More on the internal structure of Alamosaurus vertebrae

January 18, 2010

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.
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16 Responses to “More on the internal structure of Alamosaurus vertebrae”

  1. zinjanthropus Says:

    Is it possible that it’s a developmental artifact? I’m thinking of a paper on primate femora where they suggested that the bony struts in the proximal femur were a record of the advancing growth plate. I think they even called them “shadows.”

    I think this is the paper, but I can’t read it from this computer:
    http://www.ncbi.nlm.nih.gov/pubmed/15536040/

  2. Mike Taylor Says:

    That is DEEPLY weird. I wonder how many more vertebrae it happens in, and we’ve just been looking right through it.

  3. Graham King Says:

    Very interesting.
    As a quibble, in the bottom pic why is the dotted outline towards lower left shown diverging from the actual articular surface there?

    Surely it is not hard to accept that asymmetry was the actual life condition in an axial bone?

  4. Graham King Says:

    a developmental artifact

    Zinj raises an interesting concept.. What do we mean by ‘artifact’?

    Philosophy and assumptions tend to slip in often while we’re unawares..

    Isn’t EVERYTHING a developmental artifact (happenstance, how things are).. unless we adopt a view that some things are how they are because ‘they are meant to be that way’ and others are how they are ‘because it just chanced to happen that way’?
    But isn’t that a distinction implying teleology, ie purpose?

    Or perhaps it is meant to distinguish between things that are functional – ‘have to be a certain way to work’ – and things that are neither particularly functional nor dysfunctional in that respect – ‘could vary somewhat in form without significant effect on function’ – calling these, artifacts.

    Still, we may have to be tentative in making such assessments.. are such hypotheses falsifiable?

  5. Graham King Says:

    Oh, sorry if I’m being boring here but I’m intrigued now!
    I just looked up artifact/artefact and there are a variety of meanings, mostly implying human agency in modification, or distortion of what is ‘real’ by some experimental or observational technique.

    And that made me wonder.. what do we assume when we see a section?
    ..That structures we see there, are structures which extend, continuous, orthogonally through the plane of the section.
    But that is just our assumption.. and so what we infer may just be an artifact, in the above senses!

    Is that thick dark line (actually just internal to the arrow not at its tip) a relatively thick sheet of bone sectioned normal to the plane, or could it be a thin sheet of bone sectioned parallel (or obliquely within) its plane?
    ie
    Bone laid down normal to the articular surface? That would make structural sense, for resisting compressive loads.
    To be certain, we’d need several sections at different angles.

  6. davidmaas Says:

    In graphics work, the word artifact refers to visual elements that are not integral to the intended creation of the graphic work, but a result of process (fingerprints or drips), tools (brush fibers, drips, etc.)
    In the terms here – and by projecting such usage – I’d expect artifact to allude to structural elements that result from growth processes, but do not contribute to the achieved structural performance. ie. if the shadow serves a structural function in the arrived state, its not an artifact.
    Linguistic argument, I know… but that’s the nature of the discussion, right?

  7. Jon Mitchell Says:

    In reference to the above discussion on artifacts, the term “spandrel” has never seemed as apropos.

  8. Nima Says:

    It’s a very interesting bone. As for Graham King’s question, it looks like the Saltasaurus slice is in ventral view, and if so, the asymmetry is lateral, and therefore can not be natural. It’s likely due to erosion, based on the incomplete surface wall. That explains the dotted line.

    But as for that INNER cotyle wall… that thing is definitely natural IMO, but that makes it no less mysterious. Do brachiosaurids like Sauroposeidon also have this feature? It seems like something that evolved along with camellae, but perhaps highly camerate sauropods may have had them too. I admit I don’t know enough to say for sure if brachiosaurids, let alone diplodocoids and camarasaurs, also had this feature.

    My humble theory is that it may be some kind of shock-absorbing reinforcement. If it’s simply a cotyle within a cotyle (I know that’s an overly simple description…) then it doesn’t seem likely to be a weight-bearing device. But a shock-absorbing, internally reinforced neck makes a lot of sense, especially considering how titanosaurs were built like tanks and may well have used their necks in shoving matches over mates (i.e. they were so wide-bodied that the only way they could “joust” precisely without either missing or injuring each other would be tossing their necks sideways much like giraffes do today – only faster and without any cranial horns or bosses, and due to their delicate skulls, they probably only used necks as weapons in these duels, not heads).

    In the case of some larger titanosaurs, particularly Puertasaurus, the dorso-ventral range of neck movement appears very great, and reinforced cotyles could help absorb the stresses from rapid neck-raising and reaching into the treetops perhaps even a bit past vertical. Others, like Isisaurus, just seem to have freakishly hefty necks – which may be another reason for cotyle reinforcement. I don’t think it’s a growth artifact, unless these specimens are juveniles or young adults. Admittedly this is all just speculation for now, but what does everyone think?

  9. Nathan Myers Says:

    Have you spoken with any structural engineers? This might be an entirely familiar and conventional feature of some kinds of heavy equipment.

  10. Graham King Says:

    Nima, thank you for your reply, and I think your idea of inner reinforcement may have merit.
    But in regard to this portion of your answer

    As for Graham King’s question, it looks like the Saltasaurus slice is in ventral view, and if so, the asymmetry is lateral, and therefore can not be natural. It’s likely due to erosion, based on the incomplete surface wall. That explains the dotted line.

    [Graham's italics added]

    ..With respect, I disagree: No (why can’t it?), and No (to me, it doesn’t explain why the dotted line doesn’t accurately follow what continuous outline of bone there is remaining, at lower left, just as it does at upper left)..

    May I repeat my question?:

    Surely it is not hard to accept that asymmetry was the actual life condition in an axial bone?

    ..asymmetry not just as an artifact of prservation nor of post-fossilization distortion.

    Many cetacean species have notably asymmetric skulls, and possibly ‘handed’ “http://whyihatetheropods.blogspot.com/2009/08/handedness-in-feeding-behavior-of.html”>behaviour too. ‘Handedness’ has been stated for walruses, in their attacks on seabed molluscs. Owls exhibit cranial asymmetry. Human faces are often somewhat asymmetric – try mirroring each half of a well-known face, and two quite distinct different faces may result – although symmetry may serve as one criterion of facial attractiveness.

    Any long-lasting habit of ‘handedness’ in a sauropod, considering the leverage/forces involved, could surely result in developing some real asymmetries of its axial skeleton, such as this vertebra, could they not?

  11. Graham King Says:

    Oops, my first reference (above) should read

    ..Many cetacean species have notably asymmetric skulls, and possibly ‘handed’ behaviour too..

    Sorry!

  12. Mike Taylor Says:

    Asymmery of skulls is a feature found in some owls and troodontids as well as whales. But I’ve never heard of asymmetrical vertebrae. Not saying there aren’t or weren’t any; just that if there were, I’ve not heard about it. Anyone have any information on this?

    I do think, without having seen the Saltasaurus vertebra in question, that erosion is a much more likely explanation: I’ve seen A LOT of eroded vertebrae.

  13. William Miller Says:

    I can find references to asymmetrical vertebrae, but nothing that seems relevant – some about a pathology in mice where the last dorsal vertebra is asymmetrical, with one side fused to the sacrum and one side not.

    http://www.isleroyalewolf.org/overview/overview/wolf%20bones.html has a picture (bottom left of the 4 vertebra pictures) showing an extremely asymmetrical cervical. So it’s *possible* (at least in mammals) for vertebrae to be naturally asymmetrical, though it’s pathological.

    But aside from the asymmetry thing, that “shadow cotyle” is weird. If it exists in more than one specimen, that seems to make the possibility that it’s just a freak of chance much less likely…

  14. Mark Evans Says:

    Some plesiosaurs have asymmetrical vertebrae, albeit in the neural arches and spines. At least one species (as yet undescribed but I am working on it, honest) has dorsal neural spines which are “D” shaped in section with the convexity of the “D” pointing alternately left and right on successive vertebrae. The same animal dramatically reduces the postzygapophyses in the caudals, and at one point a vertbra has a left postzygapophysis and lacks a right one completely. Weird.

  15. Mike Taylor Says:

    Mark, to call their merely “weird” stretches the definition of that word! I don’t know if I’ve ever heard of anything so bizarre. Are you aware of any precedent? (And if not, then why oh why have you not published this beast yet?! :-)

  16. Graham King Says:

    Asymmetries of vertebrae: William’s (wolf) and Mark’s (plesiosaur) data: I find these details extraordinary, and fascinating!

    Mike:

    Are you aware of any precedent?

    Yes – Stegosaurus stenops! -okay not asymmetry of the vertebrae themselves (though I don’t know) but of the well-known dorsal plates (osteoderms?)
    A single row of plates has been proposed, with their bases in a continuous line covering the spinal column but with their upper large portions leaning alternately left and right. This seems most plausible to me, since it could arise developmentally simply by exaggerated growth of the distal portions of initially symmetric osteoderms (dorsal spines), which would then be displaced asymmetrically to clear their cranial/caudal neighbours.
    Among extant reptiles, iguanas and marine iguanas have a midline dorsal crest of simple spines, which do not all line up straight vertically but may lean to the sides somewhat randomly. I see Stegosaurus as exhibiting an extreme development of this reptilian trait.

    I wonder if Mark’s plesiosaur also had a dorsal crest: similarly with midline osteoderms that (enlarging) diverged alternately to left and right, influencing the neural spines beneath into their reported asymmetric form?

    Czerkas SA (1987). “A Reevaluation of the Plate Arrangement on Stegosaurus stenops”. in Czerkas SJ, Olson EC. Dinosaurs Past & Present, Vol 2. University of Washington Press, Seattle. pp. 82–99. ISBN.
    A short description of the arrangement is here


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