Sacral pneumatization in sauropods was complex

{.size-large .wp-image-15375 .aligncenter loading="lazy" attachment-id="15375" permalink="http://svpow.com/2018/10/16/sacral-pneumatization-in-sauropods-was-complex/amnh-516-diplodocus-sacrum-osborn-1904-fig-3/" orig-file="https://svpow.wordpress.com/wp-content/uploads/2018/10/amnh-516-diplodocus-sacrum-osborn-1904-fig-3.png" orig-size="1500,2350" comments-opened="1" image-meta="{"aperture":"0","credit":"","camera":"","caption":"","created_timestamp":"0","copyright":"","focal_length":"0","iso":"0","shutter_speed":"0","title":"","orientation":"0"}" image-title="AMNH 516 Diplodocus sacrum – Osborn 1904 fig 3" image-description="" image-caption="

Sacrum of Diplodocus AMNH 516 in left lateral and (for our purposes irrelevant) ventral views. (Osborn 1904 figure 3)

Here's D10 and the sacrum of Diplodocus AMNH 516 in left lateral and ventral view, from Osborn (1904: fig. 3). Note how the big lateral pneumatic foramina, here labeled 'pleurocoelia', start out up at the top of the centrum in D10 and kind of pinch out up there, seemingly entirely absent by S3 (although there is a suspicious-looking shadowed spot above and behind the sacral rib stump labeled 'r3'). Then on S4 and S5 the big foramina are back, but now they're low on the centrum, ventral to the sacral ribs. In ventral view, the foramina on D10, S1, and S2 aren't visible–they're both over the curve of the centrum, and in the case of S1 and S2, obscured by the sacral ribs. But in S4 and S5, the big lateral foramina are visible in ventral view.

I've been interested in a while in this seeming hand-off in centrum pneumatization from dorsolateral, which prevails in the dorsal vertebrae, to ventrolateral, which prevails in the posterior sacral and caudal vertebrae. Almost all sauropod dorsals have the pneumatic foramina quite high on the centrum, sometimes even encroaching on the neural arch. But if sauropod caudals have pneumatic fossae or foramina on the centrum, they're usually quite low, and almost always below the caudal rib or transverse process (there may also be pneumatic fossae on the neural arch and spine)–for evidence, see Wedel and Taylor (2013b). To me this implies two different sets of diverticula.

{.size-large .wp-image-15376 .aligncenter loading="lazy" attachment-id="15376" permalink="http://svpow.com/2018/10/16/sacral-pneumatization-in-sauropods-was-complex/haplocanthosaurus-cm-879-sacral-4-right-dorsolateral/" orig-file="https://svpow.wordpress.com/wp-content/uploads/2018/10/haplocanthosaurus-cm-879-sacral-4-right-dorsolateral.jpg" orig-size="2272,1704" comments-opened="1" image-meta="{"aperture":"2.6","credit":"","camera":"E4500","caption":"","created_timestamp":"1127743052","copyright":"","focal_length":"7.85","iso":"100","shutter_speed":"0.066666666666667","title":"","orientation":"1"}" image-title="Haplocanthosaurus CM 879 sacral 4 right dorsolateral" image-description="" image-caption="" medium-file="https://svpow.wordpress.com/wp-content/uploads/2018/10/haplocanthosaurus-cm-879-sacral-4-right-dorsolateral.jpg?w=300" large-file="https://svpow.wordpress.com/wp-content/uploads/2018/10/haplocanthosaurus-cm-879-sacral-4-right-dorsolateral.jpg?w=1024" srcset="https://svpow.wordpress.com/wp-content/uploads/2018/10/haplocanthosaurus-cm-879-sacral-4-right-dorsolateral.jpg?w=480 480w, https://svpow.wordpress.com/wp-content/uploads/2018/10/haplocanthosaurus-cm-879-sacral-4-right-dorsolateral.jpg?w=960 960w, https://svpow.wordpress.com/wp-content/uploads/2018/10/haplocanthosaurus-cm-879-sacral-4-right-dorsolateral.jpg?w=150 150w, https://svpow.wordpress.com/wp-content/uploads/2018/10/haplocanthosaurus-cm-879-sacral-4-right-dorsolateral.jpg?w=300 300w, https://svpow.wordpress.com/wp-content/uploads/2018/10/haplocanthosaurus-cm-879-sacral-4-right-dorsolateral.jpg?w=768 768w" sizes="(max-width: 480px) 100vw, 480px"}

I think that in part because sometimes you get both sets of diverticula acting on a single vert. Here's the centrum of sacral 4 of Haplocanthosaurus CM 879 in right dorsolateral view; anterior is to the right.

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Here's the same thing annotated (yeah, it does look a little like an Ent who is alarmed because his left eye has been overgrown by a huge nasal tumor). This vert has two sets of pneumatic features on the centrum: a big lateral fossa below the sacral rib articulation, presumably homologous with the same feature in S4 of the Diplodocus above; and a smaller dorsolateral fossa above and behind sacral rib articulation.

Unfortunately, CM 879 doesn't tell us much about how these two sets of diverticula might have changed along the column. The centra of S1-S3 were not found, S5 lacks both sets of fossae, the first caudal has fossae both on the centrum, below the caudal rib, and low on the arch, and the second and subsequent caudals lack both sets of fossae. (I wrote a LOT more about pneumaticity in this individual in my 2009 air sacs paper, which is linked below.)

Working out how these diverticula change serially is a tractable problem. Someone just needs to sit down with a reasonably complete, well-preserved series that includes posterior dorsals, all the sacrals, and the proximal caudals–or ideally several such series–and trace out all of the pneumatic features. As far as I know, that's never been done, but feel free to correct me if I've missed something. I'm neck deep in other stuff, so if someone wants that project, have at it. (If you happen to look into this, I'd be grateful for a heads up, so we don't run over each other if I do get a yen to investigate further myself.)

References

• Osborn, Henry F.  1904.  Manus, sacrum and caudals of Sauropoda.  Bulletin of the American Museum of Natural History 20:181-190.
• Wedel, M.J. 2009. Evidence for bird-like air sacs in saurischian dinosaurs. Journal of Experimental Zoology 311A:611-628.
• Wedel, Mathew J., and Michael P. Taylor. 2013. Caudal pneumaticity and pneumatic hiatuses in the sauropod dinosaurs Giraffatitan and Apatosaurus.PLOS ONE 8(10):e78213. 14 pages. doi:10.1371/journal.pone.0078213 [PDF]