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.
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.
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