Wheelbarrow handles for vertebrae? The cervical rib bundles of Sauroposeidon and other sauropods

September 22, 2014

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

 

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10 Responses to “Wheelbarrow handles for vertebrae? The cervical rib bundles of Sauroposeidon and other sauropods”

  1. Frosted Flake Says:

    Illuminating. I had been wondering what was up with those bones. Thank you.

  2. Alex L Says:

    This is possibly a silly question, but how flexible would the cervical ribs have been in life? I’ve always assumed that they would be quite rigid, but it looks like there’s some movement possible in the bird neck?
    Alex.

  3. Mike Taylor Says:

    “How flexible would the cervical ribs have been in life?”

    Wait for the paper! :-)

  4. Alex L Says:

    Excellent! Look forward to that.

  5. paleoaerie Says:

    You may want to contact Taka Tsuihiji. He has worked on rhea necks and dinosaur comparisons. He published a paper on that exact topic, if you recall. The paper doesn’t show MRI data, but he may have some as they did a lot of studies on rhea necks while he was in Witmer’s lab, who might also have some relevant data. Mike has the paper online. It is ten years old, but Taka has done more work since then that may be of interest. http://www.miketaylor.org.uk/tmp/papers/Tsuihiji_04_nuchal-ligament.pdf

  6. Matt Wedel Says:

    Yeah, I know Taka’s work well. In addition to that 2004 paper, he had a couple of later papers on diapsid neck muscles that are indispensable:

    Tsuihiji, T. (2005). Homologies of the transversospinalis muscles in the anterior presacral region of Sauria (crown Diapsida). Journal of Morphology, 263(2), 151-178.

    Tsuihiji, T. (2007). Homologies of the longissimus, iliocostalis, and hypaxial muscles in the anterior presacral region of extant Diapsida. Journal of Morphology, 268(11), 986-1020.

    I had never thought to ask him if they MRIed a rhea neck; I will do so. Thanks for the prod!

  7. Uri Walter Wolkowski Says:

    An addition to Alex’s question (while waiting for the paper, too):
    If cervical ribs anatomy and overlapping will be studied throughly, will it change the current perspective on how sauropods held their necks?
    Or that it will put further lines of matching evidence to your Taylor et. al. (2009) study andresults about neck stature in life, since the cervical ribs are flexible in modern examples?
    (If the notion that cervical ribs existed before your 2009 paper and maybeI missed such examples in the paper itself, please correct me :)
    Uri.

  8. Matt Wedel Says:

    If cervical ribs anatomy and overlapping will be studied throughly, will it change the current perspective on how sauropods held their necks?

    Maaaybe. I suppose it depends on how much people think that sauropod necks could bend in the first place.

    One of the curious aspects of the Graf and Vidal studies on neck posture in extant animals (on which we based much of our 2009 paper) is that animals apparently do hold their necks in an undeflected, ‘neutral’ pose–in the middle of the neck, just not at the ends. So for many animals raising and lowering the head is accomplished by moving the whole head-neck apparatus up and down at the base of the neck.

    If overlapping cervical ribs were stiff enough to force sauropod necks to be fairly straight, that wouldn’t necessarily change that scenario, the whole neck could still be tilted up and down at the base as in other animals. At least, we think that the vertebrae at the base of the neck probably had enough flexibility to accommodate that.* Kent Stevens thinks that the vertebrae at the base of the neck had much less freedom of movement, so in his scenario, a stiffer neck would further restrict an already fairly small movement envelope.

    * I am thinking here of the juvenile Apatosaurus specimen, CM 3390, where the first dorsal vertebra is extended by more than 30 degrees compared to the second one. That would tilt the whole neck up by at least 30 degrees, even if the neck was stiff as a board. if the same degree of extension was possible at C15/D1, you could have the neck angled up at 60 degrees before you even get into the cervical series proper. See Taylor and Wedel (2013c) for details, and this post.

  9. Mike Taylor Says:

    “We have good descriptions of the proximal parts of the cervical ribs for lots of sauropods.”

    Really? I can’t think of many. Osborn and Mook, of course, which has good descriptions of everything. What else? They tend to be given surprisingly short shrift in most descriptive work.

  10. Mike Taylor Says:

    “If cervical ribs anatomy and overlapping will be studied throughly, will it change the current perspective on how sauropods held their necks? Or that it will put further lines of matching evidence to your Taylor et. al. (2009) study?”

    One of the nice things about the study Matt and I are alluding to is that we genuinely don’t know what results we’re going to get. We will find out whether this line of evidence corroborates or contradicts what we currently believe.

    Go, science!


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