually did it. If you really wanted to test whether the animal did it in life, that would require other lines of evidence that aren’t in the paper.” For example, further studies could investigate the "sizes of muscle scars on the bones, [as they] might tell you how much force the muscle generated in life and in what directions," she said.

 

But the authors maintain that their "prior work on pterosaur locomotion has included measurements of bone properties that provide direct information of real in-life loads, similar to what is suggested."

 

"Given that pterosaurs are extinct," the researchers said, "we can’t really say anything more than what they were capable of. But data like muscle scar shape and size would provide additional support.”

 

Dr. Manafzadeh explained that "our capacity to go beyond saying just what extinct animals were capable of and to more accurately pinpoint what they actually did in life has dramatically improved in the past five to 10 years—so I wouldn't rule out being able to more confidently answer this question one way or another in the near future. Either way, it will be a very interesting result.”

Model flyers?

Reconstructing possible flight from creatures that lived hundreds of millions of years ago is astounding in and of itself, but this ancient flight capability may also have modern applications.

 

When Dr. Manafzadeh was asked how studying ancient articulation could help us today, she had a quick answer: robots. She is not impressed with the human range of movement, saying we "move in a kind of boring way, honestly." Modeling robot locomotion on animals—extinct and extant—would be, in her opinion, much more interesting. In this way, engineers constructing robots would be "looking at a sea of diversity of locomotor modes that have evolved over 500 million years," she said.

 

Dr. Habib mentioned that pterosaur flight capabilities might influence "next-generation rotor aircraft," i.e., helicopters. He described how helicopter blades sag when idle but rise up in motion. The performance of those blades, he noted, is improved by being flexible rather than stiff. But if the blades are too thin, there’s a very real danger of aeroelastic flutter. The Department of Defense is interested in how animals, who have much softer wings than any helicopter or airplane, solve the aeroelastic flutter problem. In other words, they are interested in the flight capabilities of birds, bats, and, yes, pterosaurs.

 

“This is not a widespread thing yet in terms of lots of DOD projects looking at pterosaurs. There is a project,” he said, “which I’m involved in.” But it has only been pitched to the DOD and hasn’t yet been formally accepted. Nevertheless, the interest is there, he said.

 

Regardless of any future utility, this sort of research offers detail rarely seen in the fossil record and opens up a world of potential new studies.

 

“If you [told] me when I was a budding young paleontology student that we would be using lasers to take photos of hidden soft tissues in dinosaurs and pterosaurs in the future, I would have asked if you were pranking me or sharing a storyline for an upcoming comic book,” Dr. Pittman said. “What I know based on LSF and other recent technological advances is that new technology will allow us to detect more and more detail from fossils, detail that we didn’t know was there and detail that we never thought we could detect.”