How thick is the cartilage in necks of extant animals?
November 6, 2013
Last time, we looked at how including intervertebral cartilage changes the neutral pose of a neck — or, more specifically, of the sequence of cervical vertebrae. The key finding (which is inexplicably missing from the actual paper, Taylor and Wedel 2013c) is that adding cartilage of thickness x between vertebrae whose zygapophyses are height y above the mid-height of the centra elevates the joint’s neutral posture by x/y radians.
But how thick was the intervertebral cartilage in sauropods?
Determining the ONP of a sauropod’s cervical vertebral column given only its bones requires is necessarily speculative since the cartilage, and thus the intervertebral spacing, is unknown.
Part of the our goal in our own PLOS collection paper (Taylor and Wedel 2013c) was to take some very tentative first steps towards estimating the cartilage thickness. To do this, we used two approaches. First, we looked at CT scans of articulated vertebrae; and second, we measured the cartilage thickness in a selection of extant animals and thought about what we could extrapolate.
Since the CT scans were Matt’s domain, I’m going to pass over those for now, in the hope that he’ll blog about that part of the paper. Here, I want to look at the extant-animal survey.
The first thing to say is that our survey is inadequate in many ways. We worked with the specimens we could get hold of, in the state we had them. This means that:
- we have a very arbitrary selection of different animals,
- they are at different ontogenetic stages, and
- their cartilage thickness was measured by a variety of methods.
Our goal was not at all to reach anything like a definitive answer, but just to get the question properly asked, and so hopefully to catalyse much a more detailed survey.
With that proviso out of the way, here are our main results (from Table 4 of the paper, though here I have removed the sauropod CT-scan rows since we’ll be writing about those separately).
|Turkey||4.56%||This study||Difference in measurements of intact neck and articulated sequence of cleaned, degreased and dried vertebrae.|
|Ostrich||6.30%||Cobley et al. (2013)||Difference in measurements of individual vertebrae with and without cartilage.|
|Rhea||2.59%||This study||Measurement of in situ cartilage in bisected neck.|
|Alligator||14.90%||This study||Measurement of in situ cartilage from photograph of cross section.|
|Horse||6.90%||This study||Measurement of in situ cartilage from photograph of cross section.|
|Camel||13.00%||This study||Crude measurement from condyle margin to cotyle lip of lateral-view X-ray. This is an interim measurement, which we hope to improve on when we obtain better images.|
|Dog||17.00%||This study||Measurement of intervertebral gaps in lateral-view X-ray, uncorrected for likely concavity of cotyles.|
|Giraffe||24.00%||This study||Difference in measurement of intact neck and closely articulated sequence of cleaned vertebrae. Young juvenile specimen.|
|Muraenosaurus||14.00%||Evans (1993)||Measurement of in situ cartilage in fossils.|
|Cryptoclidus||20.00%||Evans (1993)||Measurement of in situ cartilage in fossils.|
We’ve expressed the measurements as a ratio between cartilage thickness and the length of the bone itself — that is, cartilage/bone. Another way to think of this is that the percentage is a correction factor which you need to add onto bone length to get whole-segment length. Note that this is not the same ratio as the proportion of total segment length that consists of cartilage: that would be (cartilage thickness + bone length) / bone length.
(We also tossed in some measurements of plesiosaur neck cartilage that Mark Evans made way back when. Get that thing properly published, Mark!)
Even this small survey throws up some interesting points.
First, there is a huge range of proportional cartilage thicknesses: almost an order of magnitude from the 2.59% of the Rhea up to the 24% of the juvenile giraffe — or, even if you discard that because of its ontogenetic stage, up to 17% for the dog. And note that the 17% for the dog is probably an under-estimate, since we were working from an X-ray that doesn’t show the concavity of the vertebral cotyles.
(Two reviewers expressed scepticism that this is the usual condition for dogs, but this X-ray is consistent with those of other dogs illustrated in the veterinary literature.)
The second thing to note is that the cartilage measurements for birds (average 4.5%) are are much lower than those of crocodilians (14.9%) or mammals (15.2%). What does this mean? Among these groups, sauropods are most closely related to birds; but birds and crocs form the extant phylogenetic bracket, so we can’t tell from phylogeny alone whether to expect them to more closely approach the avian or crocodilian condition. Furthermore, in being opisthocoelous (condyle in front, cotyle at the back) sauropod cervicals most closely resemble those of mammals in gross structure — and they have the thickest cartilage of all.
The third thing to note is that there is considerable variation within groups. Although the cartilage is proportionally thin for all three birds, it’s more than twice as thick in the ostrich as in the rhea (although some of this could be due to the different measurement methods used for these two birds). More interestingly, among mammals the cartilage is twice as thick in camels as in horses. In the horse, the condyles are deeply inserted into the cotyles of the preceding vertebrae; but in camels, they don’t reach even the lip of the cotyle. This should worry us, as horse and camel cervicals are grossly similar, and no osteological correlates have been identified that would allow us to determine from the bones alone how very different the cartilage is between these two mammals. So it seems possible that there were similarly dramatic differences in the neck-cartilage thickness of different sauropods.
Note: I said that no osteological correlates have been identified. That doesn’t mean they don’t exist. One thing I would love to see is a serious attempt to analyse cartilage thickness across a broad range of mammals, and to examine the corresponding dry bones to see whether in fact there are correlates that could be informative in this respect. One lesson that Matt and I have learned over and over again is that there’s often plenty of data in places that are out in the open, but where no-one’s thought to look.
Next time: more on searching for osteological correlates of cartilage. Then, measurements of sauropod-neck cartilage from CT scans, and likely implications for cartilage thickness in life.
- Cobley, Matthew J., Emily J. Rayfield, and Paul M. Barrett. 2013. Inter-vertebral flexibility of the ostrich neck: implications for estimating sauropod neck flexibility. PLOS ONE 8(8):e72187. 10 pages. doi:10.1371/journal.pone.0072187 [PDF]
- Evans, Mark. 1993. An investigation into the neck flexibility of two plesiosauroid plesiosaurs: Cryptoclidus eurymerus and Muraenosaurus leedsii. University College London: MSc thesis. London.
- Stevens, Kent A. 2013. The articulation of sauropod necks: methodology and mythology. PLOS ONE 8(10): e78572. 27 pages. doi:10.1371/journal.pone.0078572 [PDF]
- Taylor, Michael P., and Matthew J. Wedel. 2013c. The effect of intervertebral cartilage on neutral posture and range of motion in the necks of sauropod dinosaurs. PLOS ONE 8(10): e78214. 17 pages. doi:10.1371/journal.pone.0078214 [PDF]