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

Long before Matt and others were CT-scanning sauropod vertebrae to understand their internal structure, Werner Janensch was doing it the old-fashioned way. I’ve been going through old photos that I took at the Museum für Naturkunde Berlin back in 2005, and I stumbled across this dorsal centrum:

Dorsal vertebra centum of ?Giraffatitan in ventral view, with anterior to top.

You can see a transverse crack running across it, and sure enough the front and back are actually broken apart. Here there are:

The same dorsal vertebral centrum of ?Giraffatitan, bisected transversely in two halves. Left: anterior half in posterior view; right: posterior half in anterior view. I had to balance the anterior half on my shoe to keep it oriented corrrectly for the photo.

This does a beautiful job of showing the large lateral foramina penetrating into the body of the centrum and ramifying further into the bone, leaving only a thin midline septum.

But students of the classics will recognise this bone immediately as the one that Janensch (1947:abb. 2) illustrated the posterior half of in his big pneumaticity paper:

It’s a very strange feeling, when browsing in a collection, to come across a vertebra that you know from the literature. As I’ve remarked to Matt, it’s a bit like running into, say, Cameron Diaz in the corner shop.

Reference

  • Janensch, W. 1947. Pneumatizitat bei Wirbeln von Sauropoden
    und anderen Saurischien. Palaeontographica, supplement
    7:1-25.

In the last post, we looked at some sauropod vertebrae exposed in cross-section at our field sites in the Salt Wash member of the Morrison Formation. This time, we’re going to do it again! Let’s look at another of my faves from the field, with Thuat Tran’s hand for scale. And, er, a scale bar for scale:

And let’s pull the interesting bits out of the background:

Now, confession time. When I first saw this specimen, I interpeted it as-is, right-side up. The round thing in the middle with the honeycomb of internal spaces is obviously the condyle of a vertebra, and the bits sticking out above and below on the sides frame a cervical rib loop. I figured the rounded bit at the upper right was the ramus of bone heading for the prezyg, curved over as I’ve seen it in some taxa, including the YPM Barosaurus. And the two bits below the centrum would then be the cervical ribs. And with such big cervical rib loops and massive, low-hanging cervical ribs, it had to an apatosaurine, either Apatosaurus or Brontosaurus.

Then I got my own personal Cope-getting-Elasmosaurus-backwards moment, courtesy of my friend and fellow field adventurer, Brian Engh, who proposed this:

Gotta say, this makes a lot more sense. For one, the cervical ribs would be lateral to the prezygs, just as in, oh, pretty much all sauropods. And the oddly flat inward-tilted surfaces on what are now the more dorsal bones makes sense: they’re either prezyg facets, or the flat parts of the rami right behind the prezyg facets. The missing thing on what is now the right even makes sense: it’s the other cervical rib, still buried in a projecting bit of sandstone. That made no sense with the vert the other way ’round, because prezygs always stick out farther in front than do the cervical ribs. And we know that we’re looking at the vert from the front, otherwise the backwards-projecting cervical rib would be sticking through that lump of sandstone, coming out of the plane of the photo toward us.

Here’s what I now think is going on:

I’m still convinced that the bits of bone on what is now the left side of the image are framing a cervical rib loop. And as we discussed in the last post, the only Morrison sauropods with such widely-set cervical ribs are Camarasaurus and the apatosaurines. So what makes this an apatosaurine rather than a camarasaur? I find several persuasive clues:

  • If we have this thing the right way up, those prezygs are waaay up above the condyle, at a proportional distance I’ve only seen in diplodocids. See, for example, this famous cervical from CM 3018, the holotype of A. louisae (link).
  • The complexity of the pneumatic honeycombing inside the condyle is a much better fit for an apatosaurine than for Camarasaurus–I’ve never seen that level of complexity in a camarasaur vert.
  • The bump on what we’re now interpreting as the cervical rib looks suspiciously like one of the ventrolateral processes that Kent Sanders and I identified in apatosaurine cervicals back in our 2002 paper. I’ve never seen them, or seen them reported, in Camarasaurus–and I’ve been looking.
  • Crucially, the zygs are not set very far forward of the cervical ribs. By some rare chance, this is pretty darned close to a pure transverse cut, and the prezygs, condyle (at its posterior extent, anyway), and the one visible cervical rib are all in roughly the same plane. In Camarasaurus, the zygs strongly overhang the front end of the centrum in the cervicals (see this and this).

But wait–aren’t the cervical ribs awfully high for this to be an apatosaurine? We-ell, not necessarily. This isn’t a very big vert; max centrum width here is 175mm, only about a third the diameter of a mid-cervical from something like CM 3018. So possibly this is from the front of the neck, around the C3 or C4 position, where the cervical ribs are wide but not yet very deep. You can see something similar in this C2-C5 series on display at BYU:

Or, maybe it’s just one of the weird apatosaurine verts that has cervical rib loops that are wide, but not very deep. Check out this lumpen atrocity at Dinosaur Journey–and more importantly, the apatosaur cervical he’s freaking out over:

UPDATE just a few minutes later: Mike reminded me in the comments about the Tokyo apatosaurine, NSMT-PV 20375, which has wide-but-not-deep cervical ribs. In fact, C7 (the vertebra on the right in this figure) is a pretty good match for the Salt Wash specimen:

UpchurchEtAl2005-apatosaurus-plate2-C3-6-7

NSMT-PV 20375, cervical vertebrae 3, 6 and 7 in anterior and posterior views. Modified from Upchurch et al. (2005: plate 2).

UPDATE the 2nd: After looking at it for a few minutes, I decided that C7 of the Tokyo apatosaurine was such a good match for the Salt Wash specimen that I wanted to know what it would look like if it was similarly sectioned by erosion. In the Salt Wash specimen, the prezygs are sticking out a little farther than the condyle and cervical rib sections. The red line in this figure is my best attempt at mimicking that erosional surface on the Tokyo C7, and the black outlines on the right are my best guess as to what would be exposed by such a cut (or pair of cuts). I’ve never seen NSMT-PV 20375 in person, so this is just an estimate, but I don’t think it can be too inaccurate, and it is a pretty good match for the Salt Wash specimen.

Another way to put it: if this is an apatosaurine, everything fits. Even the wide-but-not-low-hanging cervical ribs are reasonable in light of some other apatosaurines. If we think this is Camarasaurus just because the cervical ribs aren’t low-hanging, then the pneumatic complexity, the height of the prezygs, and the ventrolateral process on the cervical rib are all anomalous. The balance of the evidence says that this is an apatosaurine, either a small, anterior vert from a big one, or possibly something farther back from a small one. And that’s pretty satisfying.

One more thing: can we take a moment to stand in awe of this freaking thumb-sized cobble that presumably got inside the vertebra through one its pneumatic foramina and rattled around until it got up inside the condyle? Where, I’ll note, the internal structure looks pretty intact despite being filled with just, like, gravel. As someone who spends an inordinate amount of time thinking about how pneumatic vertebrae get buried and fossilized, I am blown away by this. Gaze upon its majesty, people!

This is another “Road to Jurassic Reimagined, Part 2″ post. As before, Part 1 is here, Part 2 will be going up here in the near future. As always, stay tuned.

References

Way back in 2009–over a decade ago, now!–I blogged about the above photo, which I stole from this post by ReBecca Hunt-Foster. It’s a cut and polished chunk of a pneumatic sauropod vertebra in the collections at Dinosaur Journey in Fruita, Colorado.

This is the other side of that same cut; you’ll see that it looks like a mirror image of the cut at the top, but not quite a perfect mirror image, because some material was lost to the kerf of the saw and to subsequent polishing, and the bony septa changed a bit just in those few millimeters.

And this is the reverse face of the section shown above. As you can see, it is a LOT more complex. What’s going on here? This unpolished face must be getting close to either the condyle or the cotyle, where the simple I-beam or anchor-shaped configuration of the centrum breaks up into lots of smaller chambers (as described in this even older post). It’s crazy how fast that can happen–this shard of excellence is only about 4 or 5 cm thick, and in that short space it has gone from anchor to honeycomb. I think that’s amazing, and beautiful.

It’s probably Apatosaurus–way too complex to be Camarasaurus or Haplocanthosaurus, not complex enough to be Barosaurus, no reason to suspect Brachiosaurus, and although there is other stuff in the DJ collections, the vast majority of the sauropod material is Apatosaurus. So that’s my null hypothesis for the ID.

Oh, back in 2009 I was pretty sure these chunks were from a dorsal, because of the round ventral profile of the centrum. I’m no longer so certain, now that I know that the anchor-shaped sections are so close to the end of the centrum, because almost all vertebrae get round near the ends. That said, the anchor-shaped sections are anchor-shaped because the pneumatic foramina are open, and having foramina that open, that close to the end of the vertebra still makes me think it is more likely a dorsal than anything else. I’m just less certain than I used to be–and that has been the common theme in my personal development over the last decade.

Ray Wilhite posted this gorgeous image on a Facebook thread, and we’re re-posting it here with his permission.

It’s taken from a poster that Ray co-authored (Roberts et al. 2016). We’re looking here at a coronal cross-section of a hen (age not specified), with anterior to the left. Latex has been injected into the air sacs and lungs, highlighting them in shocking pink.

FInding your way around: the big yellow blobs near the middle are vitelline follicles. Just to their left, the two rounded red triangles that look like networks are the lungs. All the rest of the pink is diverticula and air-sacs: the interclavicle air-sac to the left, the caudal thoracic air-sac right behind the left (lower) lung, and abdominal air-sacs running backwards from the tips of the lungs. The big white oval is a calcified egg.

More from this poster in a subsequent post!

References

  • Roberts, John, Ray Wilhite, Gregory Almond, Wallace D Berry, Tami Kelly, Terry Slaten, Laurie McCall and Drury R. Reavill. 2016. Gross and histologic diagnosis of retrograde yolk inhalation in poultry. The American Association of Avian Pathologists, San Antonio, Texas. doi:10.13140/RG.2.2.28204.26246

 

The more I look at the problem of how flexible sauropod necks were, the more I think we’re going to struggle to ever know their range of motion It’s just too dependent on soft tissue that doesn’t fossilise. Consider for example the difference between horse necks (above) and camel necks (below).

The skeletons of both consist of vertebrae that are pronouncedly opisthocoelous (convex in front and concave behind), so you might think their necks would be similarly flexible.

But the balls of horse cevicals are deeply embedded in their corresponding sockets, while those of camels have so much cartilage around and between them that the tip of the ball doesn’t even reach the rim of the socket. As a result of this (and maybe other factors), camel necks are far more flexible than those of horses.

Which do sauropod necks resemble? We don’t currently know, and we may never know. It will help if someone gets a good handle on osteological correlates of intervertebral cartilage.

 


[This post is recycled and expanded from a comment that I left on a Tetrapod Zoology post, but since Tet Zoo ate that comment it’s just as well I kept a copy.]

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?

We’ve noted that the Taylor et al. SVPCA abstract and talk slides are up now up as part of the SVPCA 2015 PeerJ Collection, so anyone who’s interested has probably taken a look already to see what it was about. (As an aside, I am delighted to see that two more abstracts have been added to the collection since I wrote about it.)

It was my privilege to present a talk on our hypothesis that the distinctive and bizarre toblerone-shaped necks of apatosaurs were an adaptation for intraspecific combat. This talk was based on an in-progress manuscript that Matt is lead-authoring. Also on board is the third SV-POW!sketeer, the silent partner, Darren Naish; and artist/ethologist Brian Engh.

Here is our case, briefly summarised from five key slides. First, let’s take a look at what is distinctive in the morphology of apatosaur cervicals:

Screen Shot 2015-09-12 at 11.22.26

Here I’m using Brontosaurus, which is among the more extreme apatosaurs, but the same features are seen developed to nearly the same extent in Apatosaurus louisae, the best-known apatosaur, and to some extent in all apatosaurs.

Now we’ll look at the four key features separately.

Screen Shot 2015-09-12 at 11.22.57

First, the cervicals ribs of sauropods (and other saurischians, including birds) anchored the longus colli ventralis and flexor colli lateralis muscles — ventral muscles whose job is to pull the neck downwards. By shifting the attachments points of these muscles downwards, apatosaurs enabled them to work with improved mechanical advantage — that is, to bring more force to bear.

Screen Shot 2015-09-12 at 11.23.06

Second, by redirecting the diapophyses and parapophyses ventrally, and making them much more robust than in other sauropods, apatosaurs structured their neck skeletons to better resist ventral impacts.

Screen Shot 2015-09-12 at 11.23.15

Third, because the low-hanging cervical ribs created an inverted “V” shape below the centrum, they formed a protective cradle for the vulnerable soft-tissue that is otherwise exposed on the ventral aspect of the neck: trachea, oesophagus, major blood vessels. In apatosaurus, all of these would have been safely wrapped in layers of connective tissue and bubble-wrap-like pneumatic diverticula. The presence of diverticula ventral to the vertebral centrum is not speculative – most neosauropods have fossae on the ventral surfaces of their cervical centra, and apatosaurines tend to have foramina that connect to internal chambers as well (see Lovelace et al. 2007: fig. 4, which is reproduced in this post).

Screen Shot 2015-09-12 at 11.23.22

Fourth, most if not all apatosaurs have distinctive ventrally directed club-like processes on the front of their cervical ribs. (It’s hard to tell with Apatosaurus ajax, because the best cervical vertebra of that species is so very reconstructed.) How did these appear in life? It’s difficult to be sure. They might have appeared as a low boss; or, as with rhinoceros horns, they might even have carried keratinous spikes.

Putting it all together, we have an animal whose neck can be brought downwards with great force; whose neck was mechanically capable of resisting impacts on its ventral aspect; whose vulnerable ventral-side soft-tissue was well protected; and which probably had prominent clubs or spikes all along the ventral aspect of the neck. And all of this was accomplished at the cost of making the neck a lot heavier than it would have been otherwise. Off the cuff, it seems likely that the cervical series alone would have massed twice as much in apatosaurines as in diplodocines of the same neck length.

Doubling the mass of the neck is a very peculiar thing for a sauropod lineage to do – by the Late Jurassic, sauropods were the leading edge of an evolutionary trend to lengthen and lighten the neck that had been running for almost 100 million years, through basal ornithodirans, basal dinosauromorphs, basal saurischians, basal sauropodomorphs, and basal sauropods. Whatever the selective pressures that led apatosaurines to evolve such robust and heavy necks, they must have been compelling.

The possibility that apatosaurs were pushing or crashing their necks ventrally in some form of combat accounts for all of the weird morphology documented above, and we know that sexual selection is powerful force that underlies a lot of bizarre structures in extant animals, and probably in extinct ornithodirans as well (see Hone et al. 2012, Hone and Naish 2013).

What form of combat, exactly? There are various possibilities, which we’ll discuss another time. But I’ll leave you with Brian Engh’s beautiful illustration of one possible form of combat: a powerful impact of one neck brought down onto the dorsal aspect of another.

ApatoNeckSmashRoughWeb

We’re aware that this proposal is necessarily somewhat speculative. But we’re just not able to see any other explanation for the distinctive apatosaur neck. Even if we’re wrong about the ventrolateral processes on the cervical ribs supporting bosses or spikes, the first three points remain true, and given how they fly in the face of sauropods’ long history of making their necks lighter, they fairly cry out for explanation. If anyone has other proposals, we’ll be happy to hear them.

References

  • Hone, D. W., Naish, D., & Cuthill, I. C. (2012). Does mutual sexual selection explain the evolution of head crests in pterosaurs and dinosaurs?. Lethaia 45(2):139-156.
  • Hone, D. W. E., & Naish, D. (2013). The ‘species recognition hypothesis’ does not explain the presence and evolution of exaggerated structures in non‐avialan dinosaurs. Journal of Zoology 290(3):172-180.
  • Lovelace, D. M., Hartman, S. A., & Wahl, W. R. (2007). Morphology of a specimen of Supersaurus (Dinosauria, Sauropoda) from the Morrison Formation of Wyoming, and a re-evaluation of diplodocid phylogeny. Arquivos do Museu Nacional, Rio de Janeiro 65(4):527-544.

Cervical rib cross-sections from Mamenchisaurus Giraffatitan and Diplodocus Klein et al 2012 fig 1

Klein et al. (2012: fig. 1)

We have good descriptions of the proximal parts of the cervical ribs for lots of sauropods. We also have histological cross-sections of a few, mostly thanks to the work of Nicole Klein and colleagues (Klein et al. 2012, Preuschoft and Klein 2013), although histological cross-sections of ribs were also figured as long ago as 1999, by Dalla Vecchia (1999: figs. 29 and 30), and as recently as this month, by Lacovara et al. (2014: supplementary figure 4).

What we have very, very few of is series of cross-sections that show how the cr0ss-section of a cervical rib changes along its length. There may be more out there (and if I have forgotten any, please remind me!), but at the moment I can only think of three such figures: two in Janensch (1950: figs. 83 and 85), both on Giraffatitan, and one in Klein et al. (2012: fig. 1), with cross-sections from Mamenchisaurus, Giraffatitan, and Diplodocus (shown at the top of the post).

Sauroposeidon cervical rib cross-sections v3

 

Rarer still are images that show cross-sections of overlapped cervical ribs, stacked in situ. You could use the information in Janensch (1950: figs. 83 and 85) to generate the stacked cross-sections, but you wouldn’t know the spacing between the ribs as they were in the ground. I think the image just above, of the cervical rib bundles in the Sauroposeidon holotype, OMNH 53062, may be the first of its kind–again, if you know of any others, please let me know. I took the notes for this figure back in 2004, sitting down with the holotype and some digital calipers to make sure I could scale everything correctly, I just hadn’t ever put it into a presentable form until now. The first C6 section (blue V-shape) is from right at the root where the capitulum and tuberculum meet and the posterior shaft of the rib begins.

It is by now well-understood that the long cervical ribs of sauropods and other dinosaurs are ossified tendons of the long hypaxial neck muscles, specifically the longus colli ventralis and flexor colli lateralis. We argued this back in 200o on comparative anatomical grounds (Wedel et al. 2000b: pp. 378-379), and it has now been demonstrated histologically (Klein et al. 2012, Lacovara et al. 2014). The system of stacked tendons is also found in most birds. Here’s the bundle of stacked tendons in a rhea neck, only slightly fanned out:

Rhea ventral tendons stacked - full

And the same neck, with both the epaxial and hypaxial muscles more fully separated:

Rhea neck muscles fanned - full

What I’d really like is an MRI of a rhea or ostrich neck, showing the stacked tendons and their associated belts of muscle, to compare with the stacked cervical ribs of Sauroposeidon and other sauropods. Anyone know of any?

Incidentally, I think the cervical ribs and cervical rib bundles of sauropods are one line of evidence for sauropod necks having been rather slenderly-muscled. The long, multi-segment muscles like the longus colli ventralis are the outermost components of the muscular envelope that surrounds the vertebrae, as you can see in the rhea dissection photos. In sauropod specimens with articulated cervical ribs, the ribs do not deviate from one another or fan out. Rather, they lie in vertically stacked bundles that run from one capitulum-tuberculum intersection to the next. So the depth of that intersection–the “root” of the cervical rib of any given vertebra–plus the thickness of the ribs stacked underneath it, is pretty much the thickness of the muscular envelope around the neck, or at least around the ventral half. And the cervical ribs are typically pretty close to the vertebral centra–only weirdos like Apatosaurus and Erketu displace them very far ventrally (see Taylor and Wedel 2013a: fig. 7 and this post). So, thin jackets of muscle around proportionally large vertebrae–or, if you like, corn-on-the-cob rather than shish-kebabs.

As for why sauropods have long cervical ribs, Mike and I discussed some possibilities in our 2013 PeerJ paper (Taylor and Wedel 2013a), and Preuschoft and Klein addressed the issue last fall in PLOS ONE (Preuschoft and Klein 2013). My favorite hypothesis is that long tendons allow an animal to shift the bulk of the muscle–and therefore the center of gravity–toward the base of the neck, but that long unossified tendons can be distorted through stretching, which wastes muscular energy. Ossifying those long tendons is like putting bony wheelbarrow handles on each vertebra, allowing the muscles to move the vertebra from a distance without so much wasted energy, and probably with finer positional control.

That’s a nifty hypothesis in need of testing, anyway. In fact, cervical ribs and their associated muscles could stand a lot more attention on both the descriptive and analytical fronts. I know that Liguo Li has some research in the works on different conformations of hypaxial muscles, tendons, and cervical ribs in birds (you know, when she’s not describing bizarre new titanosaurs like Yongjinglong — see Li et al. 2014). If you saw Peter Dodson give their talk at SVP last fall, you probably remember some stunning images of dissected bird necks. As a famous legislator once said, we shall watch her career with great interest.

References

 

You know the drill: lotsa pretty pix, not much yap.

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Our first stop of the day was the Fruita Paleontological Area, which has a fanstastic diversity of Morrison animals, including the mammal Fruitafossor and the tiny ornithopod Fruitadens.

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Plus it’s a pretty epic landscape, especially with the clouds and broken light we had this morning.

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I found a bone! Several bits, actually, a few meters away from the Fruitadens type quarry. I’d like to think that this proximal femur might be Fruitadens, but I don’t know the diagnostic characters and haven’t had time to look them up. Anyone know how diagnostic this honorary shard of excellence might be?

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After lunch, John Foster took us on a short hike to the quarry where Elmer Riggs got the back half of the Field Museum Apatosaurus. The front half came from a site in southern Utah, several decades later.

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The locals brought Riggs out in the 1930s for the dedication of two monuments–this one at the Apatosaurus quarry, and another like it at the Brachiosaurus quarry some miles away. Tragically, both monuments have the names of the dinosaurs misspelled!

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In the afternoon we visited the Mygatt-Moore Quarry and the Camarasaurus site in Rabbit Valley. Can you see the articulated Camarasaurus neck in this photo?

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Here’s a hint: the neural arches of two posterior cervical vertebrae in transverse horizontal cross-section.

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This Camarasaurus is apparently a permanent feature. If you’re wondering why no-one has excavated it, it’s because it’s buried in sandstone that is stupid-dense. The expenditure of time and resources just isn’t worth it, when right down the hill dinosaurs are pouring out of the much softer sediments of the Mygatt-Moore Quarry like water from a hydrant. This is the lesson I am learning about the Morrison: finding dinosaurs is easy. Finding dinosaurs you can get out of the ground and prepare–that’s something else.

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Our last stop of the day was Gaston Design, where Rob Gaston showed us how he molds, casts, and mounts everything from tiny teeth to good-sized skeletons.

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Like this Deinosuchus that is about to chomp on Jim Kirkland. Jim doesn’t look too worried.

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Here’s a nice cast of a busted sauropod dorsal, probably from Apatosaurus or Diplodocus, showing the pneumatic internal structure. Compare to similar views of dorsals in this post and this one. This is actually one half of a matched set that includes both halves of the centrum. I left with one of those sets of my own, a few dollars poorer and a whole lot happier.

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The end–for now.