The Potter Creek Brachiosaurus humerus, in various states of repair

Sunday, May 5, 2024 18:22

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As iconic as Brachiosaurus altithorax is, it’s known from surprisingly little material. As I cover in my 2009 brachiosaur paper (Taylor 2009:788–789), only the holotype specimen FMNH PR 25017 can be reliably considered to belong to the species: little of the material that has been referred to it over the years overlaps with the holotype, and among those elements that do, synapomorpies are hard to come by.

One referred element that does overlap is the Potter Creek humerus, which I covered in that paper as follows (Taylor 2009:788):

Potter Creek Humerus—As recounted by Jensen (1985, 1987), Eddie and Vivian Jones collected a large left humerus from the Uncompahgre Upwarp of Colorado and donated it to the Smithsonian Institution where it is accessioned as USNM 21903. It was designated Brachiosaurus (Anonymous, 1959) although no reason for this assignment was published; it was subsequently described very briefly and inadequately by Jensen (1987:606-607). Although its great length of 213 cm (pers. obs.) is compatible with a brachiosaurid identity, it is in some other respects different from the humeri of both B. altithorax and B. brancai, although some of these differences may be due to errors in the significant restoration that this element has undergone. The bone may well represent Brachiosaurus altithorax, but cannot be confidently referred to this species, in part because its true proportions are concealed by restoration (Wedel and Taylor, in prep.). It can therefore be discounted in terms of contributing to an understanding of the relationship between B. altithorax and B. brancai.

(By the way: that Wedel and Taylor (in prep.) paper has not materialized, fifteen years on. It’s titled “The humeri of brachiosaurid sauropods” and the manuscript has not been touched since 2007 — two years before the main brachiosaur paper was published! I just looked at it, and it’s 14 pages long, so I guess that’s yet another project that we really ought to exhume and push over the line.)

I first encountered this humerus in Jensen, where it’s illustrated in Figure 4:

Jensen (1985:figure 4B). Three reproductions: left, Brachiosaurus sp. rib 2.75 m (9′) long; middle, Ultrasaurus macintoshi right scapulocoracoid; right, left humerus of Brachiosaurus sp. from Potter Creek. J. A. Jensen (left) and Adrian M. Bouche (right).

Obviously you can’t make out a ton of detail in this photo, which in any case is of a replica rather than the original bone. But Jensen illustrated it better in his 1987 paper, figures 3 and 5 (as well as repeating figure 4 of his 1985 paper as figure 6 of the 1987 one).

Jensen (1987:figure 3D-E). Potter Creek Quarry brachiosaur. D, fourth or fifth dorsal vertebra; E, left humerus.

Jensen’s caption doesn’t say it, but obviously this view is anterior. (The dorsal vertebra from the same quarry is a whole nother kettle of non-tetrapod vertebrates, which we won’t discuss today.)

Jensen (1987:figure 5). Potter Creek quarry: A-D, brachiosaur humerus. A., proximal end; B, mid-shaft section; C, detail of bulbous deltoid crest; D, anterior, distal end.

This is not the clearest illustration. Part A is obviously in anterior view, matching nicely with Jensen’s figure 3E. Part B seems to be in medial view, and part C in lateral view. Part D, I can’t make much sense of: it’s described as “anterior, distal end”, but it’s not a good match for the distal end shown in figure 3E.

Some time later, I got to see the bone for myself: it’s long been on public display as a touch specimen at the NMNH in Washington DC. Here’s a photo — not one of mine, which didn’t come out too well, but one sent by Mike Brett-Surman:

Potter Creek Brachiosaurus humerus in anterior view, lateral to the bottom, in the NMNH public gallery. Photograph by Michael Brett-Surman.

Now you can probably tell from the photo, but in person it was really obvious that a great chunk in the middle was fakezilla. Here’s the drawing I did for myself back in 2007:

My notes on the Potter Creek Brachiosaurus humerus from by viewing at the NHMN in 2007.

(Yes, my sketch has the proportions horribly wrong. But it does properly capture where the faked up areas are.)

And here is one of my not-very-good photos: a close-up of part of the shaft, where damage to the surface clearly shows that what’s underneath the gloss is not bone but fibreglass or something similar:

Potter Creek Brachiosaurus humerus at the NMNH, close-up of reconstructed shaft, taken in 2007.

I think that reconstructed shaft is wider than it should be, which is why I argued back in 2009 that “it is […] different from the humeri of both B. altithorax and B. brancai, although some of these differences may be due to errors in the significant restoration that this element has undergone […] its true proportions are concealed by restoration.”

I’d since come around to thinking the humerus most likely is Brachiosaurus after all, as the main reason for finding that unlikely is down to the reconstructed thick shaft. But a little while ago, I found something really helpful: Brachiosaurus photos in the Smithsonian Institution Archives! In particular, this one showing the humerus as it used to be in 1959, shortly after it was donated to the museum sitting on a plinth in front of the mounted Diplodocus forefeet (which are really Camarasaurus forefeet, but that’s a different story).

“Dinosaur Bone on Exhibit”, 2 September 1959. Smithsonian Institution Archives, Acc. 16-126, Box 01, Image No. MNH-046.

The exciting thing about this photo is of course that it was taken before all that midshaft restoration was done. And sure enough, the shaft is noticeably narrower than in the current restored version — a much better match for the holotype Brachiosaurus humerus and even the yet-more-slender one of Giraffatitan.

(There is more I could say here, notably about the deltopectoral crest. But that can wait for another day — this post is plenty long enough as it.)

I can understand why this restoration was done: if this was to be a touch specimen, that fragile, damaged mishaft was absolutely going to flake away. The purpose of the restoration was probably just protection. But I still lament that it was done — and that it was done in this way. To me, it just says that this should never have been a touch specimen.

References

Anonymous. 1959. Brachiosaurus exhibit at the Smithsonian Institution. Nature 183:649–650.
Jensen, James A. 1985. Three new sauropod dinosaurs from the Upper Jurassic of Colorado. Great Basin Naturalist 45(4):697-709.
Jensen, James A. 1987. New brachiosaur material from the Late Jurassic of Utah and Colorado. Great Basin Naturalist 47(4):592-608.
Taylor, Michael P. 2009. A re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and its generic separation from Giraffatitan brancai (Janensch 1914). Journal of Vertebrate Paleontology 29(3):787-806. doi: 10.1671/039.029.0309