Sauropod neural canals are weird, part 1b: more offset neural arch joints

January 22, 2018

In the first installment in this series (link), we looked at a couple of weird sauropod vertebrae with neurocentral joints that were situated either entirely dorsal or ventral to the neural canals. This post has more examples of what I am calling “offset” neurocentral synchondroses.

I decided it made more sense to refer to the synchondrosis as being offset, instead of referring to the neural canal as offset. Because the neural canal in all of these vertebrae is right where it pretty much always is, just dorsal to the articular surfaces of the centrum. In an adult, fused vertebra, there’d be no sign that anything unusual had ever happened. So I think it makes more sense to talk about the neurocentral joint having migrated dorsally or ventrally relative to the canal, rather than vice versa. If you know differently, or if these weirdos have been addressed before elsewhere and I’ve just missed it, please let me know in the comments!

Here’s a plate from Marsh (1896) showing caudal vertebrae of Camarasaurus (“Morosaurus” in O.C. Marsh parlance), which echo the Alamosaurus caudal from the first post in having the neurocentral joint almost entirely ventral to the neural canal. The neural arch here doesn’t just arch over the canal dorsally, it also cuts under it ventrally, at least in part.

This plate is also nice because it shows the relationships among the arch, centrum, and caudal ribs before they fuse. Here’s the caption, from Marsh (1896):

Here’s the preceding plate, Plate 33, with illustrations of an unfused Camarasaurus sacrum.

And its caption:

This plate not only shows how the sacral ribs fuse to the arch and spine medially, and to each other laterally (forming the sacrocostal yoke), it also shows a last sacral that is very similar to the aforementioned caudals in the position of the neurocentral joint. But interestingly that neurocentral joint offset only seems to be present in the last caudal sacral – the lower figure shows widely-separated neurocentral joint surfaces in the more anterior centra, indicating that the neural arches (not figured in this dorsal view) did not wrap around the neural canal to approach the midline. (I think we’re looking at S2 through S5 here, and missing a dorso-sacral.)

So now I’m freaked out, wondering if this neural arch wrap-around in the caudals is common to most sauropods and I just haven’t looked at enough juvenile caudals to have spotted it before. As always, feel free to ablate my ignorance in the comments, particularly if you know of more published examples. I’m a collector.

The neural canal of that last sacral also has a very interesting cross-sectional shape, like a numeral 8. I have some thoughts on that, but they’ll keep for a future post in this series.


5 Responses to “Sauropod neural canals are weird, part 1b: more offset neural arch joints”

  1. liang Says:

    “But interestingly that neurocentral joint offset only seems to be present in the last caudal”
    you mean “in the last sacral”, right?

  2. Matt Wedel Says:

    Oh yes, good catch! Fixed.

  3. John Scanlon Says:

    Internal lateral ridges in the neural canal are a feature of snakes, by the way (a group that never has neurocentral ‘joints’). Being wider below the ridges (across the inap, internal neural arch pockets sensu Hecht and LaDuke), the canal cross-section is better described as a trefoil than a 8.

  4. Matt Wedel Says:

    Thanks, John, that is extremely interesting. I assume the Hecht and LaDuke ref is:

    HECHT, M. K., AND T. C. LADUKE. 1997. Limbless tetrapods. In R. F. Kay, R. H. Madden, R. L. Cifelli, and J. J. Flynn (eds.), Vertebrate Paleontology in the Neotropics—The Miocene Fauna of La Venta, Colombia, pp. 95–99. Smithsonian Institution Press, Washington, DC.

    – is that correct?

    I haven’t gotten hold of a copy of that yet, but I’m going to plunge ahead fearlessly anyway: do these internal ridges occur in extant snakes, and if so, what soft tissues are they associated with?

  5. […] few points arise. As explained in this post, it makes more sense to talk about the neurocentral joint migrating up or down relative to the […]

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