Sauropod Vertebra Picture of the Week

A section of the cotyle of a presacral vertebra of Alamosaurus (Woodward and Lehman 2009:fig. 6A). The arrow will be explained in a future post!

Last year was good for sauropod pneumaticity. In the past few months we’ve had the publication of the first FEA of pneumatic sauropod vertebrae by Schwarz-Wings et al (2009), as well as a substantial section on pneumaticity in the big Alamosaurus histology paper by Woodward and Lehman (2009). I won’t repeat here everything that Woodward and Lehman have to say about pneumaticity, I just want to draw attention to a little piece of it. Their work is observant, up-to-date, and worth reading, so if you can get access to the paper, read it.

The major brake on the growth of our knowledge and understanding of pneumaticity is sample size. I harped on this in 2005 (Wedel 2005), and Mike just brought it up again in a comment on a previous post. In fact, what he had to say is so relevant that I’m going to just cut and paste it here:

How does degree of pneumatisation vary between individuals? Here are three more: how does it vary along the neck, how does it vary long the length of an individual vertebra, and how does it vary through ontogeny? Then of course there is variation between taxa across the tree. So what we have here is a five-and-half-dimensional space that we want to fill with observations so that we can start to deduce conclusions. Trouble is, there are, so far, 22 published observations (neatly summarised by Wedel 2005:table 7.2), which is not really enough to let us map out 5.5-space! That’s one reason why, at the moment, each observation is valuable — it adds 4% to the total knowledge in the world.

To be fair, there are a few more published observations. Schwarz and Fritsch (2006) published ASPs for cervicals of Giraffatitan and Dicraeosaurus, and I have a gnawing feeling that there are a couple here and there that I’ve seen but not remembered. I’ve got some more of my own data in the as-yet-unpublished fourth chapter of my diss, which I failed to get out as part of the Paleo Paper Challenge. And, getting back to the subject of the post, Woodward and Lehman (2009:819) have some tasty new data to report:

Digital images of sections of vertebrae and ribs were imported into ArcGIS 8.1 (Dangermond, 2001; for methods see Woodward, 2005). A unitless value for the total area of the image was calculated, using the outline of the bone as a perimeter. Subtracted from this was the area value taken up by bone, as determined by color differences (lighter areas are camellate cavities, darker areas are bone). Using this method, longitudinal sections of centra are estimated to be roughly 65% air filled. The amount of open space similarly calculated for the pneumatic proximal and medial rib sections is about 52%, whereas the cancellous spongiosa in distal rib transverse sections yields an average estimate of about 44% of their cross sectional area. Hence, the camellate cavities result in an appreciably lower bone volume compared to spongiosa.

The ASP of 0.65 for centra is right in line with the numbers I’ve gotten for neosauropods, and with the results of Schwarz and Fritsch (2006) for Giraffatitan (Dicraosaurus had a much lower ASP, around 0.2 IIRC). The stuff about the ribs is particularly interesting. Using densities of 0.95 for bone marrow, 1.8 for avian (and sauropod) compact bone, and 1.9 for mammalian compact bone we get the following:

• Pneumatic Alamosaurus vertebrae – ASP of 0.65, density of 0.63 g/cm^3.
• Pneumatic Alamosaurus ribs – ASP of 0.52, density of 0.86 g/cm^3.
• Apneumatic Alamosaurus ribs – MSP (marrow space proportion) of 0.44, density of 1.43 g/cm^3.
• Pneumatic bird long bones – ASP of 0.59, density of 0.74 g/cm^3.
• Apneumatic bird long bones – MSP of 0.42, density of 1.44 g/cm^3.
• Apneumatic mammal long bones – MSP of 0.28, density of 1.63 g/cm^3.

ASPs and MSPs of bird and mammal bones are calculated from K values reported by Cubo and Casinos (2000) for birds and Currey and Alexander (1985) for mammals. I don’t know what the in vivo density of sauropod compact bone was; changing it from the avian value of 1.8 to the mammalian value of 1.9 would have a negligible effect on the outcome.

At least with the data in hand, we can make the following generalizations:

• The apneumatic bones of birds are thinner-walled than those of mammals, on average. (This has been known for a long time.)
• The apneumatic ribs of Alamosaurus were more similar in density to apneumatic bird bones than to apneumatic mammal bones.
• In both birds and Alamosaurus, pneumatization reduces the amount of bone tissue present by 15-30% in the same elements (long bones for birds, ribs for Alamosaurus). Pneumatic bones are light not just because the marrow is replaced by air, but because there is less bone tissue than in apneumatic bones, as bird people have been observing for ages.

There’s loads more work to be done on this sort of thing, so I’m going to stop blogging now and get back to it. Stay tuned!

References

• Cubo, J., and Casinos, A. 2000. Incidence and mechanical significance of pneumatization in the long bones of birds. Zoological Journal of the Linnean Society 130: 499–510.
• Currey, J. D., and Alexander, R. McN. 1985. The thickness of the walls of tubular bones. Journal of Zoology 206:453–468.
• Schwarz D, and Fritsch G. 2006. Pneumatic structures in the cervical vertebrae of the Late Jurassic Tendaguru sauropods Brachiosaurus brancai and Dicraeosaurus. Eclogae Geologicae Helvetiae 99:65–78.
• Schwarz-Wings, D., Meyer, C.A., Frey, E., Manz-Steiner, H.-R., and Schumacher, R. 2009. Mechanical implications of pneumatic neck vertebrae in sauropod dinosaurs. Proceedings of the Royal Society B. doi: 10.1098/rspb.2009.1275
• Wedel, Mathew J.  2005.  Postcranial skeletal pneumaticity in sauropods and its implications for mass estimates.  pp. 201-228 in Wilson, J. A., and Curry-Rogers, K. (eds.), The Sauropods: Evolution and Paleobiology.  University of California Press, Berkeley.
• Woodward, H.N.,  and Lehman, T.M. 2009. Bone histology and microanatomy of Alamosaurus sanjuanensis (Sauropoda: Titanosauria) from the Maastrichtian of Big Bend National Park, Texas. Journal of Vertebrate Paleontology 29(3):807-821.