The world's most amazing sauropod

By now you might have heard the thrilling news that Britain has a brand-new sauropod dinosaur: it's an animal that I've obliquely alluded to many times here at Tet Zoo (since February 2006 in fact), and its study and publication have been many months - in fact years - in the making. Yes, it's (arguably) the world's most amazing sauropod... Xenoposeidon proneneukos Taylor & Naish, 2007, an enigmatic and morphologically bizarre Lower Cretaceous neosauropod from the Wealden Supergroup rocks of East Sussex, described in the new issue of Palaeontology (Taylor & Naish 2007). At last, readers with good memories will understand why I've been making all those veiled references to the inherent coolness of names that start with 'xeno'. The story behind Xenoposeidon's massive awesomeness is a long one, and the implications of its discovery are immense (sort of).....

Let's get one thing out of the way to begin with: Xenoposeidon proneneukos is known only from a single bone - a posterior dorsal vertebra - and not a particularly big one at that (it has a preserved height of 30 cm and is about 20 cm long). We'll get to the 'naming new taxa on single bones' issue in a moment, and - yes - I'm being somewhat tongue-in-cheek hyperbolic when I refer to this taxon as 'the world's most amazing sauropod'. And, despite the fact that Xenoposeidon is a 'new' dinosaur, it most certainly isn't a recently discovered fossil. Like so many recently named taxa, it's actually been sitting in a museum collection (that of the Natural History Museum in London) for a loooong time; since the 1890s in fact.

Discovered by Philip James Rufford (best known for the good collection of fossil plants he amassed), it was later sent to the museum and then briefly described by Richard Lydekker. Palaeontologists generally know Lydekker as a prolific describer of Jurassic and Cretaceous fossil reptiles, whereas other zoologists better associate him with taxonomic work on living elephants, lions, deer and dolphins, and on biogeography (it ain't called Lydekker's Line for nothin'). Lydekker (1893) provided a very nice figure of the specimen (later given the catalogue number BMNH R2095) and suggested that it might be referable to the Wealden sauropod species that he referred to as Morosaurus brevis (must... avoid... discussion... of... hideous... taxonomic... mess) [the excellent figure from Lydekker (1893) is shown below. BMNH R2095 is on the left; the specimen on the right is an Isle of Wight brachiosaur vertebra]. Opinions on BMNH R2095 were then provided by Harry Seeley, John Hulke and Edwin Newton: all of them distinguished and experienced veterans of British dinosaur research. Furthermore, over the years just about everyone who's ever worked on sauropods has visited the excellent collection in the NHM and thus might well have seen, or examined, BMNH R2095 (though admittedly only a few of them have spent special time with the Wealden material). But despite all this, there are (so far as we know) no post-Lydekker (1893) comments on BMNH R2095 anywhere in the literature (well, it's alluded to in Naish & Martill (2007, p. 499), but that doesn't count).

Enter Mike P. Taylor, computer programmer by day and sauropod vertebra specialist by night [Mike is shown in the image at the top of the article, posing romantically with BMNH R2095]. Mike is either the best, or worst, sort of palaeontologist: the sort that is essentially devoted to a single, highly arcane, very restricted field of interest... in this case the study of sauropod presacral vertebrae*. Mike's particularly into brachiosaurs, and as well as studying the taxa from the Morrison Formation and Tendaguru deposits of Tanzania he's also been looking at the members of this group that come from the Lower Cretaceous Wealden Supergroup of southern England. But not all Wealden sauropods are brachiosaurs - there's a lot of other stuff there, and among the bits and pieces sat in the NHM collection, Mike stumbled upon, and became intrigued by, the probably non-brachiosaurian BMNH R2095. I'd never looked at R2095 (though given my work on Wealden sauropods (Naish 2005, Naish & Martill 2001, 2007, Naish et al. 2004) maybe I should have), but I did know of it from Lydekker's description and thought that it looked sort of odd. After Mike and I got corresponding on it we ended up co-authoring a description. The big deal is that BMNH R2095 isn't just some random vertebra that might have belonged to any old sauropod: it's a radically new, highly divergent specimen, representing a quite novel suite of anatomical features, and apparently indicating the presence of a hitherto undocumented sauropod lineage.

* Although that's not entirely accurate, as he's also published on phylogenetic definitions and on dinosaur diversity patterns over time, and he does sometimes look at limb bones and girdles. His published articles are available here.

Several features make BMNH R2095 morphologically distinctive: in fact the specimen is unusually well-endowed, if you will, with unique characters (or autapomorphies as they're properly known) [in adjacent image of BMNH R2095, anterior is to the left]. The neural arch extends for the entire length of the centrum (weird), the neural arch slopes forward relative to the vertebra's vertical axis (very weird), bony laminae form a sort of vaulted arch over the anterior part of the neural canal (highly weird), and the neural canal itself is asymmetrical, being small and circular posteriorly but tall and tear drop-shaped anteriorly (really weird)... there are other autapomorphies too, mostly involving the arrangement of laminae on the neural arch. If you aren't familiar with these anatomical terms and need some explanation, the good news is that you now have the awesome guide that is SV-POW! to help you.

These features aren't seen in any of the other sauropods of the Wealden Supergroup, nor indeed in any other sauropod from anywhere else in the world: accordingly, BMNH R2095 is a diagnosable taxonomic entity, and as such the right thing to do is to name it as a new taxon. We chose the name Xenoposeidon proneneukos (we're even good enough to provide a pronunciation guide in the paper: ZEE-no-puh-SYE-d'n pro-nen-YOO-koss). Since Matt Wedel and colleagues named the North American brachiosaur Sauroposeidon in 2000 (Wedel et al. 2000), Mike has been flirting with the idea of using 'poseidon' as the root of further sauropod names* - Poseidon wasn't just the god of the sea, he was also in charge of earthquakes, and associating the name with sauropods does seem apt (although it has to be said that it endorses the popular misconception that sauropods made the ground shake every time they walked). 'Xeno' means strange or alien (hence xenomorph, Xenopus, Xenosyneunitanura...) and was used to reflect the genuinely weird, unique morphology of BMNH R2095. 'Proneneukos' means something like 'forward sloping' and of course refers to one of the neural arch's most distinctive anatomical features.

* We informally refer to MIWG.7306 as 'Angloposeidon' [ver 1 articles on it start here]; would it ruin the surprise if I said that other 'poseidon' names are in the system?

With its complex system of bony neural arch laminae, large and invasive pneumatic foramina, and large size, Xenoposeidon is undoubtedly a sauropod dinosaur - there just aren't any other animals with vertebrae like this [adjacent image shows Xenoposeidon in multiple views. Clockwise from top left: left lateral, right lateral, posterior, anterior]. Furthermore, because the pneumatic foraminae are particularly deep, and the vertebra's interior consists of multiple large chambers, Xenoposeidon surely belongs to the 'higher sauropod' clade called Neosauropoda. Beyond that, however, its affinities are unclear. Xenoposeidon lacks the features that would allow it to be allied with any of the major neosauropod clades (the diplodocoids, camarasaurs, brachiosaurs or titanosaurs) and looks very different from all of them. So it seems to be out there on its own: a neosauropod doing something quite weird, apparently representing a distinct lineage that, if we still used Linnaean ranks in taxonomy, would be given its own Family and Superfamily. While we arrived at this conclusion by way of traditional comparison, we also coded Xenoposeidon for phylogenetic analysis and threw it into a large sauropod data set (the one produced by Harris (2006) in his work on the diplodocoid Suuwassea). The best conclusion that can be drawn from our results is that, again, Xenoposeidon is a neosauropod that can't be convincingly referred to any of the other major neosauropod clades.

So we seem to have an entirely new sort of sauropod. That's pretty cool. The possibility does exist, of course, that Xenoposeidon is in fact a strongly divergent member of one of those long-recognised groups, and it's also possible that this one specimen is a freak of some sort: a member of a less bizarre taxon that did some weird embryological hocus-pocus when growing its bones, or an individual that was afflicted with some unidentified Cretaceous disease. Both of these hypotheses however are far, far more radical than the conclusion that this is a new taxon representing a new group. It's also just about possible that Xenoposeidon might turn out to be the same thing as other sauropods known from contemporaneous rocks, like 'Pelorosaurus' becklesii. This sauropod, named for forelimb bones and famous for having skin impressions preserved, is a titanosaur (Upchurch 1995, Upchurch et al. 2004, Naish & Martill 2007). Then again, Xenoposeidon doesn't resemble titanosaur vertebrae in the least, so this is also not a favourable hypothesis.

Of course, there's always the fact that Xenoposeidon is based on but a single bone. Should we have been so bold as to go as far as naming a new taxon on the basis of such meagre evidence? Here we come to something that experts disagree on. Some workers argue, on principle alone, that the naming of new species on single elements is bad practise. This sounds good and sensible, but - in the case of diagnostic specimens like BMNH R2095 - naming it really is the right thing to do. As one of our reviewers noted, Xenoposeidon is diagnosed by more autapomorphies than are certain other sauropods known from near-complete skeletons! Furthermore, as Naish & Martill (2007) argued, naming taxa - even those based on poor material - is a useful exercise simply because people remember the taxa that get named, but tend to forget or ignore things like 'new taxon c' when it comes to tallying diversity. I discussed this subject before in an article on British dinosaurs (here).

As a specialist on Wealden dinosaurs I find the discovery of a new Wealden sauropod particularly exciting. But what might be missed by many people is that Xenoposeidon is not contemporaneous with all those Isle of Wight dinosaurs that the Wealden is best known for. Contrary to the idea that the Wealden represents one brief span of Lower Cretaceous time inhabited by one assemblage of creatures, the term 'Wealden' in fact applies to a unit of rocks (properly called the Wealden Supergroup) that were deposited over about 25 million years, from Berriasian times (right down at the start of the Early Cretaceous) to early Aptian times (close to the 'middle' of the Cretaceous). The Wealden Supergroup is divided into three groups: the Hastings Beds Group and Weald Clay Group (both of the Weald sub-basin of the English mainland) and the Wealden Group of the Wessex sub-basin of the Isle of Wight (this might be familiar if you read the Valdoraptor and Becklespinax article) [adjacent image shows what a partial series of Xenoposeidon dorsal vertebrae might look like... sigh, if only we had more].

Based on its locality data and on the origin of the other Wealden fossils collected by Rufford, we think that Xenoposeidon comes from the Ashdown Beds Formation, and this is part of the Hastings Beds Group. Furthermore, the part of the Ashdown Beds Formation exposed at Ecclesbourne Glen (the probable site of origin of Xenoposeidon) is Berriasian in age. So Xenoposeidon is a really, really old Wealden dinosaur - removed by about 20 million years from Isle of Wight Wealden dinosaurs like Eotyrannus and Hypsilophodon. Worth noting here is that the Hastings Beds Group includes a pretty rich and diverse dinosaur assemblage, and that the dinosaurs from Hastings and the surrounding region might now equal or exceed the Isle of Wight in terms of taxonomic diversity. That's a subject for another time.

In fact, everything else is now going to be a subject for another time, as this article is too long and I have to stop there. As you'll know if you've visited SV-POW! today, that site will be hosting a whole week of Xenoposeidon-based goodness, so Mike, Matt and I will all have much more to say about it there. You can obtain a pdf of the Taylor & Naish (2007) paper by emailing Mike or myself. Mike has prepared two webpages that provide supplementary information on Xenoposeidon: the Introducing Xenoposeidon page was written for journalists, while the supplementary information page provides other researchers with additional figures, maps and the data used in our cladistic analysis. There is now a reasonable amount of newsy coverage of Xenoposeidon on the web: highlights include Dr Vector's take on it here [the adjacent image is only half-serious and meant to show the relative position of the Xenoposeidon holotype within its owner. Note that Xenoposeidon was not a particularly big sauropod].

We hope very much that additional specimens of Xenoposeidon might be discovered or recognised in future, now that the whole world knows...

PS - by now, the new news on the African rebbachisaurid Nigersaurus is also out (in PLoS ONE here). This sauropod is known from slightly more complete remains than Xenoposeidon, but would still not be able to beat it in a fight :) And I'd like to say more, but can't do so right now. As those of you who saw the version of this Xenoposeidon article on Tet Zoo ver 1 will know (it's now been removed), I've been having extreme problems in getting this article published; something to do with cookies apparently. Still, got there in the end.

Refs - -

Harris, J. D. 2006. The significance of Suuwassea emilieae (Dinosauria, Sauropoda) for flagellicaudatan intrarelationships and evolution. Journal of Systematic Palaeontology 4, 185-198.

Lydekker, R. 1893. On a sauropodous dinosaurian vertebra from the Wealden of Hastings. Quarterly Journal of the Geological Society, London 49, 276-280.

Naish, D. 2005. The sauropod dinosaurs of the Wealden succession (Lower Cretaceous) of southern England. The Quarterly Journal of the Dinosaur Society 4 (3), 8-11.

- . & Martill, D. M. 2001. Saurischian dinosaurs 1: Sauropods. In Martill, D. M. & Naish, D. (eds) Dinosaurs of the Isle of Wight. The Palaeontological Association (London), pp. 185-241.

- . & Martill, D. M. 2007. Dinosaurs of Great Britain and the role of the Geological Society of London in their discovery: basal Dinosauria and Saurischia. Journal of the Geological Society, London 164, 493-510.

- ., Martill, D. M., Cooper, D. & Stevens, K. A. 2004. Europe's largest dinosaur? A giant brachiosaurid cervical vertebra from the Wessex Formation (Early Cretaceous) of southern England. Cretaceous Research 25, 787-795.

Taylor, M. P. & Naish, D. 2007. An unusual new neosauropod dinosaur from the Lower Cretaceous Hastings Beds Group of East Sussex, England. Palaeontology 50, 1547-1564.

Upchurch, P. 1995. The evolutionary history of sauropod dinosaurs. Philosophical Transactions of the Royal Society, London 349, 365-390.

Upchurch, P., Barrett, P. M. & Dodson, P. 2004. Sauropoda. In: Weishampel, D. B., Dodson, P. & Osmólska, H. (eds) The Dinosauria, Second Edition. University of California Press, Berkeley, pp. 259-322.