More on the internal structure of Alamosaurus vertebrae
January 18, 2010
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.
- 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.