The ribs of Brachiosaurus: stranger than you thought

As we’ve often observed, it’s a funny thing that incredibly well-known dinosaur specimens can sit around for decades, or for more than a century, before someone notices something fascinating about them. One lesson to learn from this is the importance of collections — their creation, maintenance and accessibility. Another is of course to always look at the fossils we see.

In another iteration of this old theme, today sees the publication of Taylor and Wedel (2023), a short paper on pneumatic features in the dosal ribs of our old friend FMNH PR 25107, the holotype of Brachiosaurus altithorax.

Taylor and Wedel 2023:Figure 2. Sauropod dinosaur Brachiosaurus altithorax Riggs, 1903, holotype FMNH PR 25107 from Dinosaur Quarry No. 13 near Grand Junction, Colorado, dating to the Kimmeridgian–Tithonian ages of the Late Jurassic, right dorsal rib “Rib A” in posterior view with proximal to the left. A1, the whole proximal half of the rib; a distal portion also exists, of similar length but without features relevant to this study; A2, close-up of the tuberculum, highlighting the complex network of support structures that show signs of speculative reconstruction. Circles highlight two possible sites of the “second tubercle” referred to by Riggs (1901: 549, 1903: 303, 1904: 239) based on Marsh’s illustration (1896: figs. 7, 8), reproduced here in Fig. 4; A3, close-up of the pneumatic foramen in the shaft of the rib, showing natural bone texture around the margin and no indication of breakage. Scale bars provide only a rough indication of the size of the elements: see the text for measurements. ” data-medium-file=”https://svpow.files.wordpress.com/2023/12/figure-2-rib-a.jpeg?w=300″ data-large-file=”https://svpow.files.wordpress.com/2023/12/figure-2-rib-a.jpeg?w=480″ class=”size-full wp-image-21683″ src=”https://svpow.files.wordpress.com/2023/12/figure-2-rib-a.jpeg?w=480&h=353″ alt=”" width=”480″ height=”353″ srcset=”https://svpow.files.wordpress.com/2023/12/figure-2-rib-a.jpeg?w=480&h=353 480w, https://svpow.files.wordpress.com/2023/12/figure-2-rib-a.jpeg?w=960&h=706 960w, https://svpow.files.wordpress.com/2023/12/figure-2-rib-a.jpeg?w=150&h=110 150w, https://svpow.files.wordpress.com/2023/12/figure-2-rib-a.jpeg?w=300&h=221 300w, https://svpow.files.wordpress.com/2023/12/figure-2-rib-a.jpeg?w=768&h=565 768w” sizes=”(max-width: 480px) 100vw, 480px” />

Taylor and Wedel 2023:Figure 2. Sauropod dinosaur Brachiosaurus altithorax Riggs, 1903, holotype FMNH PR 25107 from Dinosaur Quarry No. 13 near Grand Junction, Colorado, dating to the Kimmeridgian–Tithonian ages of the Late Jurassic, right dorsal rib “Rib A” in posterior view with proximal to the left. A1, the whole proximal half of the rib; a distal portion also exists, of similar length but without features relevant to this study; A2, close-up of the tuberculum, highlighting the complex network of support structures that show signs of speculative reconstruction. Circles highlight two possible sites of the “second tubercle” referred to by Riggs (1901: 549, 1903: 303, 1904: 239) based on Marsh’s illustration (1896: figs. 7, 8), reproduced here in Fig. 4; A3, close-up of the pneumatic foramen in the shaft of the rib, showing natural bone texture around the margin and no indication of breakage. Scale bars provide only a rough indication of the size of the elements: see the text for measurements.

Here’s the thing about this rib, which Riggs illustrated in two of his papers — the initial brief description of Brachiosaurus (Riggs 1903:figure 6) and the subsequent monographic osteology (Riggs 1904:plate LXXV:figure 5). It has a pneumatic condition that has not been documented in any other sauropod specimen.

The pneumatic opening is part way down the shaft — about 60 centimeters down from the tuberculum. But there are no other pneumatic features more proximally on the same face of the rib. (We don’t know what’s on the other face: it’s sitting in a plaster half-jacket, and flipping it would not be trivial.)

What does this mean? The seven-location schema of Wedel and Taylor (in revision) predicts that pneumatic features in costal elements would follow vascular foramina from the segmental and intercostal arteries. The segmental arteries pass behind the ribs on their circuit of the centrum, vacularizing the posterior aspect of the proximal portion of the ribs, the tubercula and capitula and region between them — and so providing channels for pneumatization in these regions. Meanwhile, intercostal arteries extend along and beyond the length of the rib shaft, providing opportunities for vascularization and subsequent pneumatization.

But while it’s pretty common to see pneumatization of the proximal portions of ribs, pneumatization of the shafts — likely by diverticula following the intercostal arteries — is hardly ever seen. In fact “Rib A” of the Brachiosaurus altithorax holotype provides the only documented occurrence, and that has only been recognized 120 years after the initial description.

All of this is more evidence for the opportunistic and random behaviour of pneumatizing diverticula. They always have the possibility of passing along the length of rib shafts and pneumatizin them — but either they rarely extend along the intercostal arteries, or if they do then they rarely excavate the bone that they are adjacent to. Why? We have no idea. It seems to be just the way the dice fall.

References

• Riggs, Elmer S. 1903. Brachiosaurus altithorax, the largest known dinosaur. American Journal of Science 15(4):299-306.
• Riggs, Elmer S. 1904. Structure and relationships of opisthocoelian dinosaurs. Part II, the Brachiosauridae. Field Columbian Museum, Geological Series 2(6):229-247, plus plates LXXI-LXXV.
• Taylor, Michael P., and Matthew J. WedelJ. 2023. Novel pneumatic features in the ribs of the sauropod dinosaur Brachiosaurus altithorax. Acta Palaeontologica Polonica 68(4): 709–718. doi:10.4202/app.01105.2023