Sorry for the very short post. We have some longer stuff planned, but we’ve been too busy to kick it out this week, and I wanted to leave you with something cool to ponder over the weekend. Here’s the ilium of Giraffatitan overlaid on that of Brontomerus, scaled to the same acetabulum diameter (Giraffatitan is HMN J1, left ilium, modified from Janensch 1961: pl. E, fig. 2; Brontomerus is of course OMNH 66430 from Taylor et al. 2011:fig. 2).

And here’s the same thing comparing Rapetosaurus and Brontomerus (Rapetosaurus is holotype FMNH PR 2209, left ilium, modified from Curry Rogers 2009: fig. 39B). This one was tricky to scale because the ilial margin of the acetabulum is so different in the two taxa.

Here is the same trick performed with the ilium of the canonical pretty basal neosauropod Camarasaurus — specifically, Camarasaurus supremus AMNH 5761 Il. 1, left ilium, modified from Osborn and Mook (1921: fig. 87).  In this case, the proportions are so very different that it’s hard to make a meaningful superimposition: we tried to scale to equal acetabulum size, but probably that of the Camarasaurus was proportionally larger than in the other taxa illustrated in this post.  Still, here it is:

Finally, in response to Paul Barrett’s comment on a subsequent article, here is a superimposition of the ilium of Alamosaurus on that of Brontomerus:

(Sorry about the poor quality of this one, but the only figure I could find of a complete Alamosaurus ilium was the line-drawing in Lehman and Coulson (2002:fig. 8) — none of the standard descriptive works seem to illustrate a complete or near-complete ilium.)

We had a figure like these in an early draft of the paper, but we ditched it because we felt that doing a broader comparative figure would be more valuable. But I like the kick in the brainpan that these overlays provide.

References

A section of the cotyle of a presacral vertebra of Alamosaurus (Woodward and Lehman 2009:fig. 6A).

The last time we talked about Alamosaurus, I promised to explain what the arrow in the above image is all about. The image above is a section through the cotyle (the bony socket of a ball-and-socket joint) at the end of one of the presacral vertebra. The external bone surface would have been over on the left; it was either very thin (which happens) or a bit eroded, or both. The arrow is pointing at something weird–a plate of bone inside the vertebra that forms a sort of shadow cotyle deep to the articular surface.

This is weird for a couple of reasons. First, once camellate (small-chambered) vertebrae get above a certain level of complexity, it’s hard to make any sense of the orientation of individual bony struts. Possibly I haven’t seen enough vertebrae, or played with enough 3D models, to figure it out. You would certainly expect that the struts would be oriented to resist biomechanical loads, just like the struts in the long bones of your limbs; the fact that sauropod verts were filled with air whereas your long bones are filled with marrow shouldn’t make any difference. Back in the day, Kent Sanders–who is second author on that super-important paper on unidirectional air flow in croc lungs that you’ve probably heard about (Farmer and Sanders 2010)–speculated to me that the complex of laminae we see in the vertebrae of most sauropods are still there in the inflated-looking vertebrae of titanosaurs and birds, they’re just incarnated in internal struts rather than external laminae. Cool hypothesis for somebody to test.

The other reason that this is weird is that the plate of bone is parallel to the articular surface. One place where I have seen some regularity in terms of strut orientation is in zygapophyses, where in both camerae and camellate vertebrae the internal struts are oriented at right angles to the articular surfaces of the zygs, like beams propping up a wall. In this Alamosaurus section, there are indeed smaller struts that run at right angles to both the cotyle and the internal plate, but I have no idea why they’re so wimpy and the plate is so thick; a priori I would have expected the reverse.

It turns out that this isn’t even the first time that an internal “shadow” of the cotyle has been figured–check out this figure that I redrew from Powell’s (1992:fig. 16) Saltasaurus osteology. But don’t credit me with the discovery. I’d looked at this section a hundred times and even drawn it and never noticed the shadow cotyle, until it was pointed out by Woodward and Lehman (2009)–another reason to read that paper if you haven’t yet. Kudos to Holly Woodward for spotting this and making the connection.

Now that I’ve drawn attention to the weirdness and given credit where it’s due, this is one of those times I’m going to throw up my hands in confusion and open the floor for comments.

References

  • Farmer, C.G., and Sanders, K. 2010. Unidirectional airflow in the lungs of alligators. Science 327:338-340.
  • Powell, J.E. 1992. Osteologia de Saltasaurus loricatus (Sauropoda – Titanosauridae) del Cretacico Superior del noroeste Argentino; pp. 165-230 in J.L. Sanz and A.D. Buscalioni (editors), Los Dinosaurios y Su Entorno Biotico: Actas del Segundo Curso de Paleontologia in Cuenca. Institutio Juan de Valdes.
  • 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.

ASPs for Alamosaurus

January 4, 2010

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