The 1st Palaeontological Virtual Congress is underway now, and will run through December 15. Mike and I have two presentations up:

“What do we mean by the directions ‘cranial’ and ‘caudal’ on a vertebra?” by Mike and me, which consists of a video Mike made presenting a slide show that he put together. The presentation sums up our thinking following the series of vertebral orientation posts here earlier this summer and fall, which are all available here.

“Reconstructing an unusual specimen of Haplocanthosaurus using a blend of physical and digital techniques” by me and a gang of WesternU-based collaborators, including Jessie Atterholt and Thierra Nalley, both of whom you saw in our recent pig-hemisecting adventures. Almost everything I’ve written on this blog about Haplocanthosaurus in 2018 was part of the run-up to this presentation (except, somewhat ironically, the post about pneumaticity), which also includes quite a bit that I haven’t put on the blog yet. So even if you follow SV-POW!, the 1PVC slideshow should have plenty of stuff you haven’t seen yet.

IF you can see it–you have to be a registered 1PVC ‘attendee’ to log in to the site and see the presentations. So probably you are either already registered and this post is old news, or not registered and this post seems useless. Why would I bother telling you about stuff you can’t see?

The answer is that neither Mike or I intend for our work to disappear when 1PVC comes to an end on December 15. Both of us are planning to put our abstracts and slide decks up as PeerJ Preprints, which is our default move for conference presentations these days (e.g., this, this, and this). I believe Mike is also going to post his video to YouTube. So the work will not only live on after the congress is over, it will jump to a much broader audience. We’re looking forward to letting everyone see what we’ve been up to, and I’m sure we’ll have some more things to say here when that happens.

So, er, go see our stuff if you’re a 1PVC attendee, and if you’re not, hang in there, we’ll have that stuff out to you in a few days.

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Here’s D10 and the sacrum of Diplodocus AMNH 516 in left lateral and ventral view, from Osborn (1904: fig. 3). Note how the big lateral pneumatic foramina, here labeled ‘pleurocoelia’, start out up at the top of the centrum in D10 and kind of pinch out up there, seemingly entirely absent by S3 (although there is a suspicious-looking shadowed spot above and behind the sacral rib stump labeled ‘r3’). Then on S4 and S5 the big foramina are back, but now they’re low on the centrum, ventral to the sacral ribs. In ventral view, the foramina on D10, S1, and S2 aren’t visible–they’re both over the curve of the centrum, and in the case of S1 and S2, obscured by the sacral ribs. But in S4 and S5, the big lateral foramina are visible in ventral view.

I’ve been interested in a while in this seeming hand-off in centrum pneumatization from dorsolateral, which prevails in the dorsal vertebrae, to ventrolateral, which prevails in the posterior sacral and caudal vertebrae. Almost all sauropod dorsals have the pneumatic foramina quite high on the centrum, sometimes even encroaching on the neural arch. But if sauropod caudals have pneumatic fossae or foramina on the centrum, they’re usually quite low, and almost always below the caudal rib or transverse process (there may also be pneumatic fossae on the neural arch and spine)–for evidence, see Wedel and Taylor (2013b). To me this implies two different sets of diverticula.

I think that in part because sometimes you get both sets of diverticula acting on a single vert. Here’s the centrum of sacral 4 of Haplocanthosaurus CM 879 in right dorsolateral view; anterior is to the right.

Here’s the same thing annotated (yeah, it does look a little like an Ent who is alarmed because his left eye has been overgrown by a huge nasal tumor). This vert has two sets of pneumatic features on the centrum: a big lateral fossa below the sacral rib articulation, presumably homologous with the same feature in S4 of the Diplodocus above; and a smaller dorsolateral fossa above and behind sacral rib articulation.

Unfortunately, CM 879 doesn’t tell us much about how these two sets of diverticula might have changed along the column. The centra of S1-S3 were not found, S5 lacks both sets of fossae, the first caudal has fossae both on the centrum, below the caudal rib, and low on the arch, and the second and subsequent caudals lack both sets of fossae. (I wrote a LOT more about pneumaticity in this individual in my 2009 air sacs paper, which is linked below.)

Working out how these diverticula change serially is a tractable problem. Someone just needs to sit down with a reasonably complete, well-preserved series that includes posterior dorsals, all the sacrals, and the proximal caudals–or ideally several such series–and trace out all of the pneumatic features. As far as I know, that’s never been done, but feel free to correct me if I’ve missed something. I’m neck deep in other stuff, so if someone wants that project, have at it. (If you happen to look into this, I’d be grateful for a heads up, so we don’t run over each other if I do get a yen to investigate further myself.)

References

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.

I was lucky enough to have Phil Mannion as one of the peer-reviewers for my recent paper (Taylor 2018) showing that Xenoposeidon is a rebbachisaurid. During that process, we got into a collegial disagreement about one of the autapomorphies that I proposed in the revised diagnosis: “Neural arch slopes anteriorly 30°–35° relative to the vertical”. (This same character was also in the original Xenoposeidon paper (Taylor and Naish 2007), in the slightly more assertive form “neural arch slopes anteriorly 35 degrees relative to the vertical”: the softening to “30°–35°” in the newer paper was one of the outcomes of the peer-review.)

The reason this is interesting is because the slope of the neural arch is measured relative to the vertical, which of course is 90˚ from the horizontal — but Phil’s comments (Mannion 2018) pushed me to ask myself for the first time: what actually is “horizontal”? We all assume we know horizontality when we see it, but what precisely do we mean by it?

Three notions of “horizontal”

The idiosyncratic best-preserved caudal vertebra of the Snowmass Haplocanthosaurus MWC 8028, illustrating three different versions of “horizontal”. A. horizontality defined by vertical orientation of the posterior articular surface. B. horizontality defined by horizontal orientation of the roof of the neural canal (in this case, rotated 24˚ clockwise relative to A). C. horizontality defined by optimal articulation of two instances of the vertebra, oriented such the a line joining the same point of both instances is horizontal (in this case, rotated 17˚ clockwise relative to A). Red lines indicate exact orthogonality according to the specified criteria. Green line indicate similar but diverging orientations: that of the not-quite-vertical anterior articular surface (A) and of the not-quite-horizontal base of the neural canal (B).

There are at least three candidate definitions, which we can see yield noticeably different orientations in the case of the Snowmass Haplocanthosaurus vertebra that Matt’s been playing with so much recently.

Definition A: articular surfaces vertical

In part A, I show maybe the simplest — or, at least, the one that is easiest to establish for most vertebrae. So long as you have a reasonably intact articular surface, just rotate the vertebra until that surface is vertical. If, as is often the case, the surface is not flat but concave or convex, then ensure the top and bottom of the surface are vertically aligned. This has the advantage of being easy to do — it’s what I did with Xenoposeidon — but it conceals complexities. Most obviously, what to do when the anterior and posterior articular surfaces are not parallel, in the 7th cervical vertebra of a giraffe?

Cervical vertebra 7 of Giraffa camelopardalis FMNH 34426, in left lateral view. Note that the centrum is heavily “keystoned” so that the anterior and posterior articular surfaces are 15-20˚ away from being parallel.

Another difficulty with this interpretation of horizontality is that it can make the neural canal jagged. Consider a sequence of vertebrae oriented as in part A, all at the same height: the neural canal would rise upwards along the length of each vertebra, before plunging down again on transitioning from the front of one to the back of the next. This is not something we would expect to see in a living animal: see for example the straight line of the neural canal in our hemisected horse head(*).

Definition B: neural canal horizontal

Which leads us to the second part of the illustration above. This time, the vertebra is oriented so that the roof of the neural canal is horizontal, which gives us a straight neural canal. Nice and simple, except …

Well, how do we define what’s horizontal for the neural canal? As the Haplocanthosaurus vertebra shows nicely, the canal is not always a nice, neat tube. In this vertebra, the floor is nowhere near straight, but dishes down deeply — which is why I used to the roof, rather than the floor of the canal. Rather arbitrary, I admit — especially as it’s often easier to locate the floor of the canal, as the dorsal margin is often confluent with fossae anteriorly, posteriorly or both.

And as we can see, it makes a difference which we choose. The green line in Part B of the illustration above shows the closest thing to “horizontal” as it would be defined by the ventral margin of the neural canal — a straight line ignoring the depression and joining the anteriormost and posteriormost parts of the base of the canal. As you can see, it’s at a significantly different angle from the red line — about 6.5˚ out.

And then you have human vertebrae, where the dorsal margin of the neural canal is so convex in lateral view that you really can’t say where the anteriormost or posteriormost point is.

Left sides of hemisected human thoracic vertebrae, medial view. Note how ill-defined the dorsal margin of the neural canal is.

So can we do better? Can we find a definition of “horizontal” that’s not dependent of over-interpreting a single part of the vertebra?

Definition C: same points at same height in consecutive vertebrae

I’ve come to prefer a definition of horizontal that uses the whole vertebra — partly in the hope that it’s less vulnerable to yielding a distorted result when the vertebra is damaged. With this approach, shown in part C of the illustration above, 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 — that a line drawn between any point on one vertebra and its corresponding point on the other is horizontal. We can define that attitude of the vertebra as being horizontal.

Note that, while we use two “copies” of the vertebra in this method, we are nevertheless determining the horizontality of a single vertebra in isolation: we don’t need a sequence of consecutive vertebrae to have been preserved, in fact it doesn’t help if we do have them.

One practical advantage of this definition is that its unambiguous as regards what part of the vertebra is used: all of it; or any point on it, at the measurement stage. By contrast, method A requires us to choose whether to use the anterior or posterior articular surface, and method B requires a choice of the roof or floor of the neural canal.

Discussion

I have three questions, and would welcome any thoughts:

  1. Which of these definitions do you prefer, and why?
  2. Can you think of any other definitions that I missed?
  3. Does anyone know of any previous attempts to formalise this? Is it a solved problem, and Matt and I somehow missed it?

Answers in the comments, please!

References

(*) Yes, of course we have a hemisected horse head. What do you think we are, savages?

Tired of Haplo caudals yet? No? Good – me neither. Not by a long shot.

Above is McIntosh and Williams (1988: fig. 10) showing the rearticulated and partially reconstructed tail of CMNH 10380, the holotype and only known specimen of Haplocanthosaurus delfsi, in right anterolateral oblique view. It’s not an original, I plucked it from a PDF scan of the paper. Probably an original reprint would be a lot more clear. In hopes of seeing more, I cropped out the background and tweaked the contrast:

The first 14 caudals are real, the rest are sculpted replicas. You can tell in the photo because the thickness of the supporting rods drops sharply between caudals 14 and 15. That’s not my original observation, McIntosh and Williams pointed it out.

Conclusion? It looks like a pretty good Haplo tail. The first caudal has big, plate-like caudal ribs, which grade rapidly into the normal laterally-projecting stumps in succeeding vertebrae. Caudal 1 also has a distinctly tall, backwardly-curved neural spine, which grades into shorter, straighter spines very rapidly as well. It’s as if the first caudal is built on a typical diplodocoid plan, but the rest are simple non-neosauropod or basal macronarian caudals and they have to switch over as quickly as possible. Both of those shifts happen in the first few caudals in the other Haplo tails, too, with some minor variation among specimens.

I’m sure I’ll have more to say about this specimen in the future, but I’m attending the Flugsaurier conference in LA this weekend so my head is in the clouds. Hope you’re having half as much fun.

Reference

  • McIntosh, J.S., and Williams, M. E. 1988. A new species of sauropod dinosaur, Haplocanthosaurus delfsi sp. nov., form the Upper Jurassic Morrison Fm. of Colorado. Kirtlandia 43:3-26.

Preserved bits of the Snowmass Haplocanthosaurus, MWC 8028, with me for scale. Modified from Wedel (2009: fig. 10), but not much – MWC 8028 was about the same size as CM 879.

Let’s say you had a critter with weird neural canals and super-deeply-dished-in centrum-ends, and you wanted to digitally rearticulate the vertebrae and reconstruct the spinal cord and intervertebral cartilages, in a project that would bring together a bunch of arcane stuff that you’d been noodling about for years. Your process might include an imposing number of steps, and help from a LOT of people along the way:

1. Drive to Dinosaur Journey in Fruita, Colorado, to pick up the fossils and bring them back to SoCal. (Thank you Paige Wiren, John Foster, and Rebecca Hunt-Foster for an excuse to come to the Moab area, thank you Brian Engh for the awesome road trip, and thank you Julia McHugh for access to specimens and help packing them up!).

2. Take the fossils to the Hemet Valley Medical Center for CT scanning. (Thank you John Yasmer and team.)

3. Find a colleague who would help you generate 3D models from the CT scans. (Thank you Thierra Nalley.)

4. Talk it over with your university’s 3D vizualization team, who suggest a cunning plan: (Thank you Gary Wisser, Jeff Macalino, and Sunami Chun at WesternU.)

5. They print the best-preserved vertebra at 75% scale. (50% scale resin print shown here.)

6. You and a collaborator physically sculpt in the missing bits with some Super Sculpey. (Thank you Jessie Atterholt for sculpting, and thank you Jeremiah Scott for documenting the process.)

(7.) The 3D-viz team use their fancy optical scanner (basically a photogrammetry machine) to make:

  • a second-generation digital model (digital)
  • from the sculpted-over 3D print (physical)
  • of the first-generation digital model (digital)
  • made from the CT scans (digital)
  • of the original fossil material (physical).

(8.) With some copying, pasting, and retro-deforming, use that model of the restored vert as a template for restoring the rest of the vertebrae, stretching, mirroring, and otherwise hole-filling as needed. (Prelim 2D hand-drawn version of caudal 1 shown here.)

(9.) Test-articulate the restored vertebrae to see if and how they fit, and revise the models as necessary. (Low-fi speculative 2D version from January shown here.)

(10.) Once the model vertebrae are digitally rearticulated, model the negative spaces between the centra and inside the neural canals to reconstruct the intervertebral cartilages and spinal cord.

(11.) Push the university’s 3D printers to the limit attempting to fabricate an articulated vertebral series complete with cartilages and cord in different colors and possibly different materials, thereby making a third-generation physical object that embodies the original idea you had back in January.

(12.) Report your findings, publish the CT scans and 3D models (original and restored), let the world replicate or repudiate your results. And maaaybe: be mildly astonished if people care about the weird butt of the most-roadkilled specimen of the small obscure sauropod that has somehow become your regular dance partner.

We did number 6 yesterday, so just counting the arbitrarily-numbered steps (and ignoring the fact that 7-12 get progressively more complicated and time-consuming), we’re halfway done. Yay! I’ll keep you posted on how it goes from here.

CM 879 caudal 1 in anterior view

Here’s caudal 1 in Haplocanthosaurus priscus, CM 879. Hatcher (1903) only illustrated this vert in right lateral view, in a drawing by Sydney Prentice (see this post). I showed the vert in left lateral, right lateral, and dorsal views in my 2009 air sac paper (figs. 7 and 9, here). As far as I know, no-one has ever illustrated this vert in anterior or posterior view before.

CM 879 caudal 1 in posterior view

That’s a shame, because it’s the only first caudal of Haplocanthosaurus with a combination of good preservation and accessibility. The first caudal of the holotype, CM 572, was pretty wrecked and the drawings of it in Hatcher (1903) are largely reconstructions (this is discussed in McIntosh and Williams 1988). H. delfsi, CMNH 10380, has a nice caudal 1 but it’s stuck way up in the air in the mounted skeleton in Cleveland. The Snowmass Haplo, MWC 8028, includes a probable first caudal but it’s not going to win any beauty contests:

MWC 8028 probable caudal 1 in anterior (left), posterior (middle), and right lateral (right) views. From Foster and Wedel (2014: fig. 5).

Oh, and there’s the Bilbey haplocanthosaur on display at the Utah Field House of Natural History State Park Museum in Vernal. It has a very nice caudal sequence, probably the best for any haplocanthosaur, but (1) the specimen is under study by others so I don’t want to say too much about it, and (2) I couldn’t if I wanted to because the caudals are displayed in such a way that only the centra are easily visible.

I intended to talk a bit about the morphology of the first caudal in CM 879 and the other Haplo specimens, but now I’m out of time, so I’ll have to circle back to that in the future.

References