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

• 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.