Would we know about flow-through lungs if birds were extinct?

March 26, 2023

Last night a thought occurred to me, and I wrote to Matt:

If birds had gone extinct 66 Mya along with all the other dinosaurs, would it ever have occurred to us that they had flow-through lungs? Is there — can there be, outside of amazing soft-tissue preservation — any way for bone fossils to tell us about this?

(Yes, we have evidence for air-sacs in the pneumatization of vertebrae and other bones, but I doubt that would have led us to the idea of the flow-through lung. I’m not even convinced it would have led us to the idea of air-sacs, if we didn’t have extant birds as a model.)

Matt wrote back and gave me permission to write up his reply into an SV-POW! post, which you are now, obviously, reading. Here’s what he said.

No, we’d have no idea about the flow-through lungs from fossils.

In fact, it’s particularly bad for birds. Big saurischian dinosaurs had lots of postcranial skeletal pneumaticity (PSP), and some extant birds have a lot of PSP, but most Mesozoic birds have limited to zero diagnostic PSP. A few have some external foramina on the vertebrae that might be pneumatic, but might just be lateral foramina for the equatorial arteries. It doesn’t help that most Mesozoic birds are smashed flat and often have other elements overlapping the vertebrae — most often the proximal portions of their own ribs.

So ironically, even if we somehow came up with the stacked notions that (1) PSP implied air sacs, and (2) air sacs implied flow-through lungs, we’d be much more likely to infer flow-through lungs in Diplodocus and Tyrannosaurus than in Archaeopteryx or most other Mesozoic birds.

But wait, it gets worse! The work by Colleen Farmer, Emma Schachner, and colleagues that demonstrated unidirectional flow in the lungs of crocs, monitor lizards, and iguanas would presumably still get done, but those animals have flow-through lungs without PSP and without particularly elevated metabolisms (although monitors are trying hard). Without the example of birds showing us how that primitive flow-through system can be further refined and supercharged to power tachymetabolism, we’d still learn of flow-through lungs, but we’d have no reason to connect them to PSP or any particular metabolic strategy.

I’ve probably mentioned this before, but it really irks me that we assume that birds are the pinnacle of lung evolution. Why? Birds survived the K/Pg extinction because they were small and could hide and eat seeds and grubs for a while, not because they had better lungs than everything else (otherwise mammals, lizards, etc. would have done even worse). To me it would be a heck of a coincidence if the one group of ornithodirans that survived — for reasons unrelated to lung function — just happened to have the most efficient lungs. It’s always been tantalizing to me that extant birds start out with 12 embryonic air sacs, which through development usually merge into the usual 9 (unpaired clavicular, and paired cervical, anterior thoracic, posterior thoracic, and abdominal sacs). This seems like an embryonic footprint of a greater diversity — and possibly even a greater complexity — of respiratory anatomy in the ancestral ornithodiran, saurischian, or theropod (or all of the above).

11 Responses to “Would we know about flow-through lungs if birds were extinct?”

  1. llewelly Says:

    seems to imply that if paleontologists ever do get well-preserved air-sac tissue from sauropods, it might be really surprising … and confusing!

  2. Matt Wedel Says:

    To be honest I have no idea how that would even happen. The air sac “tissue” is an extremely thin-walled balloon of epithelium, which is filled with air. How it wouldn’t deflate and collapse into goo in the first couple of days after the animal died, regardless of depositional scenario, is beyond me. I’ve joked that what we need is for a small theropod or sauropod to have somehow inhaled enough mud to fill its respiratory system, and then staggered far enough to be buried in a lens of sand, and then maybe, if we were alert, we might see something. A sand cast of the respiratory system in mudstone would be better from a preparator’s standpoint, but sand might not make it through all the little tubes in the lungs, which can be incredibly small.

    I don’t want to say we’ll never get good direct evidence of air sacs in dinosaurs — the universe is endlessly surprising — but I’m not, ahem, holding my breath.

    BUT your point that preserved a preserved sauropod respiratory system would be both surprising and confusing is well-taken!

  3. llewelly Says:

    I don’t have any good ideas about how it would happen either, but I admit I didn’t think carefully about that part before I commented.

    I take your point that a cast would be better for preparation, and probably better for understanding organization and so forth. I suppose fine loess dust *might* be better than sand, or maybe not, but anything seems really unlikely.

  4. arctometatarsus Says:

    “To be honest I have no idea how that would even happen”

    I mean, sauropod gets trapped in amber.



  5. Matt Wedel Says:


    I mean, I am 100% down for the sauropod-sized chunk of amber. Even one that would trap a hatchling sauropod would be nice. But as long as we’re dreaming, how about a geriatric world-record Maraapunisaurus? It leaned up against some kind of 100m-diameter redwood (that we don’t know about because it’s far too big to fossilize), fell asleep…for a year or so, and just never woke up.

  6. llewelly Says:

    For some reason this reminds of one of Darren Naish’s articles on strange giraffe deaths, among which was a giraffe (or maybe more than one) which had gotten its head stuck in the crotch of a high tree branch, and died that way.

    Now if a Maraapunisaurus could die the same way, and *then* get covered by amber …

    : )

    I mean, as long it’s wild speculation started by my not-too-carefully thought out comment.

    link to the internet archive version of Darren’s old blog article … gosh, I hope the wayback machine survives the current lawsuit against the internet archive.


  7. Jura Says:

    I’m going to push back a little here. If we were able to deduce that these bony excavations represented air sacs then we would almost have to infer that those air sacs were extensions off the lungs. We would still have an understanding of paranasal, paratympanic, and tracheal air sinuses / sacs, so we would still know that the diverticula responsible need an air space to already be present, and the lungs would be the most logical location.

    Further, if the FarmLab’s discoveries still happened (something I am less sure of. Avian chauvinism has played a large role in understanding reptile respiration), then our assumption that these air sacs were coming from the lungs would only be strengthened as we would still know that reptile lungs are partitioned into a respiratory (lung) portion and an avascular, sac-like portion that is responsible for respiratory drive. It wouldn’t be that difficult to connect that this sac and its offshoots would likely extend into the skeleton. The function of these sacs in respiration and the knowledge of sacs that don’t contact the skeleton would still elude us, though, which may affect inferences of activity levels in fossilized birds (small pulmonary area might suggest low respiratory capacity).

    Stiffness of the lungs and the connection to the vasculature would be difficult to figure out (we still don’t know much about the latter for extant reptiles), but if we at least reached the point of recognizing these excavations as air sacs, then I think we wouldn’t be too far off from where we currently are.

  8. Matt Wedel Says:

    All fair points!

    I’m also mindful of Emma Schachner’s work pointing out that if the parapophyses of the dorsal vertebrae are very ventral to the diapophyses, then the ribs must incise the lungs, and therefore the lungs must be dorsally attached and can’t be sliding around in the chest cavity (like our own lungs, and like those of crocs), and that at least suggests a ventilatory system different from that of mammals or crocs. That was a forehead slap moment for me, one of those things that is super obvious after it’s been pointed out. Those papers:

    Schachner, E.R., Lyson, T.R. and Dodson, P., 2009. Evolution of the respiratory system in nonavian theropods: evidence from rib and vertebral morphology. The Anatomical Record 292(9): 1501-1513.

    Schachner, E.R., Farmer, C.G., McDonald, A.T. and Dodson, P., 2011. Evolution of the dinosauriform respiratory apparatus: new evidence from the postcranial axial skeleton. The Anatomical Record 294(9): 1532-1547.

    All of this makes me think that without birds as models, we might suspect that something interesting was going on with non-avian dinosaur respiration, but without a lot of certainty about the specifics. Which is not a million miles out from where we actually are now, even with birds as models. Birds show us how respiration in non-avian dinos and other ornithodirans might have worked, but there are enough differences, and enough diversity of form through the clade, that the specifics remain tantalizingly out of reach for now.

  9. Andrea Cau Says:

    Amber is not that good for fine preservation: in most cases, what we have is just a cast of the outer body filled with a macerated carbon film. The nice preservation of Scipionyx intestine and blood vessels suggest that authigenic lithification under certain underwater conditions is the best hope for some soft tissues to be preserved in the fossil record. A sauropod preserved in some Pietraroja lagoon-like condition might eventually preserve some abdominal air sac near the intestine…

  10. Ben B Says:

    Those Schachner papers spend a lot of time talking about the impact of transverse processes morphology on the likely respiratory mechanism in use. I went looking online for good figures identifying the transverse process but came up empty.

    Just curious, any reason why Tutorial 2 on vertebral anatomy (https://svpow.com/2007/10/04/tutorial-2-basic-vertebral-anatomy/) doesn’t identify those–maybe its just not thing on Sauropods, or just less important?

  11. Matt Wedel Says:

    The transverse processes are the supports for the diapophysis. A lot of times we just skip the term “transverse process” and refer to the diapophysis specifically. Arguably though for a tutorial we might have mentioned that!

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