How Pterosaurs Filled Their Lungs

Imagine a balloon inside a cask. There’s an opening at the top to blow air into, but here’s the problem – the balloon hugs the wooden walls of the chamber. There’s nowhere for the balloon to expand into and no way for old air to be pressed out. The sides of the barrel won’t move. But there is another way to move air in and out of that balloon. If you partitioned the balloon in the front half of that barrel with a disc of wood, you could make a piston to pump forward and backward, expanding and contracting that bag of air. And if you can envision that, you can wrap your head around how pterosaurs breathed.

Commonly called “flying reptiles”, pterosaurs were close cousins of dinosaurs and the first vertebrates to take to the air. They did so on wings of skin stretched between their bodies and extraordinarily-elongated fourth fingers. The first of their kind were relatively small, but, over time, their ranks swelled to include giants such as Quetzalcoatlus with wingspans over 33 feet across. And as with many outstanding extinct organisms, paleontologists are trying to puzzle together the biological basics of these animals. Among the major mysteries – how did big pterosaurs fill their lungs with air?

Answering the question is impeded by the fact that no one knows what pterosaur lungs looked like. Such soft tissues have not been found just yet, and may never be. But, as Sonoma State University paleontologist Nicholas Geist and colleagues point out in a new Anatomical Record paper, skeletal anatomy offers a few clues as to the bony constraints pterosaurs faced as they inhaled and exhaled.

Geist and colleagues focused on large pterosaurs – species with wingspans over 9 feet across. That’s because large pterosaurs had relatively rigid torsos. Some of their vertebrae fused into a stiffened rod of bone that was reinforced by “a dense latticework of mineralized tendons”, and the large ribs at the front of large pterosaur chests fused to their supporting vertebrae to create a stiff structure the researchers call a synthorax. This strengthened the skeleton and reduced the need for heavy muscles, coming with the cost of highly-reduced torso flexibility. No wonder Geist and colleagues titled their paper “Breathing in a Box.”

All that skeletal fusion limited the ways in which pterosaurs could have filled their lungs. The ribs around their lungs couldn’t flex inward and outward to help pump air. And despite the fact that pterosaurs had a system of air sacs invading their bones – much like birds and other dinosaurs on the saurischian line – large pterosaurs wouldn’t have been able to breathe the way modern birds do. Birds rely more on up-and-down motions of the sternum to expand and contract their complex system of lungs and air sacs, but the corresponding bones in large pterosaurs were too rigid to allow this.

A model of pterosaur breathing based on piston-like movements of the liver. From Geist et al., 2014.
A model of pterosaur breathing based on piston-like movements of the liver. From Geist et al., 2014.

The answer to the problem might rest among different living relatives of pterosaurs – the crocodylians. Alligators and crocodiles expand and contract their lungs by way of what’s called a hepatic piston. The liver acts as a barrier between the lungs and the viscera – like the disk of wood in the barrel analogy – and can be retracted to squish the innards down to make room for an alligator’s lungs to expand. Muscles on the animal’s side can then act on the gastralia – belly ribs – to bring the liver back into position and compress the lungs for exhalation. Geist and colleagues suggest that this method could have worked for large pterosaurs, too, moving the guts instead of the bones.

This way of breathing may not have been a specialization of the largest pterosaurs. While they had more mobile chest bones, Geist and coauthors point out, some articulated skeletons of small pterosaurs like Rhamphorynchus hint that these little fliers kept their torsos relatively rigid. They may have established the piston-pump method early, and, if this was the case, then the trait may have been part of what allowed pterosaurs to reach truly gigantic sizes. What started off as an evolutionary option ended up as an essential feature of the largest animals ever to soar over the planet.

Reference:

Geist, N., Hillenius, W., Frey, E., Jones, T., Elgin, R. 2014. Breathing in a box: Constraints on lung ventilation in giant pterosaurs. The Anatomical Record. 297: 2233-2253. doi: 10.1002/ar.22839

5 thoughts on “How Pterosaurs Filled Their Lungs

  1. Based on the name, the artist Mark Witton would seem to also be the British paleontologist who has published articles about this line of pterosaurs. Based on his (artist’s) website, one gets the impression he makes more money sidelining as an artist and popular writer than from his research/teaching position (at U. of Portsmouth). Somehow, this makes me simultaneously happy and sad.

  2. Ian: the size and capacity of the stomach in large pterosaurs is not known in much detail (no direct fossilization), but the stomach would have been relatively modest in size simply based on the relatively small abdomen. The “empty” mass for a large azhdarchid has been estimated as 200-250 kg (work by Paul, Witton, Marden) but the launch limitations were quite a bit higher than that (still launch capable at 300 kg or so for a 10.5 meter span – see work by Palmer and myself). Since it’s highly unlikely that the large pterosaurs could fit 50-100 kg of food in their stomach, they could probably still launch after a heavy meal (i.e. even if the stomach was completely full). Steady flight was less limiting than launch, so food intake seems not to have been a limiting factor on flight.

  3. @Michael Habib
    Thanks. I understand steady flight was very calorically efficient but becoming airborne from the ground would have been fairly expensive. The balance between the calories needed to maintain itself and how much it could consume and carry I find interesting. Also, they remind me of Marabou Storks, which makes me think they were great scavangers able to soar over vast distances in search of giant dinosaurian carcasses.

  4. @ian: You do realise that the whole “azhdarchids were [specialised] scavengers” thing was debunked along with the fish eating, right? They had inflexible necks and straight bills, hardly useful to probe into dead bodies or rip flesh. Rather, much like modern ground hornbills, they stalked the ground for prey they could swallow whole.

    Istiodactylids, on the other hand, were dedicated scavengers.

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