Atlantal ribs of the Carnegie Diplodocus, Moscow and Vienna casts
Eighteen months ago, I noted that the Carnegie Museum’s Diplodocus mount has no atlantal ribs (i.e. ribs of the first cervical vertebra, the atlas). But that the Paris cast has long atlantal ribs — so long the extend past the posterior end of the axis.
There were two especially provocative comments to that post. First, Konstantin linked to a photo of the Russian cast (first mounted in St. Petersburg but currently residing in Moscow). I’ll reproduce it here:
Carnegie Diplodocus, Russian cast, head and anterior neck in left lateral view showing smallish, straight atlantal ribs. Our thanks to Konstantin for this photo.
As you can see, there are atlantal ribs on this specimen, but they do not resemble those of the Paris cast. These are much shorter, narrower, and lacking in structure. I have not to my knowledge seen anything like this on any other Diplodocus, and my guess — it’s only a guess — is that these were added by the Russians at some stage in this specimen’s very complex history.
But wait, there’s more!
In another comment on the same post, Crown House linked to a 3D model of the Vienna cast that has been posted to Sketchfab. It’s a pretty low resolution model, but if you zoom and pan, you can see that it has large and complex Paris-style atlantal ribs:
Carnegie Diplodocus, Vienna cast, head and anterior neck in left lateral view, showing large atlantal ribs.
Although these resemble the atlantal ribs of the Paris mount, they are not identical: the wavy margins face posteriorly rather than anteriorly as in both the Paris mount and Holland’s (1906) illustrations; and the proximal end has a dorsal expansion.
So we seem to have (at least) four different state of atlantal ribs in different casts of the same Diplodocus:
- Absent in Pittsburgh and London
- Small and rod-like in Moscow
- Long with a wavy dorsal margin in Paris
- Long with a wavy ventral margin and a proximal dorsal expansion in Vienna
Can anyone offer informed speculation on how this state of affairs came about?
But then things get weird. If you manoeuvre your way around the model to look up at this region from below:
Carnegie Diplodocus, Vienna cast, head and anterior neck in left ventrolateral view, showing lateral processes of the atlas.
Well, what the heck are we seeing here? There are two spiny processes, one on each side, projecting laterally from the ventral part of the atlas, and swept back at mid-length.
I have never seen anything like this in any sauropod — or, come to think of it, any other animal, but I admit I don’t pay much attention to other animals.
Does anyone have any idea what these projections are? Remember you can go to the model and look at them in 3D.
Source: https://svpow.com/2024/04/27/atlantal-ribs-of-the-carnegie-diplodocus-moscow-and-vienna-casts/
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