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

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).