Xenoposeidon week, day 2: imagining the whole thing
November 16, 2007
So, by now, most people in the known universe have heard about Xenoposeidon, know what a big deal it is, and understand its immense value and significance. A longish article on the history and anatomy of Xenoposeidon, and about how interesting it is in terms of Wealden dinosaur diversity, is now up at Tetrapod Zoology (here). And as you’ll know if you’ve seen Mike’s article from yesterday, the story was all over the national and global media yesterday. To their credit, not one reporter or interviewer said ‘but it’s just a single bone’. After Sky News advertised a piece on Xenoposeidon at 12-50pm yesterday, I kept the TV on. But by 4pm they hadn’t shown it, and I was so maddened by hearing the same several news stories about 400 times each that I could not stomach any more TV that day. So I missed Mike’s many actual appearances… bar a very brief one on the 10-30pm edition of local news programme South Today. Anyway…
There’s so much to say about Xenoposeidon; so much more ground to cover [if you need help with some of the following anatomical terms do remember you've got our excellent tutorial on vertebral anatomy to help you]. The picture shown here (it’s Fig. 5 from Taylor & Naish 2007) shows Xenoposeidon as, we think, it would have looked when complete. As interesting and anatomically revealing as the single known specimen is, it lacks the anterior condyle, the neural spine, and the zygapophyses, so we had to do some reasonable extrapolation when imagining what these missing bits looked like. Because the posterior articular surface of the centrum is concave in Xenoposeidon, it makes sense to assume that the (mostly missing) anterior condyle was somewhat convex, as it is in macronarians (the group of sauropods that includes camarasaurs, brachiosaurs and titanosaurs). The condition where the centrum is concave posteriorly and convex anteriorly is known as opisthocoely. Note, however, that the opisthocoely of Xenoposeidon is not as well developed as that of macronarians.
The neural arch of Xenoposeidon is strikingly tall: about as tall as the centrum. That’s odd. The forward-sloping of the neural arch is even more odd, and in fact is a unique feature of the taxon – we have yet to see this character in any other sauropod (making it a Xenoposeidon autapomorphy). We can see several structures on the side of the neural arch: the parapophysis (the more ventral of the two attachment points for the two rib heads) is located really high up, and it’s this high location which has led to our conclusion that this vertebra comes from the posterior part of the dorsal sequence (the articular processes for the rib heads don’t stay in the same place along the vertebral sequence. In anterior dorsals, the parapophyses [plural of parapophysis] are positioned way down on the centrum, but in mid- and posterior dorsals, they rise up onto the neural arch). Because Xenoposeidon doesn’t preserve any trace of its prezygapophyses, we have to conclude that these were located even further dorsally than were the parapophyses. This indicates that the specimen is not among the most posterior of the dorsals in the sequence for, in those vertebrae, the parapophyses are located further dorsally than are the prezygapophyses.
The least knowable thing here is how tall the neural spine was; what we depict is a generic not-too-gracile, not-too-robust conservative sort of neural spine. It will be interesting to see how this reconstruction matches reality when a complete Xenoposeidon dorsal vertebra – hopefully associated with the rest of the skeleton – is discovered, which of course it will, one day, now that everyone in the English Weald is looking (in the adjacent image I’m looking for dinosaurs in Weald Clay Group rocks of East Sussex; if Mike can include a photo that shows him posing with a dalek, I can justify one involving field work).
The Xenoposeidon image above shows the left side of the specimen. Curiously, the right side is rather different: it exhibits a shallow accessory fossa located dorsal to the large lateral pneumatic opening, and also has an extra bony boss positioned about half-way up and near the anterior margin of the neural arch. We don’t really know what’s going on here. Asymmetry in vertebrae – particularly in complicated ones like the heavily pneumatised vertebrae of some birds – is fairly common, and a small amount of asymmetry in the limbs, skulls and pelvic girdles of vertebrates is also pretty widespread and not that remarkable. Asymmetry in pneumatic openings is (so I understand) common, and – partly because of this – some workers have suggested that pneumatic diverticulae can be imagined as opportunistic structures that will ‘invade’ spaces on or in a bone when the opportunity arises. Matt might want to correct me or elaborate on this point, as he’ll know what he’s talking about. More on the wonder that is Xenoposeidon tomorrow!
Ref – -
Taylor, Michael P. and Darren Naish. 2007. An unusual new neosauropod dinosaur from the Lower Cretaceous Hastings Beds Group of East Sussex, England. Palaeontology 50 (6): 1547-1564. doi: 10.1111/j.1475-4983.2007.00728.x
Postscript, added by Mike at 16:57
I know it’s a bit cheeky to add a postscript to someone else’s post, but I don’t want to make a “day 2-and-a-half” post, and I do just want to mention a bit more media coverage. I’ve heard from Vin Morgan in America that they’ve been running Xenoposeidon on CNN, and I know that the Canadian Discovery Channel is going to do something, probably tonight. And I know from my brother-in-law, who emigrated to Spain, that it’s in Spanish newspapers today. I’ve also seen it mentioned on Greek, Russian and Chinese web sites — although not being able to read any of those languages I can’t tell whether the articles just say “Xenoposeidon stinks”. Spanish TV slot still to come. Not bad for a paper that was rejected WITHOUT REVIEW from
Name Of Journal Withheld on the grounds that “the paper is rather of a local significance”. (Sorry, was that last part a bit petty?)