What’s up with your insanely thick intervertebral discs, Snowmass Haplocanthosaurus?

Among the numerous weird features of MWC 8028, the Snowmass Haplocanthosaurus, is the extreme biconcave profile of the caudal vertebrae, in which each centrum is basically reduced to a vertical plate of bone separating two cup-shaped articular surfaces. All four available caudals — found in different parts of the quarry, in different orientations — have essentially the same cross-section. For the diagram above, I just copied caudal 3, because it’s the most complete, so I could figure out the thickness and cross-sectional shape of a single intervertebral disc.

I drew a more realistic version, with the first three caudals at approximately the right scale, for our neural canal paper last year:

The first three caudal vertebrae of Haplocanthosaurus specimen MWC 8028 in midsagittal section, emphasizing the volumes of the neural canal (yellow) and intervertebral joint spaces (blue). Anterior is to the right. Wedel et al. (2021: fig. 2B).

It’s a drawing, sure, but it’s based on a true story, because we have CT scans of all the vertebrae (and we’re going to publish them, soon, along with the reconstructed verts). 

(NB: I’m using “intervertebral disc” as a convenient shorthand for “whatever soft tissues filled the joint space”. But I do think it was a big, fat, fibrocartilaginous disc, not wildly different from the ones in the human vertebral column. It’s not totally impossible that there was some combination of crazy thick articular cartilage and a synovial cavity — there is some precedent in extant salamanders and lizards — but that seems way less likely, for reasons I’ll go into in detail elsewhere. Incidentally, the notion is floating around that reptiles have only synovial intervertebral joints, but this is simply false: intervertebral discs are present in some squamates [Winchester and Bellairs 1977] and in the tails of birds [Baumel 1988].)

I should point out that the other specimens of Haplocanthosaurus also have biconcave caudal vertebrae, but the concavities are much shallower. So what we’re seeing in MWC 8028 is an extreme version of something we see in other individuals of the same genus.

Now, because the caudal centra and joint spaces are roughly radially symmetrical, their relative cross-sectional areas, in these mid-sagittal sections, should be good proxies for their relative volumes. You can imagine the generating the volume of a centrum by rotating its cross-section through 180 degrees, ditto for the joint space (ignoring tilt since both the centrum and joint space are tilted). We’ll have this math worked out in more detail in the next paper, along with volumes from the 3D models, but the upshot is this:

The volume of the intervertebral discs is about twice that of the vertebral centra. If we ignore the neural arch and spine and the transverse processes, and focus only on the weight-bearing column formed by the proximal caudal centra and intervertebral discs, that column is 2/3 cartilage and only 1/3 bone. 

Why, tho?

I spent some time brainstorming with Alton Dooley and we came up with a whole slate of hypotheses. We don’t necessarily like any of them very much, we’re just trying to cast the widest possible net, to make sure we haven’t overlooked any possibilities, no matter how remote they might seem. Here’s what we have so far:

Non-biological:

1. taphonomic distortion

Abnormal biology:

2. congenital malformation

3. pathology

Ontogenetic:

4. incomplete ossification (animal died without laying down the ‘missing’ bone)

5. senescence (the ‘missing’ bone was removed by some process related to aging)

Functional:

6. increased or decreased movement between vertebrae

7. weight reduction

8. shock absorption

What else? 

To reiterate, we’re in the hypothesis-generating stage, not the hypothesis-evaluating stage. So we’re not interested in whether any of these hypotheses are likely. (In point of fact, I think the ones we have so far all suck.) We just want all of the ideas that aren’t impossible.

The comment field is open!

References

• Baumel, J.J. 1988. Functional morphology of the tail apparatus of the pigeon (Columba livia). Advances in Anatomy, Embryology, and Cell Biology 110: 1-115.
• Wedel, Mathew; Atterholt, Jessie; Dooley, Jr., Alton C.; Farooq, Saad; Macalino, Jeff; Nalley, Thierra K.; Wisser, Gary; and Yasmer, John. 2021. Expanded neural canals in the caudal vertebrae of a specimen of Haplocanthosaurus. Academia Letters, Article 911, 10pp.
• Winchester, L. and Bellairs, A.D.A. 1977. Aspects of vertebral development in lizards and snakes. Journal of Zoology 181(4): 495-525.