Sauropod Vertebra Picture of the Week #AcademicSpring

In the recent post on OMNH 1670, a dorsal vertebra of a giant Apatosaurus from the Oklahoma panhandle, I half-promised to post the only published figure of this vertebra, from Stovall (1938: fig. 3.3). So here it is:

And in the second comment on that post, I promised a sketch from one of my notebooks, showing how much of the vertebra is reconstructed. Here’s a scan of the relevant page from my notebook. Reconstructed areas of the vert are shaded (confusingly, using strokes going in opposite directions on the spine and centrum, and the dark shaded areas on the front of the transverse processes are pneumatic cavities), and measurements are given in mm.

Next item: is this really a fifth dorsal vertebra?

Apatosaurus louisae CM 3018 D4 and D5, in anterior (top), left lateral, and posterior views, from Gilmore (1936: plate 25).

Here are D4 and D5 of A. louisae CM 3018. They sort of bracket OMNH 1670 in terms of morphology. D4 has a broader spine, and D5 has a narrower one. The spine of D5 lacks the slight racquet-shaped expansion seen in OMNH 1670, but the overall proportions of the spine are more similar. On the other hand, the transverse processes of D4 taper a bit in anterior and posterior view, as in OMNH 1670, and unlike the transverse processes of D5 with their more parallel dorsal and ventral margins. But honestly, neither of these verts is a very good match (and the ones on either side, D3 and D6, are even worse).

Apatosaurus parvus UWGM 15556 (formerly A. excelsus CM 563) D4 (left) and D3 (right) in anterior (top), right lateral, and posterior views, from Gilmore (1936: plate 32).

Here are D3 and D4 of A. parvus UWGM 15556. D3 is clearly a poor match as well–it is really striking how much the vertebral morphology changes through the anterior dorsals in most sauropods, and Apatosaurus is no exception. D3 looks like a dorsal in lateral view, but in anterior or posterior view it could almost pass for a posterior cervical. If I was going to use the term “cervicodorsal”, indicating one of the vertebrae from the neck/trunk transition, I would apply it as far back as D3, but not to D4. That thing is all dorsal.

And it’s a very interesting dorsal from the perspective of identifying OMNH 1670. It has fairly short, tapering transverse processes. The neural spine is a bit shorter and broader, but it has a similar racquet-shaped distal expansion. I’m particularly intrigued by the pneumatic fossae inscribed into the anterior surface of the neural spine–in Gilmore’s plate they make a broken V shapen, like so \ / (or maybe devil eyes). Now, OMNH 1670 doesn’t have devil eyes on its spine, but it does have a couple of somewhat similar pneumatic fossae cut into the spine just below the distal racquet–perhaps a serially modified iteration of the same pair of fossae as in the A. parvus D4. It’s a right sod that D5 from this animal has its spine blown off–but it still has its transverse processes, and they are short and tapering as in OMNH 1670.

Apatosaurus sp. FMNH P25112, dorsal vertebrae 1-10 and sacrals 1 and 2, Riggs (1903: plate 46)

Here are all the dorsals and the first couple sacrals of FMNH P25112, which was originally described as A. excelsus but in the specimen-level analysis of Upchurch et al. 2005) comes out as the sister taxon to the A. ajax/A. parvus/A. excelsus clade. Note the striking similarity of the D5 here with D4 of the A. parvus specimen in Gilmore’s plate (until the careful phylogenetic work up Upchurch et al. 2005, that A. parvus specimen, once CM 563 and now UWGM 15556, was considered to represent A. excelsus as well). But  also notice the striking similarity of D6 to OMNH 1670. It’s not quite a dead ringer–the transverse processes are longer and have weird bent-down “wingtips” (XB-70 Valkyrie, anyone?)–but it’s pretty darned close, especially in the shape of the neural spine.

So what does this all mean? First, that trying to specify the exact serial position of an isolated vertebra is nigh on to impossible, unless it’s something that is one-of-a-kind like an axis. Second, after doing all these comparos I think it’s unlikely that OMNH 1670 is a D4–those are a bit too squat across the board–but it could plausibly be either a D5 or a D6. Third, I’m really happy that it doesn’t seem to match any particular specimen better than all the rest. What I don’t want to happen is for someone to see that this vertebra looks especially like specimen X and therefore decide that it must represent species Y. As I said in the comments of the previous post, what this Oklahoma Apatosaurus material needs is for someone to spend some quality time seeing, measuring, and photographing all of it and then doing a phylogenetic analysis. That sounds like an ambitious master’s thesis or the core of a dissertation, and I hope an OU grad student takes it on someday.

If you were intrigued by my suggestion that the big Oklahoma Apatosaurus rivalled Supersaurus in size, and wanted to see a technical comparison of the two, I am happy to report that Scott Hartman has done the work for you. Here’s one of his beautiful Apatosaurus skeletal reconstructions, scaled to the size of OMNH 1670, next to his Supersaurus silhouette. This is just a small teaser–go check out his post on the subject for a larger version and some interesting (and funny) thoughts on how the two animals compare.

References

• Gilmore, C.W. 1936. Osteology of Apatosaurus with special reference to specimens in the Carnegie Museum. Memoirs of the Carnegie Museum 11:175-300.
• Riggs, E.S. 1903. Structure and relationships of opisthocoelian dinosaurs, part I: Apatosaurus Marsh. Field Columbian Museum Publications, Geological Series 2(4): 165–196.
• Stovall, J.W. 1938. The Morrison of Oklahoma and its dinosaurs. Journal of Geology 46:583-600.