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?
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.
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!
- Holliday, Casey M., Ryan C. Ridgely, J. C. Sedlmayr and Lawrence M. Witmer. 2010. Cartilaginous Epiphyses in Extant Archosaurs and Their Implications for Reconstructing Limb Function in Dinosaurs. PLoS ONE 5(9): e13120. doi:10.1371/journal.pone.0013120
- Taylor, Michael P. 2005. Upper limits on the mass of land animals estimated through the articular area of limb-bone cartilage. p. 18 in Anonymous (ed.), Conference programme and abstracts: Progressive Palaeontology 2005, University of Leicester, 15-16 June. 26 pp.