This is one of those things that has been sitting in my brain, gradually heating up and getting denser, until it achieved criticality, melted down my spinal cord, and rocketed out my fingers and through the keyboard. Stand by for caffeine-fueled testifyin’ mode.

Part 1: Why Study Pneumaticity

Last item first: why you should study pneumaticity. The honest reason that primarily motivates me is that pneumaticity is frickin’ cool. Air inside bones! And endlessly novel — pneumatization is opportunistic and invasive (Witmer 1997), and it never quite works out the same way twice. So every time I see a pneumatic bone, inside or out, my antennae are up, because I suspect it will have its own little quirks and oddities, any one of which might unlock something new about the morphogenetic process of pneumatization or its functional importance.

If you need something more respectable than “Whoa, dude!” to put on a thesis proposal or a grant application, how’s this: we think that skeletal pneumaticity was a key innovation for both sauropods (Sander et al. 2011) and theropods (Benson et al. 2012) [edit: and pterosaurs {Claessens et al. 2009}], but our documentation of it is very poor. For a lot of sauropod genera, we’ve only CT-scanned one or two vertebrae, often from the same quarry, usually from a single individual. For a lot more, we’ve scanned none at all. As I wrote back in 2018, “Someone just needs to sit down with a reasonably complete, well-preserved series that includes posterior dorsals, all the sacrals, and the proximal caudals–or ideally several such series–and trace out all of the pneumatic features” (link). The same principle — “crawling” one or more specimens to document everything — could be extended to address intraspecific and interspecific variation, the extent to which pneumatic traces might relate to nerve and blood vessel pathways, and ontogenetic changes. We know that vertebral pneumatization got more extensive and more complex through an individual animal’s maturation, but we don’t know much about how and when that happened, or if it ever stopped in large and long-lived individuals. I don’t know what we’ll find when people get around to doing this, but there won’t be any boring answers — indeed, much of what I thought about the early evolution of pneumaticity for the last 25 years is probably wrong.

Whether you want to work on pneumaticity or not, definitely do not make the mistake of looking at the existing literature and assuming “it’s all been done“. I’ve probably spilled more ink about dinosaur pneumaticity than anyone else alive, and I’m telling you that the field is wide open. Just off the top of my head:

• Sometimes pneumatized sauropod vertebrae have more bone than they need, because fossae are embossed into otherwise flat plates of bone that would be lighter if they lacked those fossae. What’s up with that? Does it ever happen in theropods (avian or otherwise) or pterosaurs?
• I mentioned that pneumatic bones rarely look identical under the hood. Heck, they rarely look identical on the surface. Whether it’s internal or external asymmetry, or variable laminae, or some other thing, there’s a LOT of variation. How does that small-scale morphogenetic opportunism jibe with the apparent macroevolutionary importance of pneumaticity in sauropods and theropods [edit: and pterosaurs]?
• Related: my a priori assumption is that pneumaticity was functionally important in non-avian theropods, more functionally important in sauropods (because size), and most functionally important in pterosaurs (because size x flight). That’s a wild guess, totally untested — but I’ll bet someone will figure out a way to test it, and variation vs developmental constraint seems like fertile ground for that testing.
• Also related: does skeletal asymmetry (pneumatic or otherwise) have any predictable relationship with body size, either ontogenetically or phylogenetically? See this post and this one for some related noodling (but no answers).
• For internal pneumatization, do bigger and older individuals make more chambers that are about the same size as the chambers in smaller individuals, or does the subadult level of complexity stay the same through adulthood, and the chambers get bigger but not more numerous? And is there even a single answer, or do different things happen in different lineages? These seem like fundamental questions, and I have my suspicions, but AFAIK neither I nor anyone else has addressed this. Put a pin this, it will come up again later in this post.
• Barosaurus cervicals have a more complex internal structure than Diplodocus or Apatosaurus cervicals (check out the eroded condyle of this vertebra). Is that because Barosaurus cervicals are longer? Is there a functional reason we never see crazy long vertebral centra that are camerate?
• Want to work on birds? Do some injections and dissections and see how often diverticula follow nerves and blood vessels as they develop. This idea, which has a lot of circumstantial support (Taylor and Wedel 2021), is based on a single observation from a paper published nearly a century ago (Bremer 1940).
• Heck, if you’re doing injections and dissections, just document the diverticular network in a single bird, full stop. That’s a descriptive paper right there. Bird pneumaticity is so grossly understudied that whole classes of diverticula are still being described for the first time (Atterholt and Wedel 2022).
• Rather work on sauropods or non-avian theropods? We could use a lot more work on pneumosteum (Lambertz et al. 2018), and on the histological signals of pneumaticity, in basically everything from pig sinuses to the tail of Diplodocus — especially basal sauropodomorphs and early theropods where pneumaticity was just getting up and running.
• Don’t want to do histo? CT scan something. Anything. And write it up. Especially dorsals, sacrals, and caudals — the published sample is skewed toward cervicals because they’re long and skinny and fit through the machines better. Don’t have access to a CT machine? No worries, that’s what the second half of this post is about.
• Don’t want to mess with machines at all? Crawl some skeletons — or maybe just like one fairly complete diplodocid or titanosaur — and describe the pneumatic (and maybe also vascular) features on the ventral surfaces of the vertebrae. That’s a whole class of diverticula (or maybe multiple classes) about which we know basically zip, other than that sometimes cervicals and caudals have foramina on their ventral surfaces (but not dorsals or sacrals — why?). You  might be able to get a short review paper just canvasing examples in the literature — but if you don’t go look at specimens in person, you’ll miss a lot, because these features are are rarely described or illustrated.
• Want a project you can do on the couch in your jammies? Wedel (2003) is my most-cited paper by some distance, but it’s waaay out of date. Comb the literature and write an up-to-date version of that paper just based on all the new stuff that’s been published in the past two decades. Here’s a fun starter: I made a big deal in that paper about camerate vertebrae in a then-undescribed titanosaur from Dalton Wells in the Cedar Mountain Formation. In time that critter proved to be Moabosaurus, a turiasaur and not a titanosaur. The whole idea of camerate titanosaurs needs a re-look. And I didn’t write anything about turiasaurs back then because the clade hadn’t been recognized yet. My top paper, and at this point it might as well have been scratched out on clay tablets. (Note: this is a good thing. That paper is out of date because there’s been so much progress. If it was still cutting-edge, it would mean the field of sauropod pneumaticity was dead. But still — someone go knock that thing off its perch.)

Before we go on, that list is by no means exhaustive. It is the product of long familiarity but not of long intentional thought; it’s literally the stuff that I thought of on the fly while composing this post. I could probably make it four times longer if I wanted to spend a day thinking of all the projects that are crying out to be done. Also, I’m writing quickly, and using the examples that are closest to hand, which are inevitably Wedel-centric. But many more potential projects are lurking in a quantum fuzz around the papers of Richard Buchmann, Ignacio Cerda, Federico Fanti, John Fronimos, Lucio Ibiricu, Liz Martin, Pat O’Connor, Daniela Schwarz, Nate Smith, Guillermo Windholz, Virginia Zurriaguz, and their students and collaborators. Plug those names into Google Scholar and go catch the cutting edge — so you can push it further. But also go look at all the specimens you possibly can, to build the baseline you’ll need to recognize important weirdness from background-radiation weirdness.

How to Study Pneumaticity on the Cheap

I think there is an assumption, or a perception, that you need to CT scan fossils to study pneumaticity. Access to CT scanners can be logistically complex, and expensive. Can be, not has to be. And there’s a lot of crucial work to be done without a CT machine. Let’s get to it.

1. Collaborate with a radiologist. Okay, but what if you do want to CT scan some fossils? Do what I do, and ask around to see if there’s a radiologist who is interested in collaborating. Most hospital CT machines are not busy all the time — there’s usually one slow afternoon each week, or each month. And in my experience, most radiologists are down to look at something interesting and different, like a dinosaur bone, as a break from the endless parade of concussions, degenerated lumbar discs, and cirrhotic livers.  The collaboration piece is key. I’m not a radiologist, and minimally I need a professional who can write up the machine specs and scan settings for the Materials and Methods section of the paper. But often the radiologist will see interesting things in the scan that I would have missed, or I’ll see interesting things in the scans that may turn out to be mundane features that look weird in cross-section. And I’m more than happy to trade authorship on whatever papers come out of the scans, and acknowledgement and good press for the hospital, in exchange for the professional’s expertise and time on the machines. Specific advice? Be humble, be polite. Once I’m through the hospital doors I’m not the expert in anything other than safely handling the fossils, and I make it clear that I’m there to be safe, respect their turf, let them direct the logistics, and learn as much as I can. All the radiologists I’ve worked with have been happy to share their knowledge, and curious about the fossils and what we hope to learn from the scans.

2. Use broken specimens. I’ve blogged before about how breaks and erosion are nature’s CT machines (here, here, here, and here, for starters), and I’ve favorably discussed the utility of broken specimens in my papers, but I figured broken specimens would always be distant also-rans in the quest to document pneumaticity. Then I read Fronimos (2023) — hoo boy. John Fronimos set out to document pneumaticity in a Late Cretaceous titanosaur from Texas (maybe Alamosaurus, maybe not), and he crushed it. It’s one of the best danged sauropod pneumaticity papers I’ve ever read, period, and the fact that he did it all without CT scanning anything makes it all the more impressive. And it’s not only a great descriptive paper — John’s thoughts on the evolution and function of pneumaticity in sauropods are comprehensive, detailed, insightful, and forward-looking. Up above I mentioned reading broadly to get caught up; if you work on sauropod pneumaticity, or want to, or just want to understand the state of the art, the discussion section of Fronimos (2023) is the new bleeding edge. Also, remember the pin we placed up above, on the question of whether pneumatic chambers get bigger or more numerous or both over ontogeny? With the right collection you could answer that with only broken specimens.

3. Study external pneumatic features. This has already come up a few times in this post, but let me draw the threads together here. Whether it’s documenting serial changes in pneumatization along the vertebral column in a single individual, or externally-visible asymmetry, or pneumaticity on the ventral surfaces of vertebrae, or how and whether pneumatic and neurovascular features relate to each other, there is a ton of work to be done that just requires collections access, a notebook, a camera, and time. And it lends itself to collaboration; two sets of eyes will see a lot more. (If you have the freedom to choose, ideally you might want one fairly big and strong person to manhandle the bones [safely, for the sake of the bones and the humans], and one fairly slim and flexible person to scramble up ladders and fit into odd nooks and crannies.)

4. Use publicly-available CT data. Okay, admittedly there’s probably not enough of this out there yet to use on anything other than birds (or mammals, if you’re into sinuses), but hey, we need bird studies, too. Bird studies hit twice — first because birds are interesting objects of study in their own right, and second because they’re our baseline for interpreting pneumaticity in fossils. (By quick count, I’ve figured drawings, photos, or CT scans of bird vertebrae in more than dozen of my papers, and in half a dozen cases they were vertebrae I prepped myself at home.) Of the four paths, this is the one I have the least experience with, but the new “oVert” (openVertebrate) collection on MorphoSource is a good place to start. Wet specimens may have a bit of a learning curve in terms of distinguishing pneumatic and non-pneumatic bones, and most of the extra-osseous pneumatic diverticula have probably collapsed, but with free access to CT scans of “>13,000 fluid-preserved specimens representing >80% of the living genera of vertebrates” I’ll bet people will think of plenty of cool stuff to do. Here’s the oVert trailer:

Conclusion: Let’s Roll

We need more pneumaticity studies. There is just so much we don’t know. I’ve been working on sauropod pneumaticity more often than not since 1998, and I’m stoked about how much basic descriptive work remains to be done, because I’m an anatomy geek at heart, and describing weird anatomy is deeply satisfying for me, as is reading other people’s descriptions of weird anatomy. But I’m also in despair about how much basic descriptive work remains to be done, because the answers to so many questions are still over the horizon from us, and probably will be for the rest of my life.

So please, if you’re interested, come do this work. Whether you’re a grad student at a major institution with an NSF pre-doc fellowship and several years of runway in which to do unfettered research, or just some person sitting on a couch thinking about dinosaur bones (er, like me right now), now you have some ideas to work on (or reach beyond), and some inexpensive ways to work on them. If you’re curious and want to get your feet wet before you commit, remember that you can get extant dinosaur carcasses at the grocery store, and prep and section your own pneumatic dinosaur bones at the kitchen table. There is a very accessible on-ramp here for anyone who has the time and inclination. Let’s do this thing.

References

• Atterholt, Jessie, and Wedel, Mathew J. 2022. A computed tomography-based survey of paramedullary diverticula in extant Aves. The Anatomical Record, 1– 22. https://doi.org/10.1002/ar.24923
• Benson, R.B., Butler, R.J., Carrano, M.T. and O’Connor, P.M., 2012. Air‐filled postcranial bones in theropod dinosaurs: physiological implications and the ‘reptile’–bird transition. Biological Reviews, 87(1), pp.168-193.
• Bremer, John L. 1940 The pneumatization of the humerus in the common fowl and the associated activity of theelin. The Anatomical Record 77(2):197–211. doi:10.1002/ar.1090770209
• Claessens LPAM, O’Connor PM, Unwin DM (2009) Respiratory evolution facilitated the origin of pterosaur flight and aerial gigantism. PLoS ONE 4(2): e4497. doi:10.1371/journal.pone.0004497
• Fronimos, John A. 2023. Patterns and function of pneumaticity in the vertebrae, ribs, and ilium of a titanosaur (Dinosauria, Sauropoda) from the Upper Cretaceous of Texas, Journal of Vertebrate Paleontology 43:2. DOI: 10.1080/02724634.2023.2259444
• Lambertz, M., Bertozzo, F. and Sander, P.M. 2018. Bone histological correlates for air sacs and their implications for understanding the origin of the dinosaurian respiratory system. Biology Letters 14(1): 20170514.
• Sander, P.M., Christian, A., Clauss, M., Fechner, R., Gee, C.T., Griebeler, E.M., Gunga, H.C., Hummel, J., Mallison, H., Perry, S.F. and Preuschoft, H. 2011. Biology of the sauropod dinosaurs: the evolution of gigantism. Biological Reviews 86(1):117-155.
• Taylor, Michael P., and Mathew J. Wedel. 2021. Why is vertebral pneumaticity in sauropod dinosaurs so variable? Qeios 1G6J3Q. doi:10.32388/1G6J3Q
• Wedel, M.J. 2003. The evolution of vertebral pneumaticity in sauropod dinosaurs. Journal of Vertebrate Paleontology 23:344-357.
• Witmer, L.M. 1997. The evolution of the antorbital cavity of archosaurs: a study in soft-tissue reconstruction in the fossil record with an analysis of the function of pneumaticity. Journal of Vertebrate Paleontology 17(Supplement 1): 1-76.

 

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doi:10.59350/bvpaq-czq07