When is a vertebra “horizontal”, part 2

August 28, 2018

Thanks to everyone who’s engaged with yesterday’s apparently trivial question: what does it mean for a vertebra to be “horizontal”? I know Matt has plenty of thoughts to share on this, but before he does I want to clear up a couple of things.

This is not about life posture

First, and I really should have led with this: the present question has nothing to do with life posture. For example, Anna Krahl wrote on Twitter:

I personally find it more comprehensible if the measurements relate to something like eg. the body posture. This is due to my momentary biomech./functional work, where bone orientation somet is difficult to define.

I’m sympathetic to that, but we really need to avoid conflating two quite different issues here.

Taylor, Wedel and Naish (2009), Figure 1. Cape hare Lepus capensis RAM R2 in right lateral view, illustrating maximally extended pose and ONP: skull, cervical vertebrae 1-7 and dorsal vertebrae 1-2. Note the very weak dorsal deflection of the base of the neck in ONP, contrasting with the much stronger deflection illustrated in a live rabbit by Vidal et al. (1986: fig. 4). Scalebar 5 cm.

If there’s one thing we’ve learned in the last couple of decades, it’s that life posture for extinct animals is controversial — and that goes double for sauropod necks. Heck, even the neck posture of extant animals is terribly easy to misunderstand. We really can’t go changing what we mean by “horizontal” for a vertebra based on the currently prevalent hypothesis of habitual posture.

Also, note that the neck posture on the left of the image above is close to (but actually less extreme than) the habitual posture of rabbits and hares: and we certainly wouldn’t want to illustrate vertebrae as “horizontal” when they’re oriented directly upwards, or even slightly backwards!

Instead, we need to imagine the animal’s skeleton laid out with the whole vertebral column in a straight line — sort of like Ryder’s 1877 Camarasaurus, but with the tail also elevated to the same straight line.

Ryder’s 1877 reconstruction of Camarasaurus, the first ever made of any sauropod, modified from Osborn & Mook (1921, plate LXXXII).

Of course, life posture is more important, and more interesting, question than that of what constitutes “horizontal” for an individual vertebra — but it’s not the one we’re discussing right now.

In method C, both instances are identically oriented

I’m not sure how obvious this was, but I didn’t state it explicitly. In definition C (“same points at same height in consecutive vertebrae”), I wrote:

We use two identical instances of the vertebrae, articulate them together as well as we can, then so orient them that the two vertebrae are level

What I didn’t say is that the two identical instances of the vertebrae have to be identically oriented. Here’s why this is important. Consider that giraffe C7 that we looked at last time, with its keystoned centrum. if you just “articulate them together as well as we can” without that restriction, you end up with something like this:

Which is clearly no good: there’s no way to orient that such that for any given point on one instance, the corresponding point on the other is level with it. What you need instead is something like this:

In this version, I’ve done the best job I can of articulating the two instances in the same attitude, and arranged them such that they are level with each other — so that the attitude shown here is “horizontal” in sense C.

As it happens, this is also just about horizontal in sense B — the floor of the neural canal is presumably at the same height as the top of the centrum as it meets the neural arch.

But “horizontal” in sense A (posterior articular surface vertical) fails horribly for this vertebra:

To me, this image alone is solid evidence that Method A is just not good enough. Whatever we mean by “horizontal”, it’s not what this image shows.


25 Responses to “When is a vertebra “horizontal”, part 2”

  1. Kenneth Carpenter Says:

    But now you are changing the position of the goal post. The original question pertained to the slope of the neural spine for a single vertebra as used in a diagnosis. Here you are considering articulated vertebrae, which is NOT the same thing. There is no “right” answer so long as you define your method

  2. Mike Taylor Says:

    Ken, am I really being that unclear? I’m not talking about articulated sequences at all. This is a method for determining the “horizontal” position of a single vertebra, by considering its hypothetical articulation with itself.

  3. Dean Says:

    What about just drawing the longest possible transect line through the centrum and making that line horizontal?

  4. Dean Says:

    From vertical center of the anterior and posterior faces of course.

  5. Mike Taylor Says:

    Well, I could see that — but then you have to figure out where the centres of the anterior and posterior faces are. Not always obvious.

  6. Kenneth Carpenter Says:

    like I said the first time, it is how you chose to figure it. Ignore vertebrae and start with rectangles, parallelograms, trapezoids, etc. You normally orient these in an illustration with the long axis parallel to the top and bottom of the page (screen, whatever). The long axis is horizontal RELATIVE to the top and bottom. You don’t have to orient it that way – there is no “law”. It is convention to do it that way. Replace the geometric shape with your vertebra(e), Likewise, convention plays a role in how we orient vertebra in a figure. Matt could, to save space, orient his sauropod cervical vertebrae parallel to the left and right sides of the page (or screen), rather than the top and bottom. There is no “law” that says it has to be one way or the other, just convention. Now to take this back to the slope issue: all that matters is that you explain the methodology so I can replicate it. Ultimately isn’t that what you want, repeatability?

  7. Mike Taylor Says:

    But, Ken, surely the point here is that vertebrae are not simple geometrical shapes. There is not always such a thing as “the long axis”.

    That said, I do agree in principle that in the case of Xeno, what matters is that I should be explicit about exactly what I’m saying with regard to the slope of the neural arch. The thing is, I can’t just say “35˚ relative to the vertical cotyle” because in lateral view the cotyle is anything but straight: instead, it’s a smooth arc that merges with the posterior margin of the arch. Like I said: vertebrae are not geometric shapes.

    Which is why I want to land on a rigorous method for determining what is horizontal that (A) reflects our intuition on this, and (B) works for all vertebrae, from giraffe C7s to Haplocanthosaurus caudals.

  8. OK, It shall henceforth be known as “Taylor’s Law of Vertebral Horizontality” ;-)

  9. Anonymous Says:

    “works for all vertebrae, from giraffe C7s to Haplocanthosaurus caudals.”

    That’s going to be difficult. You almost never get something like that in nature. Even in situations where something works like that for 99% of cases, you’re always going to get at least one case that sticks out. Even things that seem like they should work all the time due to biomechanical principles you get exceptions in some group or another because they’ve modified that part for some other function and the initial assumptions no longer hold.

    For all we know giraffe (and maybe camel) necks are an outlier to the idea of using the border of the centrum as the definition for horizontality because most mammals don’t have long necks and most long-necked animals don’t have to work around the constraint of having seven neck vertebrae, so the angle between two individual neck vertebrae is not as pronounced as it is in mammals. Giraffe and camel necks do always seem to have sharper angles between the vertebrae in skeletal reconstructions than something like a bird, though that is just anecdotal observation on my part.

  10. Mike Taylor Says:

    Interesting thoughts, Anonymous. I think I’m OK with the idea that whatever definition of “horizontal” we land up with doesn’t work with 100% of vertebrae (despite what I said earlier), because I like the idea that in cases when it doesn’t “work” — i.e. when what it defines as horizontal doesn’t match our intuition — it’s telling us something real about the animal.

    But I don’t buy the specific notion that in “most long-necked animals […] the angle between two individual neck vertebrae is not as pronounced as it is in mammals”. See for example the rapid cranking in the neck of the parrot at the bottom of this post: it goes through nearly 180 degrees in four joints.

  11. Cloveoil Says:

    I notice the form of the neck verts blurs into that of the dorsals: Its like a dogma mammals have 7, but it seems to me a bit arbitrary to draw a neat line in the first place?

  12. Mike Taylor Says:

    Yes, it’s not always completely clear where neck ends and trunk begins. I believe it’s clearer in mammals where there tends to be more abrupt morphological transition, but for at least some sauropods, some of the shoulder-region vertebrae are arguable.

  13. ijreid Says:

    What if … you define horizontal as the orientation of the vertebrae when the bottom edge of the centrum is flat? Excluding crests etc standing a vertebra upright with both the bottom of the cotyle and bottom of the condyle level would probably make the vertebra horizontal regardless of the tilt of the articular surfaces. Not a perfect method, expecially when the vertebrae have a much smaller anterior (or posterior) end, but that is rather rare to see.

  14. ijreid Says:

    Better yet have it as the bottom edge of two articulated vertebrae being level posterior end to posterior end. Then you account for the size difference in taxa like Haplocanthosaurus (cervicals)

  15. Mike Taylor Says:

    That is actually not a bad idea. We’ll think on this. I don’t think the “excluding crests etc” clause is going to make it, though? Who can say when something is or is not a crest?

    Also, what about Barosaurus cervicals where the ventrolateral processes of the posterior part of the centrum extend further ventrally than the cotyle margin?

  16. ijreid Says:

    Ah yes I guess then bottom of cotyle to bottom of cotyle of articulated vertebra would be best, as it also accounts for ventral keels (what I meant when I said crests)

  17. Mike Taylor Says:

    But bottom-of-cotyle to bottom-of-cotyle in articulated vertebrae is the same as our Method C, isn’t it?

  18. ijreid Says:

    I guess through the time I’ve thought I’ve gotten to like method C. But when only one vertebra is known bottom of condyle to bottom of cotyle should work fairly well.

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  20. […] arsed to (1) follow up on the “Down in Flames” post, (2) add anything thoughtful to the vertebral orientation discussion, or (3) crop or color-adjust these photos. You’re getting them just as they came out of my […]

  21. […] helped me clarify my thinking on how we ought to orient vertebrae, which Mike wrote about here and here. And second, it gives me some hope, because if we can figure out why tilting your articular […]

  22. […] 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. […]

  23. […] that Matt and I have blogged various thoughts 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 […]

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  25. […] for a vertebra 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 […]

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