The revenge of the controversial hypantra of Argentinosaurus
January 8, 2008
A quick follow-up on Darren’s recent post: the reconstructed Argentinosaurus dorsal in the photo he used seems to be based on the anterior dorsal of the holotype — at the least, the proportions and most of the features are the same — so we can get some more information by looking at the figure of the same element in the description of Bonaparte and Coria (1993). Here it is:
This shows the details better than the photo, though admittedly it’s rather less spectacular. But what’s most noticable, to me at least, is that the centrum and the lower part of the neural arch is completely missing … which means that the “hypantrum” of the reconstructed vertebra in the photo that Darren used is pretty much a complete fiction. Or let’s be more charitable and say “involves a certain amount of interpretation”.
So! What’s the story? The dorsal figured here is the one that Bonaparte and Coria considered to be ?first, but the ?second and ?third (and more posterior dorsals) are also preserved — and more fully, at least in the relevant area. The ?second dorsal, shown in their figure 3, is not illustrated in anterior view, but the ?third is shown in their figure 4:
So now we can see what we came here for: the distinctly ventrolaterally sloping borders of the triangular hollow below the prezygapophyses. But is it a hypantrum? Well, the purpose of a hypantrum is to accept the hyposphene of the vertebra in front: but if this vertebra’s alleged hypantrum were filled by a hyposphene, its neural canal would be almost completely blocked off. So maybe not. And indeed the ?second dorsal of the Argentinosaurus type specimen, figured by B&C93 in posterior view as fig. 3A, doesn’t seem to have anything resembling a hyposphene. So that supports Salgado and Martínez’s (1993) assertion that the “hyposphenes” of Argentinosaurus are in fact just big old centropostzygapophyseal laminae. (Salgado and Bonaparte 2007 cited and reaffirmed this reidentification, but noted that the posterior dorsals of Big-A do seem to have big hyposphenes and hypantra. Unfortunately these don’t seem to be figured in any of the papers we’ve cited here, so who can say?)
At this point, I am going to stop (A) posting all the figures from Bonaparte and Coria 1993, and (B) pontificating … at least until I’ve read Apesteguía (2005) which — for shame — I haven’t, yet. Well, it’s a truly frightening piece of work.
References are the same as for Darren’s post.