Some noodling about pneumaticity and body size

October 26, 2018

In a comment on the last post, Mike wrote, “perhaps the pneumaticity was intially a size-related feature that merely failed to get unevolved when rebbachisaurs became smaller”.

Caudal pneumaticity in saltasaurines. Cerda et al. (2012: fig. 1).

Or maybe pneumaticity got even more extreme as rebbachisaurids got smaller, which apparently happened with saltasaurines  (see Cerda et al. 2012 and this post).

I think there is probably no scale at which pneumaticity isn’t useful. Like, we see a saltasaurine the size of a big horse and think, “Why does it need to be so pneumatic?”, as if it isn’t still one or two orders of magnitude more massive than an ostrich or an eagle, both of which are hyperpneumatic even though only one of them flies. Even parakeets and hummingbirds have postcranial pneumaticity.

Micro CT of a female Anna’s hummingbird. The black tube in the middle of the neck is the supramedullary airway. Little black dots in the tiny cervical centra are air spaces.

We’re coming around to the idea that the proper way to state the dinosaur size question is, “Why are mammals so lousy at being big on land?” Similarly, the proper way to state the pneumaticity question is probably not “Why is small sauropod X so pneumatic?”, but rather “Why aren’t some of the bigger sauropods even more pneumatic?”

Another thought: we tend to think of saltsaurines as being crazy pneumatic because they pneumatized their limb girdles and caudal chevrons (see Zurriaguz et al. 2017). Those pneumatic foramina are pretty subtle – maybe their apparent absence in other sauropod clades is just because we haven’t looked hard enough. Lots of things have turned out to be pneumatic that weren’t at first glance – see Yates et al. (2012) on basal sauropodomorphs and Wedel and Taylor (2013b) on sauropod tails, for example.

Back of the skull of a bighorn sheep, showing the air spaces inside one of the broken horncores.

Or, even more excitingly, if the absence is genuine, maybe that tells us something about sauropod biomechanics after all. Maybe if you’re an apatosaurine or a giant brachiosaurid, you actually can’t afford to pneumatize your coracoid, for example. One of my blind spots is a naive faith that any element can be pneumatized without penalty, which I believe mostly on the strength of the pneumatic horncores of bison and bighorn sheep. But AFAIK sauropod girdle elements don’t have big marrow cavities for pneumaticity to expand into. Pneumatization of sauropod limb girdles might have come at a real biomechanical cost, and therefore might have only been available to fairly small animals. (And yeah, Sander et al. 2014 found a pneumatic cavity in an Alamosaurus pubis, but it’s not a very big cavity.)

As I flagged in the title, this is noodling, not a finding, certainly not certainty. Just an airhead thinking about air. The comment thread is open, come join me.

References

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4 Responses to “Some noodling about pneumaticity and body size”

  1. Brad Lichtenstein Says:

    The following is handwaving: it makes sense that smaller animals with smaller masses producing smaller forces on bone surfaces, will be able to afford more pneumaticity. A long bone that is functionally just a tube connecting 2 joints, can effectively transmit the forces of those joints (and nearby muscle attachments) even if it looks like your pelican specimen – if the creature is the size of a pelican. But a limb girdle is subject to forces across much of its surface, and by the double-square-cube rule, even the long bones of large sauropods can’t resemble the household tissue paper cores of a pelican.

    Clearly, a good mechanical engineer could apply numbers to take into account mass, forces, strength of materials, and how the forces would be distributed through a complex system, pneumatized or not.

  2. Matt Wedel Says:

    The following is handwaving

    Perhaps, but it all makes sense to me.

    Clearly, a good mechanical engineer could apply numbers to take into account mass, forces, strength of materials, and how the forces would be distributed through a complex system, pneumatized or not.

    Yeah, maybe. The problem is ‘forces’. For most bones in the skeletons of sauropods, we know where some of the major muscle groups attached, but not all. We know the limb bone cartilage was probably thick, but not how thick. It’s pretty hard to convincingly narrow mass estimates to less than a factor of 1.5 or 2, even for the best-known individuals, of which there are very few.

    The problem with approaching animals as mechanical engineering problems is that there’s not a lot of middle ground. You can model things very simply, like limb bones as cylinders, and sometimes learn interesting things. But once you take on more complex problems, the number of things you need to take into account becomes almost limitless. Michael Fagan’s group at the University of Hull has been working on modeling the skull of Sphenodon for years. They’ve shown that if you don’t account for the soft tissue in the sutures of the skull, or if you don’t roof the muscles with fascia in your FEA model, you get answers that are sometimes not just wrong but opposite of reality. If I was a biomechanist working on extinct animals, that would terrify me. I’m not saying that people shouldn’t attempt to do biomechanical analyses of fossil organisms, just that we should have a very healthy humility about the results.

  3. Mike Taylor Says:

    The following is handwaving: it makes sense that smaller animals with smaller masses producing smaller forces on bone surfaces, will be able to afford more pneumaticity.

    I don’t buy that at all. But, OK, it depends on how you frame the question. If we’re taking at as our invariant that the external volume of a bone is constant, then, yes, pneumatising it internally does somewhat weaken that bone. But if our invariant is that the total mass of bone is constant, then building a pneumatic bone with it makes it stronger, as azhdarchid wing phalages demonstrate. And this is how it happen with (at least) sauropod necks: the vertebrae are effective “inflated” with the result that the tensile and compressive forces acting on the walls of the bone do so further away from the fulcrum: see Sauropods were corn-on-the-cob, not shish kebabs and indeed the “Extent of soft-tissue relative to size of vertebrae” sub-section of our 2013 PeerJ paper.

    Clearly, a good mechanical engineer could apply numbers to take into account mass, forces, strength of materials, and how the forces would be distributed through a complex system, pneumatized or not.

    I’m even more pessimistic about this than Matt is. Another part of the problem is that even “solid” bone is a terrifyingly complex material, with its own internal structure and anisotropy, which makes it very hard to model at all well — as the repeated failure of even the most sophisticated FEA models to match reality shows.

  4. Andy Farke Says:

    I studied bovid frontal sinuses (not postcranial pneumaticity) for my Ph.D., but did find an interplay between size of the pneumatized element as well as phylogeny — “Other analyses indicated that frontal sinus size was correlated most closely with the size of the frontal bone itself, rather than with the overall skull size or horn size. These results may be partially consistent with the hypothesis of sinuses being the result of ‘opportunistic pneumatization’, in which sinus size depends on the quantity of bone available for pneumatization as well as the mechanical demands placed on the skull. Additional evidence also indicates a strong phylogenetic correlation with sinus morphology, particularly with regard to the presence of paranasal diverticula, as well as the ability of sinuses to cross sutural boundaries.” (Farke 2010, JZLS, “Evolution and functional morphology of the frontal sinuses in Bovidae (Mammalia: Artiodactyla), and implications for the evolution of cranial pneumaticity”).

    So to parallel the issue with sauropods, I wonder if some things don’t pneumatize certain elements because…well, they just don’t pneumatize those elements. Whatever little genetic switch that brings a diverticulum into the right area is turned off, or doesn’t exist in the first place. Going back to bovids, there were some species that had plenty of bone to pneumatize, but the relevant diverticulum off the nasal passage just never developed.


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