Xenoposeidon week, day 3: the basic beast inside

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.

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).

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.

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.