Xenoposeidon week, day 3: the basic beast inside

November 17, 2007

Welcome to our continuing coverage of the wackiness that is Xenoposeidon. I drew the ‘pneumaticity’ straw, not surprisingly, so I get to introduce the anterior and posterior views of the vertebra, which reveal some of the internal structure. But they also reveal another bit of weirdness, which is the neural canal, so let’s start there.

Neural Canal

As you’ll recall from an earlier post, the neural canal is the hole in a vertebra through which the spinal cord passes. As you can see above, in posterior view the opening of the neural canal is a nicely-behaved, nearly circular hole. It would be completely unremarkable if it wasn’t so different from the opening on the front side, a scant 6 inches away. The anterior opening is slightly wider, about three times as tall, and vaulted like a cathedral.

Now, admittedly the anterior opening is filled with matrix, so it’s quite likely that we’re just seeing some kind of antrum and that lurking within that bony cathedral there is another nicely-behaved, nearly circular hole. But I’ve never seen a sauropod vertebra with such divergent neural canal openings, and neither have Mike or Darren, and they’ve both looked at a lot more dorsals than I have.

Still, I can offer a pretty good guess about why the anterior canal opening is vaulted that way. In birds, pneumatic diverticula not only run alongside the vertebrae, they also pass through the neural canal above the spinal cord. So birds occasionally have pneumatic fossae and foramina at the openings of the neural canal or even in the walls of the tube. We are fairly certain that sauropods also had these supramedullary airways, partly because they often have pneumatic features around the openings of the neural canals, and also because a handful of vertebrae actually show connections from the neural canal to the surrounding pneumatic cavities. More on that another time. For now, our inference is that the anterior opening of the neural canal of Xenoposeidon is so big because it held pneumatic diverticula in addition to the spinal cord. The posterior opening probably held the spinal cord alone. Why that should be is beyond us for now, and it will probably stay that way until someone does a big comparative study and maps the precise location of the diverticula in a bunch of sauropods.

Symmetry, Schmymmetry

William Blake asked, “What immortal hand or eye / could frame thy fearful symmetry?” None, in this case, because symmetry is in short supply. In the first Xenoposeidon post you probably noticed that the pneumatic features are also different on the left and right sides of the centrum. On both sides you have a pneumatic foramen (air hole) sitting inside a larger pneumatic fossa (depression or excavation), but the similarity ends there. The foramen on the left is twice as big as the one on the right, and the fossa on the right is partly divided by a small lamina and has a smaller accessory fossa above it, to boot.

Now, this asymmetry is also weird, but it’s expected weirdness. Pneumaticity seems to just be inherently variable, whether we’re talking about human sinuses or the facial air sacs of whales or the vertebrae of chickens. It appears that the form of pneumatic features is entirely determined by local tissue interactions, with little or no genetic control of the specific form. Think of it this way: genes prescribe certain developmental events, and those events bring tissues into contact–such as pneumatic epithelium and bone. The morphology of the bone arises out of that interaction, and each interaction of bone and pneumatic epithelium has the potential to produce something new. In this case, the diverticula on the left side of the vertebral column come from the lungs or air sacs on the left, and those on the right side come from the lungs or airs sacs on the right, so it’s really two sets of diverticula contacting the bone independently. The wonder, then, is not that pneumatic bones are so variable, but that we see any regularities at all.

Mike and Darren didn’t count the left-right asymmetry when they listed the diagnostic features of Xenoposeidon. If these features can vary so much from one side of a single bone to the other, they probably also vary among bones and among individuals and populations in increasing proportion, which makes them fairly useless for taxonomy. They did count the differences in the shape of the neural canal openings, but those are midline structures with contributions from diverticula on both sides (at least in birds, and we have no reason to suspect otherwise in sauropods), so the presence of a big pneumatic antrum on the front and the absence of one on the back is much more likely to represent a real, heritable, taxonomically useful difference.

How do we know? Partly because we’ve looked at a lot of vertebrae and have seen a lot more left/right asymmetry than anything else, and also because we have a plausible, testable explanation for why that should be so. But, like everything we tell you, this is a hypothesis, and it’s open to falsification. We’re not just cool with that–we prefer it that way.

Internal Affairs

The anterior part of the centrum is sheared off at an angle to reveal some of the ‘guts’ of the vertebra. The centrum was hollow in life, with a vertical midline septum separating the left and right halves, and a couple of projections sticking down and in at roughly 11:00 and 1:00. What is all this mess?

In most sauropods, the pneumatic diverticula on either side of the vertebra invaded the centra and hollowed out a pair of big chambers, called camerae. ‘Camera’ is Latin for ‘chamber’; a variant spelling forms part of the name of Camarasaurus, the “chambered reptile”, a dorsal vertebra of which is shown below.

camarasaurus-internal-structure.jpg

You can see from the horizontal section (slice 2, above) that big chambers are all you get in this vertebra. But you’ve seen pictures of more complicated stuff here before–vertebrae with lots of little chambers inside. This is something else I’ve beat to death in my papers, so here’s the short, short version: primitive sauropods have simple pairs of chambers; in most derived sauropods the vertebrae have a mix of big and small chambers, and in the most derived (and longest necked) sauropods the vertebrae are completely filled with small chambers.

To see what the internal structure of Xenoposeidon might have looked like when the vertebra was complete, let’s look at a couple of relatives that are probably comparable in terms of internal structure (recall that we have very little idea where Xenoposeidon actually fits into the evolutionary tree of sauropods; that’s part of what makes it so cool).

apatosaurus-omnh-500.jpg

brachiosaurus-byu-500.jpg

These vertebrae of Apatosaurus and Brachiosaurus have large, paired camerae in the middle of the centrum, but the ends of the centra are divided into smaller chambers by radial walls of bone that form septa between the cavities. The Apatosaurus vert is shown in anterior and lateral view. The Brachiosaurus vert might be a bit tougher to interpret. The roof of the centrum and the neural spine are detached (preserved, just as a separate piece) and we’re looking at the vertebra in left antero-dorso-lateral view. What’s left of the condyle is visible in the lower left foreground, and you can see the prominent median septum connecting it to the cotyle, which is facing away toward the stack of books on the upper right. Anyway, the Xenoposeidon vertebra probably had a whole ring of those septa, but when the front of the centrum was sheared off they were all lost, except for the two closest to the top.

xeno-septa-500.jpg

This type of internal structure, with a combination of big camerae and smaller chambers, is only found in mamenchisaurids and neosauropods, and it’s one of the pieces of evidence that Xenoposeidon is a neosauropod, albeit a strange one. If you want the full phylogenetic story, don’t forget that you can read the paper for free, but also stay tuned for the other 4/7 of Xenoposeidon Week here at your number one sauropod vertebra news source.

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17 Responses to “Xenoposeidon week, day 3: the basic beast inside”

  1. Mike from Ottawa Says:

    Just because Mike Taylor (I hate to be so formal, but as Mike Traynor, it’s not easy finding shorter distinctions) make a plaintive cry for comments so you know we’re out there (can’t you hear us breathing?), I’ll say I’ve just looked at the pictures for this one. For someone like myself, an outsider to paleontology, it is work getting through these and Xenoposeidon week is a bit overwhelming. It is work, but worth it, as these are fascinating and you guys do an excellent job explaining what before I wouldn’t have believed could be made interesting. I’m just thinking how lucky it would be to be a student of any of the three of you.

  2. Mike from Ottawa Says:

    I should add that while I just looked at the pictures, that is so far. I will go through this post properly in the next little while.


  3. […] would be known if it weren’t snapped in two; and of course, we’d not have known about the internal structure of the Xenoposeidon vertebra were the condyle not blasted […]


  4. […] there). What is true of straight lines is true in spades of perfect cylinders, especially in the asymmetric osteological playgrounds of sauropod vertebrae.  Like cold fusion and cheap mortgages, the foramen was Too Good To Be […]


  5. […] to pick favourites. But from my own very biased perspective, I particularly enjoyed all eight days of the extended Xenoposeidon week, a rather exhausting series of posts that may make Xeno the most […]


  6. […] may remember this image from Xenoposeidon week–almost two years ago […]


  7. […] fossa on lateral face of the centrum. What’s up with that? Here’s an explanation from an old post (about another sauropod) that still […]


  8. […] or Diplodocus, showing the pneumatic internal structure. Compare to similar views of dorsals in this post and this one. This is actually one half of a matched set that includes both halves of the centrum. […]

  9. ijreid Says:

    I have just found all the extra pictures on Mike’s website (http://www.miketaylor.org.uk/dino/xeno/extras.html#3) and see that many of them show views that I have not seen before. Are these images, like those of Brontomerus freely licensed, or are they copyrighted? If they are in fact free to use Mike, and you wouldn’t mind them going into a wikipedia article on Xeno, could you email me what license and under what conditions the images can be used, so that they are not removed from wikipedia? Thanks for a reply.

  10. Mike Taylor Says:

    Hmm. Well, so far as I am concerned anyone is welcome to use those photos under the terms of the Creative Commons Attribution licence (CC By). But the Natural History Museum’s terms state that all photographs taken of their material falls under their own copyright, which means I am not allowed to make that offer. In fact, I’m not sure we’re technicall even allowed to show photos of NHM material here on the blog — see Credit where it’s due.

    The more I think about that, the more wrong it seems. I may try to start up a broad discussion of museum photography policies.

    But for now, I think that, ridiculously, it’s only fully safe to include NHM specimen photos in Wikipedia articles if the museum has given explicit permission.

  11. ijreid Says:

    Well, the Freedom of Panorama in UK, according to wiki, is “OK, including public interiors”. This means that the image are freely licensed, and while the may museum still retain copyright, depending of course, they cannot control what license is used for the work.

  12. Mike Taylor Says:

    I’m wasn’t familiar with the term freedom of panorama — thanks for introducing it to me. I don’t see anything in that article about “public interiors”, though.

    It’s interesting to think about how this applies to the Natural History Museum. The prose in section 62 of Copyright, Designs and Patents Act 1988 refers to “sculptures, models for buildings and works of artistic craftsmanship”, which doesn’t mention fossils by name though arguably was intended to cover such things. But it also mentions “if permanently situated in a public place or in premises open to the public”, which covers specimens in the public galleries but arguably not those in collections. They are not “situated in a public place” but are, by the letter of the law, “in premises open to the public”.

    I have no idea how any of that would work out in law if it was ever pursued. We need our old friend Mike From Ottawa to chime in.

  13. ijreid Says:

    Oh, you have the wrong wiki. http://commons.wikimedia.org/wiki/Commons:Freedom_of_panorama is the proper link, as all image that are either CC licensed or 70 + years old are housed there, and lack pages on the english wiki. It defines what a “public interior” is as “buildings, and
    sculptures, models for buildings and works of artistic craftsmanship (if permanently situated in a public place or in premises open to the public). Even if the fossil is not permanently situated in a public place, the or shows that as long as its premises are open to public, it is freely licensed.

  14. ijreid Says:

    Oh, side note now that I have informed you that the images are under your copyright. Are the Brontomerus images also free to use, and would an emailed permission be possible so I can forward the license to wiki OTRS?

  15. Mike Taylor Says:

    Yes, the Brontomerus images are all copyright me or Matt (we don’t care which, maybe call them joint copyright) and licenced CC By. The Oklahoma Museum of Natural History has no impediment clauses.

  16. ijreid Says:

    Is an emailed permission stating that all Brontomerus and Xenoposeidon images on your website (miketaylor.co.uk) are freely licensed under CC-by possible, because last time this popped up in a discussion on a blog, the image was deleted because (apparently) the logins are not secure enough to verify it truly is the author.

  17. Mike Taylor Says:

    I have changed the wording on the Brontomerus extras page to make it explicit that the photos are CC By (and therefore available to Wikimedia).

    Unfortunately I can’t do the same for the Xenoposeidon photos because of the complications outlined above. I will contact the museum and see if anything can be done.


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