Did sauropod necks have intervertebral discs?

November 8, 2013

One aspect of sauropod neck cartilage that’s been overlooked — and this applies to all non-avian dinosaurs, not just sauropods — is the configuration of the cartilage in their necks. It’s not widely appreciated that birds’ necks differ from those of all other animals in this respect, and we don’t yet know whether sauropods resembled birds or mammals.

Here’s a classic sagittal view of a mammal neck — in this case a human — from The Basics of MRI (Joseph P. Hornak, 1996-2013):

sagittal-neck

You can see two distinct kinds of structure alternating along the neck: the big, square ones are vertebral centra (slightly hollow at each end), and the narrower lens-shaped ones are the intervertebral discs.

In mammals, and most animals, we find this distinct fibrocartilaginous element, the disc, between the centra of consecutive vertebrae. These discs have a complex structure of their own, consisting of an annulus fibrosus (fibrous ring), made of several layers of fibrocartilage, surrounding a nucleus pulposus (pulpy centre) with the consistency of jelly.

IntervertebralDisc

But in birds, uniquely among extant animals, there is no separate cartilaginous element. Instead, the articular surfaces of the bones are covered with layers of hyaline cartilage which articulate directly with one another, and are free to slide across each other. The adjacent articular surfaces are enclosed in synovial capsules similar to those that enclose the zygapophyseal joints. You can see this in the hemisected Rhea neck from last time:

Figure 18. Cartilage in the neck of a rhea. Joint between cervicals 11 (left) and 10 (right) of a rhea, sagittally bisected. Left half of neck in medial view. The thin layers of cartilage lining the C11 condyle and C10 cotyle are clearly visible.

Taylor and Wedel (2013c: Figure 18). Cartilage in the neck of a rhea. Joint between cervicals 11 (left) and 10 (right) of a rhea, sagittally bisected. Left half of neck in medial view. The thin layers of cartilage lining the C11 condyle and C10 cotyle are clearly visible.

The difference between these two constructions is very apparent in dissection: in birds, adjacent vertebrae come apart easily once the surrounding soft tissue is removed; but in mammals, it is very difficult to separate consecutive vertebrae, as they are firmly attached to the intervening intervertebral disc.

Figure 19. Alligator head and neck. Sagittally bisected head and neck of American alligator, with the nine cervical vertebrae indicated. Inset: schematic drawing of these nine vertebrae, from ([62]: figure 1), reversed.

Taylor and Wedel (2013c: Figure 19). Alligator head and neck. Sagittally bisected head and neck of American alligator, with the nine cervical vertebrae indicated. Inset: schematic drawing of these nine vertebrae, from ([62]: figure 1), reversed.

To complicate matters further, thin articular discs occur in the necks of some birds — for example, the ostrich (see illustration below), the swan, and the king penguin. But these discs do not occur in all birds — for example, they are absent in the turkey and the rhea. When they are present, these articular discs divide the synovial cavity and prevent the (cartilage-covered) bones on either side from ever articulating directly with each other, just like the articular discs in the human temporomandibular and sternoclavicular joints. These discs are thinner than the true intervertebral discs of mammals and crocodilians; and they are different in composition, lacking the annulus/nucleus structure and consisting of a simple sheet of fibrocartilage.

Taylor and Wedel (2013: Figure 4). Intervertebral articular discs of an ostrich (not to scale). Left: first sacral vertebra in anterior view, showing articular disc of joint with the last thoracic vertebra. Right: posterior view view of a cervical vertebra, with probe inserted behind posterior articular disc. The cervical vertebra is most relevant to the present study, but the the sacral vertebra is also included as it shows the morphology more clearly. These fibrocartilaginous articular discs divide the synovial cavity, like the articular discs in the human temporomandibular and sternoclavicular joints, and should not be confused with the true intervertebral discs of mammals and other animals, which consist of a nucleus pulposus and an annulus fibrosus.

Taylor and Wedel (2013: Figure 4). Intervertebral articular discs of an ostrich (not to scale). Left: first sacral vertebra in anterior view, showing articular disc of joint with the last thoracic vertebra. Right: posterior view view of a cervical vertebra, with probe inserted behind posterior articular disc. The cervical vertebra is most relevant to the present study, but the the sacral vertebra is also included as it shows the morphology more clearly. These fibrocartilaginous articular discs divide the synovial cavity, like the articular discs in the human temporomandibular and sternoclavicular joints, and should not be confused with the true intervertebral discs of mammals and other animals, which consist of a nucleus pulposus and an annulus fibrosus.

Crucially, the extant phylogenetic bracket (EPB) does not help us to establish the nature of the intervertebral articulations in sauropods, as the two extant groups most closely related to them have different articulations. As noted, birds have synovial joints; but crocodilians, like mammals, have fibrocartilaginous intervertebral discs. So their most recent common ancestor, the ur-archosaur, could equally have had either condition, and so could its various descendants.

vertebral-joint-type-cladogram

This seems like a mystery well worth solving. For one thing,  in the wholly inadequate database that we assembled for the paper, the birds had much thinner cartilage than the other animals. Since they are also the only animals with synovial neck joints, thin cartilage correlates with this kind of joint — at least across that tiny database. Is that correlation reliable? Does it hold out across a bigger sample? Is there a causation? If so, then finding out what kind of intervertebral joints sauropods had would help us to determine how thick their cartilage was, and so what their actual neutral posture was.

But we can’t tell this directly unless we find sensationally well preserved specimens that let us see the structure of the cartilage. We might speculate that since birds have unique saddle-shaped joints and sauropods have ball-and-socket joints like those of mammals and crocs, they’d be more likely to resemble the latter in this respect, too, but that’s rather hand-wavey.

Can we do better?

If we can, it will be through osteological correlates: that is, features of the bones (which are preserved in fossils) that are consistently correlated with features of the soft tissues (which are not). We’d want to find out from analysis of extant animals what correlates might exist, then go looking for them in the bones of extinct animals.

A couple of times now, I’ve pitched this as an abstract for a Masters project, hoping someone at Bristol will work on it with me as co-supervisor, but so far no-one’s bitten. Maybe next year. It would be a very specimen-based project, which I’d think would be a plus in most people’s eyes.

Figure 8. Cervical vertebra 7 from a turkey. Anterior view on the left; dorsal, left lateral and ventral views in the middle row; and posterior on the right.

Taylor and Wedel (2013: Figure 8). Cervical vertebra 7 from a turkey. Anterior view on the left; dorsal, left lateral and ventral views in the middle row; and posterior on the right.

Anyway, the awful truth is that at the moment we know spectacularly little about the cartilage in the necks of sauropods. We don’t know whether they had true intervertebral discs. If not, we don’t know whether they had articular discs like those of ostriches. We don’t know how thick these elements, if present, were. We don’t know how thick the hyaline cartilage on the bones’ articular surfaces was, or how evenly it covered its those surfaces.

And until we know those things, we don’t really know anything about neck posture or range of movement.

There’s lots of work to be done here!

Advertisements

10 Responses to “Did sauropod necks have intervertebral discs?”


  1. […] time: more on searching for osteological correlates of cartilage. Then, measurements of sauropod-neck cartilage from CT scans, and likely implications for cartilage […]

  2. Duane Nash Says:

    As someone who has suffered the pains and setbacks of lower back pain- ruptured disc/stenosis/bulging disc- this post is of some interest to me. Here is my thought: If they had intervertebral discs might sauropods occasionally have suffered slipped discs/disc degeneration? And if so, could such ailments have left tell-tail signs on the bone? Disc issues leave bone spurs (osteophytes) around facets and in the intervertebral space in humans. Whether or not you can prove that the presence of such features is diagnostic of disc problems I don’t know. And also there is the limited data set as well. But disc problems are common enough in humans that it stands to reason they may have been common in multi-tonne archosarus if they had discs.

  3. Mike Taylor Says:

    I think it’s inevitable that any animal will sometimes suffer damage to its cartilage, and that’s especially the case with something like a sauropod that has that cartilage under heavy compression much of the time.

    I don’t know anything about the osteological correlates of such conditions, and I’ve never noticed the bone spurs you mention — but then again I’ve never been deliberately looking for them. Often the cotyle rim gets eroded away from sauropod vertebrae anyway — see for example the left posterolateral view of the Xenoposeidon holotype, NHM R5937 — and I have a horrible feeling that when they’re preserved they might sometimes get prepped away. So it’s quite possible that they’re out there.

  4. Cameron Pahl Says:

    Maybe this wouldn’t help for sauropod research, but has anyone used any fancy imaging techniques on the famous Trachodon Mummy? I bet some intervertebral material is preserved in that specimen, not to mention some of the other dino mummies of the past 20 years or so. How hard is it to access the imaging machines they used for Sue’s skull and other CT scanned dinosaurs? More importantly, is it difficult to get access to the specimens? I know duckbills are a lot different than sauropods, but they could provide a starting point.

  5. Mike Taylor Says:

    Yes, CT-scanning well-preserved vertebral sequences will help. I hope we’ll have a post up soon about the CT-scanning part of our paper. Finding suitable material is the trick: getting articulated sequence of adult sauropod cervicals through a scanner isn’t easy.

  6. Jura Says:

    “Crucially, the extant phylogenetic bracket (EPB) does not help us to establish the nature of the intervertebral articulations in sauropods, as the two extant groups most closely related to them have different articulations.”

    True, this limits our inferences to a level 2 or 2′. To better gain perspective on what the likely character would be we need to move further down the tree. Further down, however, does not mean looking immediately at mammals. What do the neck articulations in squamates and turtles look like? Checking the CT data I have for savannah monitors and iguanas, they look remarkably similar to the tight synovial articulations of birds. The super-low resolution CT images on Digimorph appear to verify this for other squamates as well (e.g., chameleons [http://digimorph.org/specimens/Chamaeleo_calyptratus/whole/], gila monster [http://digimorph.org/specimens/Heloderma_suspectum/juvenile/whole/], and thorny devil [http://digimorph.org/specimens/Moloch_horridus/whole/], though it seems less so for geckos). These same tight articulations seem true for turtles too. I’ve looked at scans of sulcatas and red-foot tortoises. They seem to have this. Again, turning to Digimorph, it looks like other turtles have it too (Pig-nosed turtle [http://digimorph.org/specimens/Carettochelys_insculpta/] and bog turtle [http://digimorph.org/specimens/Carettochelys_insculpta/]). It’s difficult to say using the scans on Digimorph, but this tight articulation may also be true for salamanders. If so then it seems that tight intervertebral articulation is a plesiomorphy for Tetrapoda, and the intervertebral disks of mammals and crocs are both independent apomorphies. Bringing this back to sauropods we might expect tight intervertebral joints to be the default for dinosaurs. At the very least it leads to the interesting question of what selective factors lead to intervertebral disk evolution. Apparently one hypothesis is that it offers an energy saving benefit by increasing elastic recoil (Buchholtz 2007). I would assume it probably offers a weight bearing benefit too.

    Ref.

    Buchholtz, E.A. 2007. Modular evolution of the cetacean vertebral column. Evolution & Development. 9(3):278–289

  7. Mike Taylor Says:

    Very interesting, Jura, thanks for that. Can anyone else shed light on the form of intervertebral cartilage in these other groups?

  8. Mark Robinson Says:

    Interesting stuff Mike (and Jura!). I don’t know why this didn’t occur to me before but is there any correlation between the type of vertebral joint and the type of cartilage? Am I correct in thinking that extant crocs have both ball-and-socket cervical joints and cartilage discs, similar to mammals?

    More importantly, it’s pleasing to see that cyborgs are finally starting to be included in phylogenetic trees. I look forward to a silhouette of a Cyberdyne Systems Model 101 being used as an indication of scale in a skeletal reconstruction soon.


  9. […] [This is part 4 in an ongoing series on our recent PLOS ONE paper on sauropod neck cartilage. See also part 1, part 2, and part 3.] […]


  10. […] I’d forgotten, when I wrote this post, that I’d previously written a more detailed post about the discs-in-sauropod-necks problem. If you’re interested in the problem, you should read […]


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: