Now that the new Wilson and Allain (2015) paper has redescribed Rebbachisaurus, we can use it to start thinking about some other specimens. Particularly helpful is this beautiful rotating animation of the best dorsal vertebra (here captured at the point of the rotation where we’ve viewing it in right anterolateral):

rebbachisaur-dorsal-rotation

As I briefly discussed on Twitter, seeing this made me think of my baby, Xenoposeidon. Now that specimen, beautiful though it is, preserves only the lower one third of the vertebra. But there are some clear commonalities, and they’re clearer if you look at the animation.

Xenoposeidon-TNF

Most obviously, there are laminae running up and down the anterior and posterior margins of the lateral face of Xeno’s neural arch, and those same laminae seem to exist in Rebbachisaurus. We didn’t name these laminae in the Xeno paper, but if they’re the same thing as in Rebbachisaurus, then they’re ACPLs and PCDL — anterior centroparapophyseal and posterior centodiapophyseal laminae.

If that’s right, then we misinterpreted the site of the parapophysis in Xenoposeidon. We (Taylor and Naish 2007) thought it was at the cross-shaped junction of laminae near the anterodorsalmost preserved part of the vertebra. In Rebbachisaurus, this cross exists, but it’s merely where the CPRL (centroprezygapophyseal lamina) intersects the ACPL.

But there’s more. In Xenoposeidon, the base of the CPRL (if that’s what it is) forms a “V” shape with an accessory lamina that proceeds posterodorsally from the same origin. (This is one of the features that’s apparent on the more damaged right side of Xeno as well as the nicer left side.) That lamina also seems to exist in Rebbachisaurus — but with the whole vertebra to consider, we can see that it’s not an accessory lamina, but a perfectly well-behaved CPOL (centropostzygapophseal lamina).

So if Xeno is indeed a rebbachisaurid, then the two branches of the “V” go to support the pre- and postzygs, and the laminae running up the anterior and posterior margins of the centrum support the parapophyses and diapophyses respectively. There are actually two crosses on each side of the neural arch: one at the intersection of CPRL and ACPL, the other at the intersection of CPOL and PCDL; but in the Xeno specimen, the posterior cross is lost, having been just above where the break occurs at the top of the neural arch.

Here’s what I mean:

reconstructed-as-rebbachisaur

In case it’s not clear, the grey lines are an (extremely crude) reconstruction, the blue lines label the important laminae, and the red circles highlight the two crosses.

Hmm. The more I look at this, the more convincing I find it.

But there’s more! The anterior aspect of the Rebbachisaurus vertebra also bears a notable resemblance to what we see in Xeno, with a pair or arched laminae forming a vaulted roof to the neural canal.

 

rebbachisaur-dorsal-rotation2

Jeff Wilson spotted the same thing in a sequence of comments on my tweets, saying:

That’s not a bad call—the infrazygapophyseal region of that vert is elongate, and there is a nice CPRF and those closely positioned TPRLs could mean that prz’s are close to one another or even conjoined. It’s tantalizing, but not much to go on. Would be nice to prep out CPRF & work out laminae on lat sfc.

Jeff is right that more preparation would help to figure this out.

Not that everything about the Rebbachisaurus dorsal is Xeno-ish. Most notably, the lateral foramen is nothing like that of Xeno, being an uninspiringly dull and simple oval rather than the much more elegant foramen-within-a-fossa arrangement that we see in Xeno. But there are other points of commonality, too, such as the flat stretch of bone above the fossa and the way the posterior margin of the neural arch reaches the posterior margin of the centrum.

All in all — while there is plenty of work yet to do — I am increasingly inclined to think that the evidence we currently have suggests Xenoposeidon is a rebbachisaurid. If that’s right, it would be quite an exciting result. It would be the earliest known rebbachisaur, and the only named one from the UK. (Mannion 2009 described, but did not name, a rebbachisaurid scapula from Wessex formation of the Wealden). Could Mannion’s scapula be Xenoposeidon? Unlikely, as it’s 10 million years more recent. But it could be a close relative.

Exciting times!

References

 

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Sauropod guru Jeff Wilson is on Twitter, as of a couple of weeks ago. In one of his earliest tweets, he showed the world this gorgeous photo of a Rebbachisaurus dorsal:

Jeff Wilson (left) and Ronan Allain (right), with dorsal vertebra of Rebbachisaurus.

Jeff Wilson (left) and Ronan Allain (right), with dorsal vertebra of Rebbachisaurus. Photograph by MNHN photographer, Copyright © Muséum National d’Histoire Natural.

I’m pretty certain this is the dorsal vertebra that’s been on exhibition in Paris for some time, and which is part of the holotype of Rebbachisaurus garasbae, which is in turn the type specimen of Rebbachisaurus and so of Rebbachisauridae as a whole. In which case it’s pretty darned important as defining a major group of sauropods.

This specimen was initially described, very briefly and without illustrations, by Lavocat (1954). The species (though a different specimen) was given a slightly better treatment by Russell (1996), as part of a larger work on isolated dinosaur bones. Russell included some line-drawings of the material (his figures 29-31), of which the pick is definitely this one of the bottom part of a dorsal vertebra:

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Rebbachisaurus garasbae, NMC 50844, anterior dorsal vertebra in (a) right lateral, (b) anterior, (c) left lateral and (d) posterior aspect. From Russell (1996:figure 30).

As noted by Russell (p388), this vertebra is similar, but not identical, to the one in Wilson’s photo. Russell says of his specimen that “the greater length of the centrum relative to the height and width of the intercentral articulations and less steeply projecting transverse processes imply that the vertebra occupied a more anterior position in the column.”

Will Rebbachisaurus ever get the detailed treatment that such gorgeous material deserves? Yes! here’s another tweet from Jeff:

Fantastic to think that Rebbachisaurus is in the works, and Nigersaurus to follow! Happy days!

References

  • Lavocat, R. 1954. Sur les Dinosauriens du continental intercalaire des Kem-Kem de la Daoura. [On the dinosaurs of the Continental Intercalaire of the Kem Kem of the Daoura].Comptes Rendus 19th Intenational Geological Congress 1952 (1):65-68. [English translation]
  • Russell, D.A. (1996). Isolated dinosaur bones from the Middle Cretaceous of the Tafilalt, Morocco. Museum Natl. d’Histoire Nat. (Paris) Bull. Ser. 4 18 (Section C, Nos. 2-3), 349-402.