Caudal pneumaticity in sauropods: in praise of actually looking at fossils

January 27, 2014

“Look at all the things you’ve done for me
Opened up my eyes,
Taught me how to see,
Notice every tree.”

So sings Dot in Move On, the climactic number of Stephen Sondheim’s Pulitzer Prize-winning music Sunday in the Park with George, which on the surface is about the post-impressionist painter Georges Seurat, but turns out to be a study of obsession and creativity.


Un dimanche après-midi à l’Île de la Grande Jatte – 1884 [A Sunday Afternoon on the Island of La Grande Jatte – 1884]

“Taught me how to see”? What kind of talk is that? One the surface, it seems silly — we all know how to see. We do it constantly, without thinking. Yet it’s something that artists talk about all the time. And anyone who’s sat down and seriously tried to paint or draw something will have some understanding of what the phrase means. We have such strong implicit ideas of what things look like that we tend to reproduce what we “know” is there rather than what’s actually there. Like I said, we see without thinking.

In fact, the psychology of perception is complicated and sophisticated, and the brain does an extraordinary amount of filtering of the visual signals we get, to save us the bother of having to consciously process way too much data. This is a whole scientific field of its own, and I’m going to avoid saying very much about it for fear of making a fool of myself — as scientists so often do when wandering outside their own field. But I think it’s fair to say that we all have a tendency to see what we expect to see.


Phylogeny of Sauropoda, strict consensus of most parsimonious trees according to Wilson (2002:fig. 13a)

In the case of sauropods, this tendency has meant that we’ve all been startlingly bad at seeing pneumaticity in the caudal vertebrae of sauropods. Because the literature has trained us to assume it’s not there. For example, in the two competing sauropod phylogenies that dominated the 2000s, both Wilson (2002) and Upchurch et al. (2004) scored caudal pneumaticity as very rare: Wilson’s character 119, “Anterior caudal centra, pneumatopores (pleurocoels)”, was scored 1 only for Diplodocus and Barosaurus; and  Upchurch et al. (2004:286) wrote that “A few taxa (Barosaurus, Diplodocus, and Neuquensaurus) have pleurocoel-like openings in the lateral surfaces of the cranial [caudal] centra that lead into complex internal chambers”. That’s all.

And that’s part of the reason that every year since World War II, a million people have walked right past the awesome mounted brachiosaur in the Museum Für Naturkunde Berlin without noticing that it has pneumatic caudals. After all, we all knew that brachiosaur caudals were apneumatic.

But in my 2005 Progressive Palaeontology talk about upper limits on the mass of land animals estimated through the articular area of limb-bone cartilage, I included this slide that shows how much bigger the acetabulum of Giraffatitan is than the femoral head that it houses:

Screenshot from 2014-01-24 17:30:30

And looking at that picture made me wonder: those dark areas on the sides of the first few caudals (other than the first, which is a very obvious plaster model) certainly look pneumatic.

Then a few years later, I was invited to give a talk at the Museum Für Naturkunde Berlin itself, on the subject “Brachiosaurus brancai is not Brachiosaurus“. (This of course was drawn from the work that became my subsequent paper on that subject, Taylor 2009) And as I was going through my photos to prepare the slides of that talk, I thought to myself: darn it, yes, it does have pneumatic caudals!

So I threw this slide into the talk, just in passing:

Screenshot from 2014-01-24 17:32:06

Those photos were pretty persuasive; and a closer examination of the specimen on that same trip was to prove conclusive.

Meanwhile …

Earlier in 2009, I’d been in Providence, Rhode Island, with my Index Data colleagues. I’d managed to carve a day out of the schedule to hop along the coast to the Yale Peabody Museum in New Haven, Connecticut. My main goal was to examine the cervicals of the mounted Apatosaurus (= “Brontosaurus“) excelsus holotype (although it was also on that same trip that I first saw the Barosaurus holotype material that we’ve subsequently published a preprint on).

The Brontosaurus cervicals turned out to be useless, being completely encased in plaster “improvements” so that you can’t tell what’s real and what’s not. hopefully one day they’ll get the funding they want to take that baby down off its scaffold and re-prep the material.

But since I had the privilege of spending quality time with such an iconic specimen, it would have been churlish not to look at the rest of it. And lo and behold, what did I see when I looked at the tail but more pneumaticity that we thought we knew wasn’t there!

Wedel and Taylor (2013b: Figure 10).

An isolated pneumatic fossa is present on the right side of caudal vertebra 13 in Apatosaurus excelsus holotype YPM 1980. The front of the vertebra and the fossa are reconstructed, but enough of the original fossil is visible to show that the feature is genuine. (Wedel and Taylor 2013b: Figure 10).

What does this mean? Do other Giraffatitan and Apatosaurus specimens have pneumatic tails? How pervasive is the pneumaticity? What are the palaeobiological implications?

Stay tuned! All will be revealed in Matt’s next post (or, if you can’t wait, in our recent PLOS ONE paper, Wedel and Taylor 2013b)!


17 Responses to “Caudal pneumaticity in sauropods: in praise of actually looking at fossils”

  1. Wait, I have to wait before commenting on the last paragraph of your discussion? Fudge!

  2. Mike Taylor Says:

    You can go right ahead :-)

  3. Andrea Cau Says:

    Something very similar to what is in this post happened to me when I begun to study the Tataouinea specimen. I had noticed the large foramen in the medial surface of the iliac peduncle of the ischium, but at first I assumed it was not pneumatic in origin, since I expected large elliptical foramina to be found in the vertebrae only. Just after we found the extensive pneumatisation in the ilium, I returned to that feature, compared in depth with vertebral pleurocoels and realised that it was a pneumatic foramen.

  4. Nathan Myers Says:

    At what point will paleontologists be obliged to justify a belief that any particular sauropod vertebra was not, in fact, pneumatic?

  5. Distal caudal pneumaticity is likely reduced due to a functional need for tail mass, even in the cantilevered organ. This need, if it truly exists, would be for the purposes of transmission of forces along the tail during movement. The faster the tail moves, the greater its mass, the greater the force it transmits if any part of that tail encounters another object. Crack! of a tail whipping by at supersonic speeds, or the force of a bulkier tail whap! when the dino decides some theropod is being too pesky.

    The irregularity of tail pneumaticity suggests a conflict in developing the AAS into the tail bones as opposed to a need to maintain mass for transmitting whipcrack forces. Shorter tails would have a similar constraint and thus conflict by using similar but slower force transmission which would prioritize mass closer to the hip. More extensive, and pervasive (and consistent) pneumaticity would suggest less of a function. Force transmission through the tail would cause mostly or partially pneumatic tails to more readily distort unless they were braced in the direction of travel.

    Potential evidence of non-pneumatized tails used for this purpose can be the damaging and fusion of caudal vertebrae, broken and rehealed distal caudal vertebrae, more irregular distortion of pneumatic vertebrae over non-pneumatic vertebrae (amount of asymmetry should increase). Exostoses of bones, or more rugose regions of enthesis, avulsion pathologies, should also be more apparent in tails in which exhibit high-force translation. These create a testable framework to permit discussion a functional constrain on caudal pneumaticity in sauropods.

    Anyways, that was what I was musing about.

  6. Mike Taylor Says:

    Well, Jaime, our growing sense generally is the skeletal pneumaticity is not strongly controlled by natural selection, and often seems to happen or not according to other factors, including sheer randomness. We see this partly in the extremely inconsistent patterns of pneumatisation in tails (which we discuss in the recent paper and which Matt will shortly blog about), but also in the early evolution of postcranial skeletal pneumaticity, in which the lightening effect of PSP would have been absolutely negligible (Wedel 2006).

    Of course that’s not to say that selection didn’t play a role, especially in neck lightening as necks grew longer (though even there the pattern is not what you might expect). But my feeling is it’s much more likely that apneumatic distal caudals are that way just because the diverticula never happened to make it down to the end of the tail rather than because apneumaticity was actively selected for.

  7. Mike Taylor Says:

    BTW., Matt, in your 2006 Integrative Zoology paper (pp. 83-84), you wrote:

    The bone removed by pneumatization accounted for <1% of the total body volume in both Coelophysis and Thecodontosaurus (Mathew J. Wedel, unpubl. data)

    Is that “unpublished” data in your dissertation? If not, you’d better publish it — here on SV-POW!, if not elsewhere.

  8. Mike, thanks for responding. I’d say if there are clades where caudal pneumaticity is the rule, not the exception (diplodocids) and then caudal pneumaticity is absent generally (basal titanosauriforms) but again then is the rule (titanosaurids) wouldn’t we be expecting that pneumaticity is being selected for in those clades merely because it is being selected against prior to the rise of the titans? This question may have to await a good model for pneumatic invasion of bone and its processes for creating internal cavities, so I guess i am not going to get a reasonable answer. But I did suggest a testable model, at least!

  9. Err, “reasonable answer” there is not a stab at you, Mike, it’s rather “there isn’t a reasonable answer to my question” and not “you won’t give ME a reasonable answer.”

  10. Mike Taylor Says:

    Yes, I understood the “reasonable” point! :-)

    It’s certainly true that there seems to be a phylogenetic signal involved in the degree and extent of caudal pneumatisation (and PSP generally). But that signal is weak and homoplastic, and even if it were clear then it wouldn’t follow that it had been selected for, merely that pneumaticity is heritable.

    For what it’s worth, my _guess_ is that the apparent homoplasy we see in pneumatic features is largely driven by the very small sample sizes we have of nearly every sauropod, and what we’re seeing is actually mostly just individual variation. But we don’t have enough data to either support or oppose that guess.

  11. Mark Robinson Says:

    I wouldn’t be surprised if pneumatic invasion (at least as far as caudal pneumaticity is concerned) proceeded chaotically and that there would be significant variation even between individuals of the same species and maturity. By chaotically I mean unpredictably rather than strictly random. A bit like knowing that summer will (very probably) be warmer than winter but not knowing whether it will be sunny four weeks from today.

  12. Matt Wedel Says:

    Mark, I think you have characterized it perfectly. As far as I’ve been able to determine, every pneumatic system in extant animals is highly variable–human sinuses, sacral pneumatization in chickens, facial air sacs in whales, you name it. It would be peculiar if this quality, which transcends very different anatomical regions across huge clades of extant animals, did not also apply in the past.

  13. Mark Robinson Says:

    Ah, thanks Matt. I had an inkling that there was considerable variability within and throughout various clades (prob garnered from here or TetZoo) but didn’t know to what extent.

    Makes me think that this is a result of the way that the process occurs at the molecular level; and yet it’s far from random.

  14. Matt Wedel Says:

    Makes me think that this is a result of the way that the process occurs at the molecular level; and yet it’s far from random.

    I agree with both parts of that. To me, one of the most interesting things about skeletal pneumaticity is that we still don’t know a ton about how it is mediated at the level of cellular interactions. The pneumatic epithelium itself does not resorb the bone in front of it, instead there is an uptick in parathyroid hormone and osteoclasts come dissolve the bone in front of the pneumatic epithelium, which expands to fill the vacated space. But how the pneumatic epithelium activates the parathyroid or how the osteoclasts, which are blood-borne, know to go dissolve the bone there and not somewhere else (and also when to stop), are things that I don’t think anyone knows yet.

    So this is one place where you can figure out something about the morphogenetic rules that govern a complex tissue interaction by looking at the kinds of outcomes it produces: a shallow fossa here, a pair of foramina there, one big chamber, a network of tiny chambers. CT scans are nice, but one can make a real contribution just by staying alert and taking pictures. That’s basically the moral of the paper, and this post.

  15. […] will be strangely familiar to anyone who remembers our experience with caudal pneumaticity in sauropods, which was: the more we looked for it, the more we found […]

  16. Matt Wedel Says:

    To VERY belatedly answer Mike’s question about the calculations for Coelophysis and Thecodontosaurus (now Pantydraco), those were unpublished in my very short 2006 paper because of space limitations, but I put them in the Appendix of my 2007 prosauropod paper (link).

  17. […] feature obscured because it’s still full of matrix? This is probably part of the reason that caudal pneumaticity so often goes unobserved, and it will very often obscure foramina within the neural canal. […]

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