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.

Figure 14. Geometry of opisthocoelous intervertebral joints. Hypothetical models of the geometry of an opisthocoelous intervertebral joint compared with the actual morphology of the C5/C6 joint in Sauroposeidon OMNH 53062. A. Model in which the condyle and cotyle are concentric and the radial thickness of the intervertebral cartilage is constant. B. Model in which the condyle and cotyle have the same geometry, but the condyle is displaced posteriorly so the anteropos- terior thickness of the intervertebral cartilage is constant. C. the C5/C6 joint in Sauroposeidon in right lateral view, traced from the x-ray scout image (see Figure 12); dorsal is to the left. Except for one area in the ventral half of the cotyle, the anteroposterior separation between the C5 cotyle and C6 condyle is remarkably uniform. All of the arrows in part C are 52 mm long.

Figure 14. Geometry of opisthocoelous intervertebral joints. Hypothetical models of the geometry of an opisthocoelous intervertebral joint compared with the actual morphology of the C5/C6 joint in Sauroposeidon OMNH 53062. A. Model in which the condyle and cotyle are concentric and the radial thickness of the intervertebral cartilage is constant. B. Model in which the condyle and cotyle have the same geometry, but the condyle is displaced posteriorly so the anteroposterior thickness of the intervertebral cartilage is constant. C. the C5/C6 joint in Sauroposeidon in right lateral view, traced from the x-ray scout image (see Figure 12); dorsal is to the left. Except for one area in the ventral half of the cotyle, the anteroposterior separation between the C5 cotyle and C6 condyle is remarkably uniform. All of the arrows in part C are 52 mm long.

But how thick was the intervertebral cartilage in sauropods?

At the moment, no-one really knows. As Kent Stevens (2013) points out in his contribution to the PLOS ONE sauropod gigantism collection:

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.

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.

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 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).

Taxon Thickness Reference Notes
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.

Figure 22. Dog neck in X-ray. Neck of a dog (dachsund), in X-ray, with the seven cervical vertebrae indicated. This photo has been used with permission from the Cuyahoga Falls Veterinary Clinic.

Figure 22. Dog neck in X-ray. Neck of a dog (dachsund), in X-ray, with the seven cervical vertebrae indicated. This photo has been used with permission from the Cuyahoga Falls Veterinary Clinic.

(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.

References

Posted by Mike Taylor
Filed in cartilage, cervical, necks, Sauroposeidon, stinkin' every thing that's not a sauropod
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The importance of cartilage in neck reconstructions

November 3, 2013

As I mentioned a few days ago, Matt and I have a couple of papers in the new PLOS ONE Sauropod Gigantism collection. We were each lead author on one and second author on the other, so for convenience’s sake we’ll refer to them as my paper (Taylor and Wedel 2013c on neck cartilage) and Matt’s paper (Wedel and Taylor 2013b on caudal pneumaticity.)

Mine is very simple in concept (although it ended up at 17 pages and 23 figures). It’s all about addressing one of the overlooked variables in reconstructing the postures of the necks of sauropods (and indeed of all tetrapods). That is, the spacing between consecutive vertebrae, and the effect this has on “neutral pose”.

The concept of “neutral pose” goes back to the DinoMorph work of Stevens and Parrish (1999). They defined it (p. 799) as follows: “We determined the neutral poses for each animal, wherein the paired articular facets of the postzygapophyses of each cervical vertebra were centered over the facets of the prezygapophyses of its caudally adjacent counterpart.”

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Taylor and Wedel (2013c: Figure 3). Articulated sauropod vertebrae. Representative mid-cervical vertebra of Giraffatitan brancai, articulating with its neighbours. The condyle (ball) on the front of each vertebra’s centrum fits into the cotyle (socket) at the back of the preceding one, and the prezygapophyses articulate with the preceding vertebra’s postzygapophyses. These vertebrae are in Osteological Neutral Pose, because the pre- and postzygapophyseal facets overlap fully.

One of the more fundamental flaws in Stevens and Parrish (1999) is the assumption that animals habitually rest their necks in neutral pose — an assumption that is unsupported by evidence and, as it turns out, false (Vidal et al. 1986, Taylor et al. 2009). But let’s leave that aside for the moment, and consider what neutral pose actually represents.

The fact that there is even such a thing as neutral articulation between two consecutive vertebrae is due to there being three points of contact between those vertebra: as with the legs of a tripod, three points is the minimum number you need to fix an object in three-dimensional space. Two of these points are at the zygapophyses, as noted in the original definition above. The third point is the articulation between the centra.

The centrum has been curiously overlooked in discussions of neutral pose, but needless to say its length is crucial in establishing what is neutral. In the image above, if the centrum was longer, then the angle between the consecutive vertebrae would need to be raised in order to keep the zygapophyses articulated.

And of course it was longer in life, because of the cartilage in between the consecutive centra. (The use of the more specific term “osteological neutral pose” goes some way to recognising that tissues other than bone have been overlooked, but the problem has not really been addressed or even properly acknowledged in published works before our paper.)

xx

Taylor and Wedel (2013c: Figure 5). Intervertebral gaps in camel necks. Head and neck of dromedary camels. Top: UMZC H.14191, in right lateral view, posed well below habitual posture, with apparently disarticulated C3/C4 and C4/C5 joints. Photograph taken of a public exhibit at University Museum of Zoology, Cambridge, UK. Bottom: OUMNH 17427, in left lateral view, reversed for consistency with Cambridge specimen. Photograph taken of a public exhibit at Oxford University Museum of Natural History, UK. Inset: detail of C4 of the Oxford specimen, showing articulations with C3 and C5. The centra are separated by thick pads of artificial ‘‘cartilage’’ to preserve spacing as in life.

You simply can’t ignore cartilage when modelling neck postures and expect to get anything resembling a meaningful result. That is, presumably, the reason why the habitual posture of rabbits in life exceeds the most extended posture we were able to obtain when manipulating dry vertebrae of a hare: compare Vidal et al. (1986: fig. 4) with Taylor et al. (2009: fig. 1).

How big is the effect? That depends on the thickness of the cartilage and the height of the zygapophyses above the center of rotation. Here is an illustration that we should have put in the paper, but which inexplicably neither of us thought of:

figNEW-angle-at-zygs

Influence of intervertebral cartilage on vertebral articulation angle. Consider the posterior vertebra (black) as fixed. The blue vertebra represents neutral pose of the preceding vertebra with centra abutting and zygapophyseal facets maximally overlapped. The red vertebra indicates neutral pose once intervertebral cartilage is added between the vertebra (where else?) The green lines show the angle by which the more anterior vertebra must be inclined in order to accommodate the cartilage, and the magenta line shows the height of the zygapophyseal articulation above the center of rotation between the two vertebrae.

Here’s some elementary trigonometry. Suppose the intervertebral cartilage is x distance thick at mid-height of the centra, and that the height of the zygs above this mid-height point (the magenta line) is y. The triangle between the middle of the condyle of the posterior vertebra, the middle of the cotyle of the anterior one and the zygapophyseal articulation is near enough a right-angled triangle as makes no odds.

Consider the angle θ between the green lines. Sin(θ) = opposite/hypotenuse = x/y, and by similarity, the additional angle of inclination of the anterior vertebra is also θ.

But for small angles (and this is generally a small angle), sin(θ) ≈ θ. So the additional inclination in radians = cartilage thickness divided by zygapophyseal height. For example, in vertebrae where the zygs are 23 cm above the mid-height of the centra, adding 4 cm of intervertebral cartilage adds about 4/23 = 0.174 radians = 10 degrees of extra inclination. (That’s pretty similar to the angle in the illustration above. Eyeballing the cartilage thickness and zyg height in the illustration suggests that 23:4 ratio is about right, which is a nice sanity-check of this method.)

millionaire-stupid-contestant4

At this point, I am cursing my own stupidity for not putting this diagram, and the very simple calculation, into the paper. I guess that can happen when something is written in a hurry (which to be honest this paper was). The formula is so simple — and accurate enough within tolerances of inevitable measurement error — that we really should have used it all over the place. I guess that will have to go in a followup now. [Update, 5th November 2014. It’s long overdue, but that followup paper has finally been submitted and is available as a preprint.]

Anyway — next time, we’ll address this important related question: how thick, in fact, was the cartilage between the cervicals of sauropods?

References

Posted by Mike Taylor
Filed in cartilage, cervical, Giraffatitan, I'm stupid, necks, papers by SV-POW!sketeers
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Two SV-POW! papers in the new PLOS Collection!

October 30, 2013

This is an exciting day: the new PLOS Collection on sauropod gigantism is published to coincide with the start of this year’s SVP meeting! Like all PLOS papers, the contents are free to the world: free to read and to re-use. (What is a Collection? It’s like an edited volume, but free online instead of printed on paper.)

There are fourteen papers in the new Collection, encompassing neck posture (yay!), nutrition (finally putting to bed the Nourishing Vomit Of Eucamerotus hypothesis), locomotion, physiology and evolutionary ecology. Lots for every sauropod-lover to enjoy.

x

Taylor and Wedel (2013c: Figure 12). CT slices from fifth cervical vertebrae of Sauroposeidon. X-ray scout image and three posterior-view CT slices through the C5/C6 intervertebral joint in Sauroposeidon OMNH 53062. In the bottom half of figure, structures from C6 are traced in red and those from C5 are traced in blue. Note that the condyle of C6 is centered in the cotyle of C5 and that the right zygapophyses are in articulation.

Matt and I are particularly excited that we have two papers in this collection: Taylor and Wedel (2013c) on intervertebral cartilage in necks, and Wedel and Taylor (2013b) on pneumaticity in the tails of (particularly) Giraffatitan and Apatosaurus. So we have both ends of the animal covered. It also represents a long-overdue notch on our bed-post: for all our pro-PLOS rhetoric, this is the first time either of has had a paper published in a PLOS journal.

Wedel and Taylor (2013b: Figure 4). Giraffatitan brancai tail MB.R.5000 (‘Fund no’) in right lateral view. Dark blue vertebrae have pneumatic fossae on both sides, light blue vertebrae have pneumatic fossae only on the right side, and white vertebrae have no pneumatic fossae on either side. The first caudal vertebra (hatched) was not recovered and is reconstructed in plaster.

It’s a bit of a statistical anomaly that after a decade of collaboration in which there was never a Taylor & Wedel or Wedel & Taylor paper, suddenly we have five of them out in a single year (including the Barosaurus preprint, which we expect to eventually wind up as Taylor and Wedel 2014). Sorry about the alphabet soup.

Since Matt is away at SVP this week, I’ll be blogging mostly about the Taylor and Wedel paper this week. When Matt returns to civilian life, the stage should be clear for him to blog about pneumatic caudals.

Happy days!

References

Posted by Mike Taylor
Filed in Apatosaurus, cartilage, caudal, cervical, Giraffatitan, necks, open access, papers by SV-POW!sketeers, PLoS, pneumaticity, Tails, vomit
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Cartilage accounts for nearly a fifth of the length of the neck of a baby giraffe

March 26, 2011

In a comment on the previous post, Dean asked: “What was the difference in length between the neck with its cartilage and the bones flush together?”

I’m glad you asked me that.  You’ll recall from last time that the fully fleshed neck — intact apart from the removal of the skin and maybe some superficial muscle — was 51 cm in length from the front of the atlas to the back of the centrum of the seventh cervical vertebra.  When I pose the cleaned and cartilage-free bones together, the total length of the series is only 41 cm — 10 cm shorter, coming in at just over 80% of the live length.  Don’t believe me?  Here are the photos!

I’m sure I need hardly say, but the top image is the neck as we got it, the second is the cleaned bones posed in more or less the arrangement they must have been in life (both of these taken from the previous post) and the bottom image the bones fully abutting.

So!  The neck of Wallace the baby giraffe was very nearly a quarter as long again as the bones alone suggest.  Does this mean that the neck of Giraffatitan was really 10.6 m long instead of 8.5 m?

It’s an exciting prospect, but I’m afraid the answer is no.  As I hinted last time, while it’s perfectly acceptable, indeed obligatory, to recognise the important role of cartilage in sauropod necks qualitatively, we can’t blindly apply the numbers from Wallace the baby giraffe to adult sauropods for two reasons: 1, Wallace is a baby; and 2, Wallace is a giraffe.

The first of these reasons is part of why I am keen to do this all over again with an adult giraffe when I get the opportunity; but there’s not much we can do about the second.  One might think that a more closely related extant animal such as an ostrich might have a neck that is more homologous with those of sauropods; and that’s true, but my feeling is that the giraffe is more analogous.  That is, although the birds share more recent common ancestry with sauropods, giraffes’ more similar size seem to have encouraged them to evolve cervicals that are in some ways more similar to those of sauropods, most notably in the possession of ball-and-socket intervertebral joints rather than the saddle-shaped joints that are ubiquitous in birds.

How big a deal is Wallace’s juvenile status?  Well, take a look at his fifth cervical vertebra in posterior view:

If this bone were found in 150 million years by competent palaeontologists, in a world where there were no extant artiodactyls to compare with, what would they make of it?  Most of the articular area of the centrum is very obviously damaged, exposing the internal spongy texture of cancellous bone — presumably the bone surface was attached more firmly to the cartilaginous posterior end of the element than to the inner part of the bone, so it came away with the cartilage during simmering.  So it would be obvious to our future palaeontologists that the articular surface was missing, and that the complete vertebra would have been somewhat longer — but it would be hard to judge by how much.

But the state of this bone is particularly interesting because the middle part of the centrum does have a preserved bone surface.  It would be easy to extrapolate that out across the whole area of the posterior end of the centrum, and assume that this was the maximum posterior extent of the element’s functional length in life — an assumption that we know, having taking the neck apart ourselves, would be completely wrong.

Are we making similar incorrect assumptions with our sauropod vertebrae?

An even more interesting case is the postzygapophyses.  The posterodorsal surface of the left postzyg is slightly damaged, but the bone of the right postzyg has a nice, perfectly preserved surface.  But I can tell you that the functional articular surface of the postzyg was totally different from this: different size, different shape, different position, different orientation.  If we tried to calculate range of movement from these zygapophyseal facets, the results we got would be literally meaningless.

The good news is, there’s a clue that would prevent us from making this mistake — a really nice, obvious one.  The texture of the bone on the postzyg is irregularly crenellated in a way that strongly indicates a cartilaginous extension: it’s the same texture you see on the ends of the long-bones of (even mature) birds if you peel off the cartilage caps.  (It’s also what you see, at a much bigger scale, on the ends of the sauropod long-bones.)

But while the presence of this texture indicates the presence of cartilage, I don’t know whether the converse is true.  In the absence of such a texture, can we assume the absence of cartilage?  I just don’t know.  Anyone?

Posted by Mike Taylor
Filed in cartilage, cervical, giraffe, juvenile, necks, stinkin' mammals
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♫ This is the dawning of the age of articular cartilage ♫

October 6, 2010

Isn’t it funny how often an idea seems to pop up all over the place at about the same time?  The classic example is the independent and more or less simultaneous invention of calculus by both Isaac Newton and Wilhelm Leibniz, but similar kinds of things seem to happen quite often.

And there’s something similar going on right now.  After a century of everyone ignoring the role of cartilage in dinosaur anatomy, suddenly everyone’s up and running all at once:

  • Here at SV-POW!, Matt, Darren and I have been running the series on camel necks (which by the way isn’t over yet — stay tuned!)  In that series we have repeatedly made the point that “it is useless to try to reach conclusions about neck posture based on osteology alone. We need to understand the soft-tissue systems — especially the articular cartilage — as well”.
  • Meanwhile, over on his blog Jurassic Journeys, Matt Bonnan has been writing about “long bones and the space between“, emphasising how we can’t really understand sauropod locomotion when we don’t know the true sizes and shapes that the long-bones had in life.
  • Independently of that, Heinrich Mallison, on the Palaeontologia Electronica blog, wrote about the importance of cartilage in his Plateosaurus digital modelling projects.  I highly recommend reading this very relevant article if only for its section headings, which sum up the state of play perfectly: Ask your doctor for advice // Palaeontology is an interdisciplinary science — we just tend to forget // Have you ever read the Journal of International Orthopaedics? // How do these go together? Where’s the manual? HELP!
  • The next thing we know, Casey Holliday and his colleagues wrote about the same issue — not merely blogging, but producing a long-awaited peer-reviewed article in PLoS ONE, “Cartilaginous Epiphyses in Extant Archosaurs and Their Implications for Reconstructing Limb Function in Dinosaurs“.  Casey and his group have gone much further than the rest of us: rather than just whining about the problem of cartilage, they’ve taken steps to solve it — see below for details.
  • Finally, it turns out that Dave Hone has had a blog entry on this subject in the works at Archosaur Musings for a year or more.

It’s a pretty amazing confluence of thought, and the Holliday et al. paper really couldn’t have come at a better time.  It gives us, for the first time, qualitative estimates of the thickness of articular cartilage in limb-bones.  They dissected birds and alligators, measured their limb bones before and after the removal of their cartilage caps, compared the measurements, and determined what they called cartilage correction factors (CCFs) that quantify the increase in limb length when cartilage is included.  They also examined the osteological correlates of extensive articular cartilage, and drew conclusions about the likely form and function of these structures in sauropods (and, yes, I suppose, other dinosaurs as well).

This all ties in nicely with a long-running background project of mine, first presented at Progressive Palaeontology in 2005, and then again at the German sauropod-fest in 2008.  While Holliday et al. were investigating the thickness of articular cartilage, I was thinking in a very naive way about its area as part of a study tentatively entitled Upper limits on the mass of land animals estimated through the articular area of limb-bone cartilage.  The slides for the talk are available, and contain a Godzilla joke that will be hauntingly familiar to anyone who saw my talk on neck elongation at SVPCA this year.

Poorly executed slide from my 2005 Progressive Palaeontology talk. Despite the clumsy graphics, the point should be clear: that the area of articular cartilage available to withstand static and locomotory forces depends hugely on how extensive the cartilage caps are, and on their shape.

I ought to be clear that my work on this was very preliminary and that I am, as usual, years behind where I wanted to be in terms of getting this written up rigorously.  In fact the talk ended with a slide in which I pointed that I was pretty confident that “my figures are correct within a factor of 756”.  And I stand by that :-)

My point is just this: suddenly there’s a visible swell of palaeontologists all saying the same thing: that we can’t expect to understand how the skeletons of extinct animals worked by looking only at their bones, which is a bit of a shame when their bones are usually all we have.  The Holliday et al. paper (2010) is a very welcome first step towards wrasslin’ with this problem as it deserves.

Oh, and it’s open access — go read it!

References

Posted by Mike Taylor
Filed in cartilage, stinkin' appendicular elements
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