What’s up with your insanely thick intervertebral discs, Snowmass Haplocanthosaurus?

February 28, 2022

Among the numerous weird features of MWC 8028, the Snowmass Haplocanthosaurus, is the extreme biconcave profile of the caudal vertebrae, in which each centrum is basically reduced to a vertical plate of bone separating two cup-shaped articular surfaces. All four available caudals — found in different parts of the quarry, in different orientations — have essentially the same cross-section. For the diagram above, I just copied caudal 3, because it’s the most complete, so I could figure out the thickness and cross-sectional shape of a single intervertebral disc.

I drew a more realistic version, with the first three caudals at approximately the right scale, for our neural canal paper last year:

The first three caudal vertebrae of Haplocanthosaurus specimen MWC 8028 in midsagittal section, emphasizing the volumes of the neural canal (yellow) and intervertebral joint spaces (blue). Anterior is to the right. Wedel et al. (2021: fig. 2B).

It’s a drawing, sure, but it’s based on a true story, because we have CT scans of all the vertebrae (and we’re going to publish them, soon, along with the reconstructed verts). 

(NB: I’m using “intervertebral disc” as a convenient shorthand for “whatever soft tissues filled the joint space”. But I do think it was a big, fat, fibrocartilaginous disc, not wildly different from the ones in the human vertebral column. It’s not totally impossible that there was some combination of crazy thick articular cartilage and a synovial cavity — there is some precedent in extant salamanders and lizards — but that seems way less likely, for reasons I’ll go into in detail elsewhere. Incidentally, the notion is floating around that reptiles have only synovial intervertebral joints, but this is simply false: intervertebral discs are present in some squamates [Winchester and Bellairs 1977] and in the tails of birds [Baumel 1988].)

I should point out that the other specimens of Haplocanthosaurus also have biconcave caudal vertebrae, but the concavities are much shallower. So what we’re seeing in MWC 8028 is an extreme version of something we see in other individuals of the same genus.

Now, because the caudal centra and joint spaces are roughly radially symmetrical, their relative cross-sectional areas, in these mid-sagittal sections, should be good proxies for their relative volumes. You can imagine the generating the volume of a centrum by rotating its cross-section through 180 degrees, ditto for the joint space (ignoring tilt since both the centrum and joint space are tilted). We’ll have this math worked out in more detail in the next paper, along with volumes from the 3D models, but the upshot is this:

The volume of the intervertebral discs is about twice that of the vertebral centra. If we ignore the neural arch and spine and the transverse processes, and focus only on the weight-bearing column formed by the proximal caudal centra and intervertebral discs, that column is 2/3 cartilage and only 1/3 bone. 

Why, tho?

I spent some time brainstorming with Alton Dooley and we came up with a whole slate of hypotheses. We don’t necessarily like any of them very much, we’re just trying to cast the widest possible net, to make sure we haven’t overlooked any possibilities, no matter how remote they might seem. Here’s what we have so far:

Non-biological:

1. taphonomic distortion

Abnormal biology:

2. congenital malformation

3. pathology

Ontogenetic:

4. incomplete ossification (animal died without laying down the ‘missing’ bone)

5. senescence (the ‘missing’ bone was removed by some process related to aging)

Functional:

6. increased or decreased movement between vertebrae

7. weight reduction

8. shock absorption

What else? 

To reiterate, we’re in the hypothesis-generating stage, not the hypothesis-evaluating stage. So we’re not interested in whether any of these hypotheses are likely. (In point of fact, I think the ones we have so far all suck.) We just want all of the ideas that aren’t impossible.

The comment field is open!

References

8 Responses to “What’s up with your insanely thick intervertebral discs, Snowmass Haplocanthosaurus?”

  1. Mike Taylor Says:

    “Intervertebral discs are present in some squamates [Winchester and Bellairs 1977] and in the tails of birds [Baumel 1988].)”

    Crocs have ’em, too.

  2. Mike Taylor Says:

    “6. increased or decreased movement between vertebrae”

    I love the hedging here. Reminds me of the fact that I have seen (but can’t remember references) papers that say the increased opisthocoely in posterior dorsals of titanosaurs was an adaptation for stiffening to the torse, and that it was an adaptation for making the torso more flexible.

    We don’t know enough about this stuff, do we? (And by “we”, I mean the human race.)

  3. Brad Lichtenstein Says:

    I’ll throw my engineer’s non-anatomist caveat here: the following post is more or less bio- and paleo-ignorant hand waving.

    Weight savings seems like the obvious one, and I don’t see why that would suck so much, given the extent sauropods “had” to focus on that. Assuming not much motion of any one joint in the column, at least this close (right?) to the sacrum, having the weight bearing ring …

    … maybe they don’t even fill that space with heavy cartilage, just a ring to accommodate the load bearing bits? Then you can air sac the rest. The stuff in the middle really just doesn’t bear any meaningful load, assuming limited articulation. I have no idea if anything extant has “tried” this, but to hand wave even more, if they’ve removed the bone in the middle, it seems plausible they’ve also removed (much of) the cartilage in the middle.

    And if diplodocids were, as has been hypothesized, swinging their tail as a weapon, shock absorption also doesn’t suck.

    If all the bones from all locations show this, then post-death distortion does pretty well suck. Unless these were arid animals? I remember trying to compost chicken bones. They obviously didn’t in the timeframe I had to observe them (half year?), but they did get – I tried breaking them into smaller pieces, and found them much much harder to break, very bendy and seemingly even tougher than before. So in a subsequently arid setting, it’s plausible the non-load-bearing middle shrank like a drying sponge, or was preferentially compressed before fossilization. But then you’d see similar things in most other large creatures.

  4. Brad Lichtenstein Says:

    A ring of cartilage is probably just as implausible as a hollow centrum, but the meniscus could easily be biconcave to, literally, match the bone.

  5. LeeB Says:

    If this genus was in the habit of rearing up to get food like a Gerenuk maybe the intervertebral disks helped support the weight of the body which would then be along the spine rather than at right angles to it.

  6. Andrew Says:

    Damn! That’s a helluva lot of not-bone!

    No idea why that might be, but I guess I shouldn’t be surprised stinkin’ sauropods found another way to have weird vertebrae…

    I mean, my first guess was weight-related, then ontogeny. Then my other guess was taphonomy, but that seems less likely, given similar distortion in unassociated/scattered bones. Do the CTs show any weird interior structures that would make the bones similarly taphonomically collapsible? How big was this critter, in relation to other Haplos? Ontogeny just doesn’t feel right. Immature animals would be building up big-kid bones, and these seem like some major chunks missing without some biologically engineered alternative support. Same goes for senescence: an adult with this much bone loss, sans alternative support, would probably be suffering from a pretty serious case of broken back.

    What other animals have spooly vertebrae like these? Sharks, fish, herps? IIRC from working on the baby apatosaur mount at the SNOMNH, the first caudals were a bit spoolier than the other verts, and tilted, but not this dramatically.

    Good luck puzzling this one out!

  7. Jura Says:

    Given the extensive articular cartilage in the limbs of sauropods, having large cartilaginous intervertebral spaces almost seems par for the course for this group. I wonder if this was a way for sauropods to stay more “cushiony” without evolving expansive fat deposits.

  8. Don Ohmes Says:

    Probably to compensate for compressive forces is my thought. Don’t bipeds have thicker discs than quadrupeds? And do quads have thicker cartilage in joints subject to constant compression as opposed to shearing forces?

    Compression related to rearing has been mentioned – and l agree. Whether for food, sex, display, dominance – or even surveillance…


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