New paper: pneumaticity in a rebbachisaurid caudal vertebra

Fig. 2. Rebbachisauridae indet. (MDPA-Pv 007) from the Sierra Chata locality (Candeleros Formation) Cenomanian (Upper Cretaceous). Anterior caudal vertebra in anterior (A1, A3), posterior (A4, A6), and left lateral (A7, A9) views. Close ups showing lateral spinal laminae (A2), accessory bony lamina located inside of spof (A5), foramina in the lateral surface of the centrum, arrowheads indicate the presence of foramina (A8). Abbreviations: acdl, anterior centrodiapophyseal lamina; amedl, anterior medial lamina; cdf, centrodiapophyseal fossa; cpol, centropostzygapophyseal lamina; cprl, centroprezygapophyseal laminae; nc, neural canal; pcdl, posterior centrodiapophyseal lamina; pmedl, posterior medial lamina; pocdf, postzygapophyseal centrodiapophyseal fossa; pocdf-l, postzygapophyseal centrodiapophyseal fossa lamina; posdf, postzygapophyseal spinodiapophyseal fossa; prcdf, prezygapophyseal centrodiapophyseal fossa; prcdf-l, prezygapophyseal centrodiapophyseal fossa lamina; prdl, prezygodiapophyseal lamina; prsdf, prezygapophyseal spinodiapophyseal fossa; pz, postzygapophyses; spof, spinopostzygapophyseal fossa; spdl, spinodiapophyseal lamina; spol-f, spinopostzygapophyseal lamina fossa; sprl, spinoprezygapophyseal laminae; sprl-f, spinoprezygapophyseal lamina fossa. Windholz et al. (2024: fig. 2).

I have a new paper out in Acta Paleontologica Polonica, with Guillermo Windholz, Juan Porfiri, Domenica Dos Santos, and Flavio Bellardini, on the first CT scan of a pneumatic caudal vertebra of a rebbachisaurid:

Windholz, G.J., Porfiri, J.D., Dos Santos, D., Bellardini, F., and Wedel, M.J. 2024. A well-preserved vertebra provides new insights into rebbachisaurid sauropod caudal anatomical and pneumatic features. Acta Palaeontologica Polonica 69(1):39-47. doi: 10.4202/app.01104.2023

This will be a short post because I’m on the road right now, but I’m pretty darned happy about this paper. Like many of my recent publications, this is primarily a descriptive paper, but with interesting implications.

Drawings of an Isle of Wight rebbachisaurid anterior caudal vertebra (MIWG 5384). A, anterior view; B, right lateral view; C, posterior view. Scale bar represents 200 mm. Mannion et al. (2011: fig. 2).

I’ve been interested in caudal pneumaticity in rebbachisaurids for a long time. As far as I can remember, the first paper that clued me in on the subject was Mannion et al. (2011), on Early Cretaceous rebbachisaurid material from the Isle of Wight. The deep, subdivided, often asymmetric fossae on the neural spines and transverse processes showed that at least some rebbachisaurids evolved caudal pneumaticity comparable to that of diplodocids. I’ve been wanting to see CT scans of a rebbachisaurid caudal ever since, and last summer, Guillermo Windholz wrote to offer me that very opportunity.

Fig. 4. Selected computed tomographic sections of Rebbachisauridae indet. (MDPA-Pv 007) from the Sierra Chata locality (Candeleros Formation) Cenomanian (Upper Cretaceous). Vertebra in anterior view (A1), transverse section taken at mid-length of the element (A2), parasagittal section (A3), frontal sections (A4–A10). Abbreviations: cdf, centrodiapophyseal fossa; nc, neural canal; pocdf, postzygapophyseal centrodiapophyseal fossa; prcdf, prezygapophyseal centrodiapophyseal fossa; spol-f, spinopostzygapophyseal lamina fossa; sprl-f, spinoprezygapophyseal lamina fossa. Windholz et al. (2024: fig. 4).

The scans are beautiful, but the revealed anatomy is wacky. The neural spine and transverse processes are shown to be formed of thin, intersecting laminae that bound deep fossae, which is always cool to see but also expected at this point — Osborn figured similarly-excavated neural spine cr0ss-sections from Diplodocus back in 1899. Internally, the centrum shows a network of large, interconnected chambers, but the internal structure is wildly asymmetric. This is particularly evident in parts A2 and A10 of Figure 4, shown above.

So what’s going on here? Why is pneumatization of the neural spine and transverse processes so complete, while pneumatization of the centrum is so haphazard? I’m a big fan of asymmetric pneumatization, but this is ridiculous. And the bottom half of the centrum is basically a brick, in stark contrast to the extensive pneumatization of the upper works. I have some thoughts on this, but they’ll keep for a future post.

Also worth noting: although CT scanning fossils is becoming so common that it’s almost de rigueur these days, our global pool of CT-scanned sauropod vertebrae is tiny. Most of what we think we know — what I think I know, what I’ve built a good chunk of my career on — is connecting some very widely-spaced dots. Until last year, in all of human history we’d not managed to scan a single pneumatic caudal of a rebbachisaurid. Now we’ve scanned exactly one — which AFAIK is one more than the number of scanned vertebrae of any kind from Barosaurus, to pick an example at random. I wonder how much we’ll have learned when that number (in either category, Barosaurus vertebrae or rebbachisaurid caudals) is 5, or 10, or 50?

References

• Mannion PD, Upchurch P, Hutt S (2011) New rebbachisaurid (Dinosauria: Sauropoda) material from the Wessex Formation (Barremian, Early Cretaceous), Isle of Wight, United Kingdom. Cretaceous Research 32(6): 774–780.
• Osborn, H. F. 1899. A skeleton of Diplodocus. Memoirs of the American Museum of Natural History 1:191–214.
• Windholz, G.J., Porfiri, J.D., Dos Santos, D., Bellardini, F., and Wedel, M.J. 2024. A well-preserved vertebra provides new insights into rebbachisaurid sauropod caudal anatomical and pneumatic features. Acta Palaeontologica Polonica 69(1):39-47. doi: 10.4202/app.01104.2023