Friday, July 14, 2006

‘Angloposeidon’, the unreported story, part II

In the previous post I introduced the long, tedious, much-delayed technical project on MIWG.7306, a giant brachiosaurid cervical vertebra from the Isle of Wight. Here we look at more details of this story. You should regard all of this as the back-story to the paper eventually published as Naish et al. 2004 (free pdf available here).

So, by February 2002 we had a provisionally complete, submitted manuscript. Mathew Wedel (well known by 2002 as an expert on giant sauropods) and Paul Upchurch (a sauropod expert specialising on phylogenetic relationships) were chosen as reviewers. Their reviews were back by June 2002, and both were highly positive. Pending minor corrections and additions, this all meant green light to publication.

Looking at it now, I can see that the 2002 version of the manuscript wasn’t too bad, but that it could do with improvement and expansion on a few areas. Little known (and perhaps I should have mentioned this earlier) is that a second vertebra was actually discovered at the same place as MIWG.7306, though it’s substantially less complete and consists only of a poorly preserved centrum 640 mm long (see adjacent photo - and that's Dave in the photo, not me). This second specimen was mentioned in the pre-review version of the manuscript, but not until November 2003 did I obtain its specimen number (IWCMS : 2003.28). You might be wondering why some specimens at Dinosaur Isle are ‘MIWG’ while others are ‘IWCMS’. Essentially, the former accession code was applied prior to the closure of the old Museum of Isle of Wight Geology, while the latter code was applied to specimens accessioned after the 2001 opening of Dinosaur Isle (IWCMS stands for Isle of Wight County Museum Service).

The figures of the 2002 pre-review version of the manuscript were a bit of a mess. Given that we’d now described the anatomy of MIWG.7306 in detail, a major shortcoming was that the figures didn’t point out which bits of the specimen had which names. You really need to do this on sauropod vertebrae, as their anatomy is complex.

In MIWG.7306, three large concavities (termed fossae) occupy most of the lateral surfaces, and these fossae themselves are subdivided into smaller fossae, with some of these subdivisions having further subdivisions. Importantly, the fossae are connected (via openings on their bony walls) to large internal chambers. Bony struts, called laminae, divide the fossae and connect the main ‘landmarks’ of the bone (such as the pre- and postzygapophyses), and each of these laminae has a name. We presently use a system devised by sauropod expert Jeff Wilson, and his paper on this subject is obligatory reading if you want to understand this nomenclature properly (Wilson 1999).

What are all these fossae and laminae for? Perhaps the most remarkable thing about sauropod vertebrae is how pneumatic they were – the lateral fossae, and the interior of the vertebrae, were occupied, in life, by air sacs. That is, by pneumatic sacs that were connected by tubes both to one another, and to the animal’s lungs. How do we know this? Essentially because the detailed anatomy is identical to that of living birds. In birds, bony openings on the sides of the vertebrae also lead into large internal chambers, just as they do in sauropods, and given that the bony openings and internal chambers of bird vertebrae house air sacs, they almost certainly did so in sauropod vertebrae as well. To date no one has proposed a better explanation for the structures seen in sauropods, quite simply because there isn’t one. This has obvious implications for mass, as it means that the animals weighed a lot less than you might otherwise think: Wedel (2005: free pdf here) showed that, by calculating for the presence of air sacs, a sauropod comes out at 8-10% lighter than it would have if its mass were uncorrected. This is actually a conservative estimate, as it doesn’t account for air sacs that would have been distributed among the soft tissues.

Extensive pneumatisation also has implications for various aspects of sauropod palaebiology (Wedel 2003: free pdf here). Don Henderson (2003: free pdf here) has worked out what difference it would make for buoyancy were sauropods to go swimming (which they presumably did at times, even though they weren’t amphibious or otherwise strongly tied to aquatic environments). Pneumaticity also indicates that sauropods enjoyed the relatively high oxygen extraction levels seen in birds, suggests that sauropods were quite capable of efficient heat dumping, and overall is highly suggestive of an elevated metabolism (as is indicated by the amazingly fast growth style now well documented for sauropods: see Erickson et al. 2001, Sander et al. 2004). Biologists today seem to think that they’re being particularly good, sceptical scientists if they outright reject the silly idea of dinosaurian endothermy, but it’s very clear that none of the work that has been done on dinosaur physiology accounts for the amazing degree of pneumatisation seen in sauropods and other groups. There’s an awful lot that could be said on the debate over dinosaur physiology however, and I don’t want to tackle it in depth here.

In its degree of pneumaticity, MIWG.7306 appears intermediate between Brachiosaurus and Sauroposeidon. I worked out that about 60% of each of its lateral surfaces was occupied by the fossae that housed lateral air sacs, and because the specimen is broken in half I could also see that very little of its interior was filled up by internal bony struts: most of it would have been air. The jaw-droppingly amazing thing about the degree of pneumatisation within MIWG.7306 and other brachiosaurids is that their vertebrae consisted of about 80-90% air. Reread that last sentence for emphasis, and you might like to memorise it and trot it out at cocktail parties.

In the previous post we saw how the first published measurement of MIWG.7306 gave its length as 920 mm. Measuring a giant sauropod vertebra isn’t as simple as you might think: firstly, because the long articular processes (the zygapophyses) may be incomplete, secondly, because the zygapophyses might have become bent or distorted during preservation, and, thirdly, because there’s more than one way to gain a vertebra’s ‘total length’! While MIWG.7306 includes a complete, intact left postzygapophysis, its left prezygapophysis is missing, and its right is broken off. Dave Martill and I re-articulated the broken right prezygapophysis and, measuring from its tip to the posterior-most rim of the posterior articular condyle, came up with a new total length of 1060 mm. That’s immense.

In 2002 I was asked to contribute an article on sauropods to an issue of The Quarterley Journal of the Dinosaur Society, and I sent it off in late February. For reasons entirely unrelated to the long delay that beset the MIWG.7306 manuscript, this article was also delayed and wasn’t published until 2005. It included a few paragraphs on MIWG.7306, stated its total length as 1060 mm, and even included a rather fetching photo of me posing with the specimen (Naish 2005). In 2002 I was commissioned to write an article on Isle of Wight dinosaurs for the excellent Japanese magazine Dino Press (now, sadly, defunct). Here again I discussed MIWG.7306, again gave its total length as 1060 mm, and again included a silly photo of me, alongside the specimen (Naish 2002). Amusingly, I’ve just noticed that the latter article states of MIWG.7306 ‘It is due to be published some time in 2002, so look out for it!’ (English text supplement, p. 25).

Alas, the 1060 mm that I gave in those two articles is, while not technically incorrect, not the standardised ‘total length’ of the specimen for, rather than including prezygapophysis length, the standard way of measuring a sauropod vertebra is to stick to centrum length alone. And the centrum length of MIWG.7306 is 745 mm, which is still highly respectable, and on par with some of the longer cervical vertebrae of HM SII (the larger of the two Brachiosaurus brancai specimens described by Janensch). In that specimen, the sixth cervical has a centrum length of 780 mm, while the longest in the series (C10 and C11) each have centrum lengths of 870 mm long. In extra-long Sauroposeidon, C8 has a centrum length of 1250 mm while C6 is 1220 mm. Luckily this oversight was caught long before the MIWG.7306 manuscript was resubmitted.

And that’s not the end of the story: far from it. More to come in the next thrilling instalment (go here).

Refs - -

Erickson, G. M., Curry Rogers, K. & Yerby, S. A. 2001. Dinosaurian growth patterns and rapid avian growth rates. Nature 412, 429-433.

Henderson, D. M. 2003. Tipsy punters: sauropod dinosaur pneumaticity, buoyancy and aquatic habits. Proceedings of the Royal Society of London B (Supp.) 271, S180-S183.

Naish, D. 2002. Thecocoelurians, calamosaurs and Europe’s largest sauropod: the latest on the Isle of Wight’s dinosaurs. Dino Press 7, 85-95.

- . 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., 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.

Sander, P. M., Klein, N., Buffetaut, E., Cuny, G., Suteethorn, V. & Le Loeuff, J. 2004. Adaptive radiation in sauropod dinosaurs: bone histology indicates rapid evolution of giant body size through acceleration. Organisms, Diversity & Evolution 4, 165-173.

Wedel, M. J. 2003. Vertebral pneumaticity, air sacs, and the physiology of sauropod dinosaurs. Paleobiology 29, 243-255.

- . 2005. Postcranial skeletal pneumaticity in sauropods and its implications for mass estimates. In Wilson, J. A. & Curry Rogers, C. (eds) The Sauropods: Evolution and Paleobiology. University of California Press, Berkeley, pp. 201-228.

Wilson, J. A. 1999. A nomenclature for vertebral laminae in sauropods and other saurischian dinosaurs. Journal of Vertebrate Paleontology 19, 639-653.


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