Cross-sectional asymmetry of sauropod vertebrae

March 13, 2021

 

FIGURE 7.1. Pneumatic features in dorsal vertebrae of Barapasaurus (A–D), Camarasaurus (E–G), Diplodocus (H–J), and Saltasaurus (K–N). Anterior is to the left; different elements are not to scale. A, A posterior dorsal vertebra of Barapasaurus. The opening of the neural cavity is under the transverse process. B, A midsagittal section through a middorsal vertebra of Barapasaurus showing the neural cavity above the neural canal. C, A transverse section through the posterior dorsal shown in A (position 1). In this vertebra, the neural cavities on either side are separated by a narrow median septum and do not communicate with the neural canal. The centrum bears large, shallow fossae. D, A transverse section through the middorsal shown in B. The neural cavity opens to either side beneath the transverse processes. No bony structures separate the neural cavity from the neural canal. The fossae on the centrum are smaller and deeper than in the previous example. (A–D redrawn from Jain et al. 1979:pl. 101, 102.) E, An anterior dorsal vertebra of Camarasaurus. F, A transverse section through the centrum (E, position 1) showing the large camerae that occupy most of the volume of the centrum. G, a horizontal section (E, position 2). (E–G redrawn from Ostrom and McIntosh 1966:pl. 24.) H, A posterior dorsal vertebra of Diplodocus. (Modified from Gilmore 1932:fig. 2.) I, Transverse sections through the neural spines of other Diplodocus dorsals (similar to H, position 1). The neural spine has no body or central corpus of bone for most of its length. Instead it is composed of intersecting bony laminae. This form of construction is typical for the presacral neural spines of most sauropods outside the clade Somphospondyli. (Modified from Osborn 1899:fig. 4.) J, A horizontal section through a generalized Diplodocus dorsal (similar to H, position 2). This diagram is based on several broken elements and is not intended to represent a specific specimen. The large camerae in the midcentrum connect to several smaller chambers at either end. K, A transverse section through the top of the neural spine of an anterior dorsal vertebra of Saltasaurus (L, position 1). Compare the internal pneumatic chambers in the neural spine of Saltasaurus with the external fossae in the neural spine of Diplodocus shown in J. L, An anterior dorsal vertebra of Saltasaurus. M, A transverse section through the centrum (L, position 2). N, A horizontal section (L, position 3). In most members of the clade Somphospondyli the neural spines and centra are filled with small camellae. (K–N modified from Powell 1992:fig. 16.) [Figure from Wedel 2005.]

Here’s figure 1 from my 2005 book chapter. I tried to cram as much pneumatic sauropod vertebra morphology into one figure as I could. All of the diagrams are traced from pre-existing published images except the horizontal section of the Diplodocus dorsal in J, which is a sort of generalized cross-section that I based on broken centra of camerate vertebrae from several taxa (like the ones shown in this post). One thing that strikes me about this figure, and about most of the CT and other cross-sections that I’ve published or used over the years (example), is that they’re more or less bilaterally symmetrical. 

We’ve talked about asymmetrical vertebrae before, actually going back to the very first post in Xenoposeidon week, when this blog was only a month and a half old. But not as much as I thought. Given how much space asymmetry takes up in my brain, it’s actually weird how little we’ve discussed it.

The fourth sacral centrum of Haplocanthosaurus CM 879, in left and right lateral view (on the left and right, respectively). Note the distinct fossa under the sacral rib attachment on the right, which is absent on the left.

Also, virtually all of our previous coverage of asymmetry has focused on external pneumatic features, like the asymmetric fossae in this sacral of Haplocanthosaurus (featured here), in the tails of Giraffatitan and Apatosaurus (from Wedel and Taylor 2013b), and in the ever-popular holotype of Xenoposeidon. This is true not just on the blog but also in our most recent paper (Taylor and Wedel 2021), which grew out of this post.

Given that cross-sectional asymmetry has barely gotten a look in before now, here are three specimens that show it, presented in ascending levels of weirdness.

First up, a dorsal centrum of Haplocanthosaurus, CM 572. This tracing appeared in Text-fig 8 in my solo prosauropod paper (Wedel 2007), and the CT scout it was traced from is in Fig 6 in my saurischian air-sac paper (Wedel 2009). The section shown here is about 13cm tall dorsoventrally. The pneumatic fossa on the left is comparatively small, shallow, and lacks very distinct overhanging lips of bone. The fossa on the right is about twice as big, it has a more distinct bar of bone forming a ventral lip, and it is separated from the neural canal by a much thinner plate of bone. The fossa on the left is more similar to the condition in dorsal vertebrae of Barapasaurus or juvenile Apatosaurus, where as the one on the right shows a somewhat more extensive and derived degree of pneumatization. The median septum isn’t quite on the midline of the centrum, but it’s pretty stout, which seems to be a consistent feature in presacral vertebrae of Haplocanthosaurus.

 

Getting weirder. Here’s a section through the mid-centrum of C6 of CM 555, which is probably Brontosaurus parvus. That specific vert has gotten a lot of SV-POW! love over the years: it appears in several posts (like this one, this one, and this one), and in Fig 19 in our neural spine bifurcation paper (Wedel and Taylor 2013a). The section shown here is about 10cm tall, dorsoventrally. In cross-section, it has the classic I-beam configuration for camerate sauropod vertebrae, only the median septum is doing something odd — rather than attaching the midline of the bony floor of the centrum, it’s angled over to the side, to attach to what would normally be the ventral lip of the camera. I suspect that it got this way because the diverticulum on the right either got to the vertebra a little ahead of the one on the left, or just pneumatized the bone faster, because the median septum isn’t just bent, even the vertical bit is displaced to the left of the midline. I also suspect that this condition was able to be maintained because the median septa weren’t that mechanically important in a lot of these vertebrae. We use “I-beam” as a convenient shorthand to describe the shape, but in a metal I-beam the upright is as thick or thicker than the cross bits. In contrast, camerate centra of sauropod vertebrae could be more accurately described as a cylinders or boxes of bone with some holes in the sides. I think the extremely thin median septum is just a sort of developmental leftover from the process of pneumatization.

EDIT 3 days later: John Whitlock reminded me in the comments of Zurriaguz and Alvarez (2014), who looked at asymmetry in the lateral pneumatic foramina in cervical and dorsal vertebrae of titanosaurs, and found that consistent asymmetry along the cervical column was not unusual. They also explicitly hypothesized that the asymmetry was caused by diverticula on one side reaching the vertebrae earlier than diverticula on other other side. I believe they were the first to advance that idea in print (although I should probably take my own advice and scour the historical literature for any earlier instances), and needless to say, I think they’re absolutely correct.

Both of the previous images were traced from CTs, but the next one is traced from a photo of a specimen, OMNH 1882, that was broken transversely through the posterior centrum. To be honest, I’m not entirely certain what critter this vertebra is from. It is too long and the internal structure is too complex for it to be Camarasaurus. I think an apatosaurine identity is unlikely, too, given the proportional length of the surviving chunk of centrum, and the internal structure, which looks very different from CM 555 or any other apatosaur I’ve peered inside. Diplodocus and Brachiosaurus are also known from the Morrison quarries at Black Mesa, in the Oklahoma panhandle, which is where this specimen is from. Of those two, the swoopy ventral margin of the posterior centrum looks more Diplodocus-y than Brachiosaurus-y to me, and the specimen lacks the thick slab of bone that forms the ventral centrum in presacrals of Brachiosaurus and Giraffatitan (see Schwarz and Fritsch 2006: fig. 4, and this post). So on balance I think probably Diplodocus, but I could easily be wrong.

Incidentally, the photo is from 2003, before I knew much about how to properly photograph specimens. I really need to have another look at this specimen, for a lot of reasons.

Whatever taxon the vertebra is from, the internal structure is a wild scene. The median septum is off midline and bent, this time at the top rather than the bottom, the thick ventral rim of the lateral pneumatic foramen is hollow on the right but not on the left, and there are wacky chambers around the neural canal and one in the ventral floor of the centrum. 

I should point out that no-one has ever CT-scanned this specimen, and single slices can be misleading. Maybe the ventral rim of the lateral foramen is hollow just a little anterior or posterior to this slice. Possibly the median septum is more normally configured elsewhere in the centrum. But at least at the break point, this thing is crazy. 

What’s it all mean? Maybe the asymmetry isn’t noise, maybe it’s signal. We know that when bone and pneumatic epithelium get to play together, they tend to make weird stuff. Sometimes that weirdness gets constrained by functional demands, other times not so much. I think it’s very seductive to imagine sauropod vertebrae as these mechanically-optimized, perfect structures, but we have other evidence that that’s not always true (for example). Maybe as long as the articular surfaces, zygapophyses, epipophyses, neural spine tips, and cervical ribs — the mechanically-important bits — ended up in the right places, and the major laminae did a ‘good enough’ job of transmitting forces, the rest of each vertebra could just sorta do whatever. Maybe most of them end up looking more or less the same because of shared development, not because it was so very important that all the holes and flanges were in precisely the same places. That might explain why we occasionally get some really odd verts, like C11 of the Diplodocus carnegii holotype.

That’s all pretty hand-wavy and I haven’t yet thought of a way to test it, but someone probably will sooner or later. In the meantime, I think it’s valuable to just keep documenting the weirdness as we find it.

References

8 Responses to “Cross-sectional asymmetry of sauropod vertebrae”

  1. dale m. Says:

    Not that I can add to this but I do remember one occasion down in Provo in the 1970s where Jensen noted, while touring me around in his lab, that some of his sauropod vertebrae were asymmetric along one side of an entire column. What column I don’t know. He simply mentioned it in passing. I barely saw 1/10th of his collections. But I always remembered that comment.

  2. Mike Taylor Says:

    Ha! that is interesting! Do you mean that all the vertebrae in the relevant stretch of the column were asymmetric in the same way (e.g. with pneumatic foramina on the right but not on the left)?

  3. Andrea Cau Says:

    In Tataouinea holotype, the left side of the vertebrae and the pelvis is more pneumatic than the right side (see Fanti et al. 2015: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0123475).

  4. John Whitlock Says:

    Virginia Zurriaguz looked at this back in 2014 in saltasaurines, at least with regard to the shape of lateral pneumatic fossae. Might be worth a look if you’re interested in this as a phenomenon.

  5. Matt Wedel Says:

    Oh, yeah, that was a cool paper! How obtuse of me to not have thought of it sooner. Thanks for the reminder!

  6. llewelly Says:

    those cross sections look like characters from a fantasy alphabet – Sauropod Glyphs.

    Since you mentioned I-beams, I couldn’t help thinking of human engineered structures, and how they don’t necessarily inform us particularly well when it comes to natural structures. (Also, human-engineered stuff is much closer to what I studied in college than sauropods are.)

    I think a great deal of the regularity present in human-engineered structures is there for ease of analysis, ease of explaining, ease of justifying, and ease of manufacturing all the parts in large volumes.

    I-beams get used in a huge variety of different buildings, for many purposes – and they need to do well in all those different situations, and do so in a way that human construction people and engineers can understand and predict.

    (some fairly odd buildings do occasionally get built, when someone is willing to spend the resources necessary to overcome those issues. I ought to be able to name some architects here, but Gehry is the only that comes to mind. )

    Those kinds of concerns don’t apply to natural structures; no one will force a diplodocus to do a bunch of forces analysis and differential equations to prove their neck won’t fall apart while they’re stripping a branch, and diplodocus verts are not going to made on an assembly line.

    So sauropod verts can be very strange and asymetrical – but still strong in the ways that they need to be, and that can be really hard for us to figure out. Like reverse-engineering a Frank Gehry building without his company’s special software.

    ( Imagine if a skyscraper fell over, and somebody said “It’s sad, but mama skyscraper lays 50 skyscraper eggs a year, so we’ll always have plenty of skyscrapers around.” )

    ( This reminds me of a thing that bugs me about the many beloved analogies between computer code and DNA – computer code, actually, usually does need to be understood by other humans, and this constrains it in many ways that don’t apply to DNA, which never needs to be understood. But that’s unrelated to this topic, because sadly, we’ll never have sauropod DNA )

  7. Matt Wedel Says:

    Interesting thoughts, llewelly. (BTW I tidied away your now-unnecessary pre- and post-comments.)

    That got me thinking about the scale of repeating biological structures. Vertebrates don’t have I-beams or 2x4s, but we do have osteocytes, osteons (most of us, anyway), nerve fascicles, muscle fascicles, capillary beds, and so on. Other than repeating epidermal structures like feathers or scales, most of those are down on the histological level, about one level of organization lower than gross morphologists usually deal with.

    And that got me thinking about the scale of pneumatic features, and what that might mean for the diverticula that produced them. In animals like Haplocanthosaurus and Camarasaurus, the pneumatic features are pretty large, typically several centimeters across. In Brachiosaurus and Giraffatitan, the external pneumatic features can be huge, but the ‘subfossae’ on the external surface, and the smallest internal chambers, are on the scale from several millimeters to just a handful of centimeters. In a lot of titanosaurs, the camellae are uniformly small, often 1cm or less. So there seems to have been a consistent trend toward osteological expressions of pneumaticity at progressively finer spatial scales.

    In the papers I’ve published to date, I’ve tended to discuss that in different terms, mainly complexity of the internal chambers. Which is really just the other side of the same coin: if the individual features are smaller, you can pack more of them into the same volume. So I wonder if the increasing ‘complexity’ of those more highly-subdivided spaces is really a secondary thing–an effect–and the cause is the ability for bone and diverticulum to play together on finer spatial scales over evolutionary time. (And what governs that? I have no idea.)

  8. llewelly Says:

    Thank you for tiding up my comments.

    fine structured pneumaticity makes me think of honeycombed steel, which has sometimes been used for aircraft (famously F-14 tomcat of the distant past, but lots of others), and sometimes ships, because it has different structural properties; I seem to recall it has much higher bending stiffness for a given weight of material.

    I wonder if there is a practical way to compare the two.

    Of course – fine structured pneumaticity might be there for other reasons, such as developmental reasons (which seems to be along the lines you’re thinking of) , or for noisemaking reasons (maybe the aHOOOgha of every titanosaur was unique due to individual variation in the camellae of its vertebrae) , and if only titanosaurs had it, that might be a count against it being structurally important.

    And none of the above are mutually exclusive; fine structured pneumaticity could be there for all three reasons.


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