Is Xenoposeidon a rebbachisaur?

July 14, 2015

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):


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.


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:


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.



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!




6 Responses to “Is Xenoposeidon a rebbachisaur?”

  1. Andrea Cau Says:

    Not surprised by this, as I’ve been suspecting Xenoposeidon to be a rebbachisaurid.
    There is also a Wessex Formation posterior caudal vertebra referred to a Rebbachisauridae (and I suspect also to Rebbachisaurinae due to its similarities with Demandasaurus, Nigersaurus and Tataouinea):

    Now you have to rediagnose Xenoposeidon.
    I’d be interested to include Xenoposeidon in the phylo/palaeogeographic of Rebbachisauridae discussed in the Tataouinea PloS One paper.

  2. Mike Taylor Says:

    Interesting, Andrea. What were the features that made you suspect Xeno had rebbachisaur affinities?

    Yes, Mannion, Upchurch and Hutt’s caudal is also relevant and of interest here. Thanks for the reminder.

    It would indeed be interesting to code Xeno up into the matrix of Fanti et al., and indeed the more recent one of Wilson and Allain. I doubt I’ll do it, though — it would be a lot of work to come up to speed with TNT, and we’d not be likely to learn anything. I know from experience that Xeno, like other very fragmentary taxa, just tends to bounce around trees destroying the resolution for everyone else until you have to delete it a posteriori.

  3. Andrea Cau Says:

    Well, the marked dorsoventral extent of the neural arch pedicel compared to the relatively depressed centrum reminded me to rebbachisaurids. In some rebbachisaurids the zygapophyseal facets are comparably high. As you note, the fragmentary nature of Xenoposeidon makes any placement very weak. I’d score it in our phylogeny in the feature, just to see if turns among rebbachisaurids and how its spatio-temporal placements affects the palaeogeographic analysis.

  4. David Marjanović Says:

    “It would indeed be interesting to code Xeno up into the matrix of Fanti et al., and indeed the more recent one of Wilson and Allain. I doubt I’ll do it, though — it would be a lot of work to come up to speed with TNT”

    …Analyse it in PAUP*, then?

  5. ijreid Says:

    Something else to consider, a titanosaurian position. Compare to the dorsals of Titanosaurus nanus ( fig. 6)

  6. Mike Taylor Says:

    Well, I don’t think those Titanosaurus nanus dorsals are very Xeno-like at all; but you’ll recall that in the original paper we did entertain (and have still not rules out) a titanosaurian identity. What I really ought to do when I have time is code it up for one (or more) of the big new matrices and see where it falls out in 2015. Strange to think that Xeno is eight years old now.

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