Vertebral orientation, part 3: Matt weighs in

October 5, 2018

WOW! I knew I was dragging a bit on getting around to this vertebral orientation problem, but I didn’t realize a whole month had passed. Yikes. Thanks to everyone who has commented so far, and thanks to Mike for getting the ball rolling on this. Previous posts in this series are here and here.

First up, this may seem like a pointlessly picky thing to even worry about. Can’t we just orient the vertebrae in whichever way feels the most natural, or is easiest? Do we have to think about this?

The alarmingly 3D pelvis of the mounted brontosaur at the AMNH. Note that sauropod pubes are usually illustrated lying flat, so what usually passes for ‘lateral’ view would be roughly from the point of view of the animal’s knee.

I think we do. For sauropods, vertebrae are usually oriented for illustration purposes in one of two ways. The first is however they sit most easily on their pallets. This is similar to the problem Mike and I found for ‘lateral’ views of sauropod pelvic elements when were on our AMNH/Yale trip in 2012. In an articulated skeleton, the pubes and ischia usually lean inward by 30-45 degrees from their articulations with the ilia, so they can meet on the midline, but when people illustrate the “lateral view” of a sauropod pubis or ischium, it’s often the ventro-lateral aspect that is face-up when the element is lying on a shelf or a pallet. Photographic lateral does not equal biological lateral for those elements. Similarly, if I’m trying to answer biological questions about vertebrae (see below), I need to know something about their orientation in the body, not just how they sit comfortably on a pallet.

The other way that vertebrae are commonly oriented is according to what we might call the “visual long axis” of the centrum—so for example, dorsoventrally tall but craniocaudally short proximal caudals get oriented with the centrum ‘upright’, whereas dorsoventrally short but craniocaudally long distal caudals get oriented with the centrum ‘horizontal’, even if they’re in the same tail and doing so makes the neural canals or articular faces be oriented inconsistently down the column. (I’m not going to name names, because it seems mean to pick on people for something I just started thinking about myself, but if you go plow through a bunch of sauropod descriptions, you’ll see what I’m talking about.)

Are there biological questions where this matters? You bet! There are some questions that we can’t answer unless we have the vertebrae correctly oriented first. One that comes to mind is measuring the cross-sectional area of the neural canal, which Emily Giffin did a lot of back in the 90s. Especially for the Snowmass Haplocanthosaurus, what counts as the cross-sectional area of the neural canal depends on whether we are looking at the verts orthogonal to their articular faces, or in alignment with the course of the canal. I think the latter is pretty obviously the way to go if we are measuring the cross-sectional area of the canal to try and infer the diameter of the spinal cord—we’d want to see the canal the same way the cord ‘sees’ it as it passes through—but it’s less obvious if we’re measuring, say, the surface area of the articular face of the vertebra to figure out, say, cartilage stress. It doesn’t seem unreasonable to me that we might want to define a ‘neural axis’ for dealing with spinal-cord-related questions, and a ‘biomechanical axis’ for dealing with articulation-related questions.

Caudal 3 of the Snowmass Haplocanthosaurus, hemisected 3D model.

With all that in mind, here are some points.

To me, asking “how do we know if a vertebra is horizontal” is an odd phrasing of the problem, because “horizontal” doesn’t have any biological meaning. I think it makes more sense to couch the question as, “how do we define cranial and caudal for a vertebra?” Normally both the articular surfaces and the neural canal are “aimed” head- and tail-wards, so the question doesn’t come up. Our question is, how do we deal with vertebrae for which the articular surfaces and neural canal give different answers?

For example. Varanus komodoensis caudal.

(And by the way, I’m totally fine using “anterior” and “posterior” for quadrupedal animals like sauropods. I don’t think it causes any confusion, any more than people are confused by “superior” and “inferior” for human vertebrae. But precisely because we’re angling for a universal solution here, I think using “cranial” and “caudal” makes the most sense, just this once. That said, when I made the image above, I used anterior and posterior, and I’m too lazy now to change it.)

I think if we couch the question as “how do we define cranial and caudal”, it sets up a different set of possible answers than Mike proposed in the first post in this series: (1) define cranial and caudal according to the neural canal, and then describe the articular surfaces as inclined or tilted relative to that axis; (2) vice versa—realizing that using the articular surfaces to define the anatomical directions may admit a range of possible solutions, which might resurrect some of the array of possible methods from our first-draft abstract; (3) define cranial and caudal along the long axis of the centrum, which is potentially different from either of the above; (4) we can imagine a range of other possibilities, like “use the zygs” or “make the transverse processes horizontal” (both of which are subsets of Mike’s method C) but I don’t think most of those other possibilities are sufficiently compelling to be worthy of lengthy discussion.

IF we accept “neural canal”, “articular surfaces”, and “centrum long axis” as our strongest contenders, I think it makes most sense to go with the neural canal, for several reasons:

  • In a causative sense, the neural tube/spinal cord does define the cranial/caudal axis for the developing skeleton. EDIT: Actually, that’s a bit backwards. It’s the notochord, which is later replaced by the vertebral column, that induces the formation of the brain and spinal cord from the neural plate. But it’s still true that the vertebrae form around the spinal cord, so it’s not wrong to talk about the spinal cord as a defining bit of soft tissue for the developing vertebrae to accommodate.
  • The neural canal works equally well for isolated vertebrae and for articulated series. Regardless of how the vertebral column is oriented in life, the neural canal is relatively smooth—it may bend, but it doesn’t kink. So if we line up a series of vertebrae so that their neural canals are aligned, we’re probably pretty close to the actual alignment in life, even before we look at the articular surfaces or zygs.
  • The articulated tails of Opisthocoelicaudia and big varanids show that sometimes the articular surfaces simply are tilted to anything that we might reasonably consider to be the cranio-caudal axis or long axis of the vertebra. In those cases, the articular surfaces aren’t orthogonal to horizontal OR to cranio-caudal. So I think articular surfaces are ruled out because they break down in the kinds of edge cases that led us to ask the question in the first places.

Opistocoelicaudia caudals 6-8, stereopair, Borsuk-Bialynicka (1977:plate 5).

“Orient vertebrae, isolated or in series, so that their neural canals define the cranio-caudal axis” may seem like kind of a ‘duh’ conclusion (if you accept that it’s correct; you may not!), but as discussed up top, often vertebrae from a single individual are oriented inconsistently in descriptive works, and orientation does actually matter for answering some kinds of questions. So regardless of which conclusion we settle on, there is a need to sort out this problem.

That’s where I’m at with my thinking. A lot of this has been percolating in my hindbrain over the last few weeks—I figured out most of this while I was writing this very post. Is it compelling? Am I talking nonsense? Let me know in the comments.

11 Responses to “Vertebral orientation, part 3: Matt weighs in”

  1. aquadraco Says:

    I must admit, I can’t see where this should even be a question. Since the alignment of the neural canal controls the position of the vertebrae regardless of the animal involved, shouldn’t this be the accepted standard?

  2. Matt Wedel Says:

    Well, I think so. but right now there isn’t a standard.

  3. Matt Wedel Says:

    And I should add that because there isn’t a standard, it took Mike and me a loooooong time to realize that there was a need for one, and it took me even longer to come around to the idea that the neural canal should define cranial and caudal. Like many things in science, if it’s obvious, it’s only obvious in hindsight.

  4. Allen Hazen Says:

    Yes. Certainly seems right. We’d like an orientation that is biologically significant, and this is the obvious candidate. Still, there are places where spinal cords come close to “kinking”: where the cervical series meets the thoracic in many mammals. When you write the textbook on “Correct orientation of vertebrae,” include some awful C7 and T1 as an exercise!

  5. Matt Wedel Says:

    Good point on the C7/T1 transition. Although:

    1. I suspect that the transition is often more gentle than the external appearance might indicate. The radiographs I’ve seen of mammals apparently “kinking” their necks actually show the vertebrae doing J-hooks or S-curves inside (for example), while the external fleshy envelope lies about what’s going on under the hood.

    2. I think one reason why most animals have MUCH larger neural canals than cords is so that the cord can still have a smooth bend inside the wide and tall neural canal even if the vertebrae themselves are kinked. The spinal cord really, really does not fare well when sharply bent. It’s my “privilege” to teach the med students about what happens when the spinal cord does get bent, and it’s mostly pretty awful.

    I like your formulation “biologically significant”–that’s a good way to put it. I’m with Mike–we don’t want our vertebral orientation scheme to be based on inferred life posture, because the latter is often unknowable, variable among taxa, or variable within an individual. But I would like to have it based on something biologically real. And, crucially, easy to assess. Vertebral orientation is a tool to help us think about things, and I don’t think there’s any harm in making sure the tool we pick is easy to use, or at least hard to misuse. I’m also not claiming that the neural canal is some kind of shining essential truth. It’s the least worst option I’ve found so far.

  6. Mark Evans Says:

    I guess ultimately it’s the anteroposterior (sorry, cranial-caudal) axis of the notochord, which then goes on to induce the formation of the neural tube and therefore the neural canal before further development make things more complicated.

  7. Matt Wedel Says:

    Yes, good point. I updated the post to reflect the complexity.

  8. Mike Taylor Says:

    Yay, I managed to see the Opisthocoelicaudia caudal-sequence sterogram! (It was disappointingly uninformative. But still.)

  9. Mike Taylor Says:

    1. I suspect that the transition is often more gentle than the external appearance might indicate. The radiographs I’ve seen of mammals apparently “kinking” their necks actually show the vertebrae doing J-hooks or S-curves inside (for example), while the external fleshy envelope lies about what’s going on under the hood.

    Doesn’t the Necks Lie sequence of posts rather show the opposite? That the transition between orientations of successive vertebrae is more extreme than you would guess from the fleshy envelope?

  10. […] about how to orient vertebra (part 1, part 2, relevant digression 1, relevant digression 2, part 3) and presented a talk on the subject at the 1st Palaeontological Virtual Congress, it’s time […]

  11. […] to be “horizontal”? That post and subsequent posts on the same topic (one, two, three) provoked interesting discussions in the comment threads, and convinced us that there was something […]

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