Sauropod Vertebra Picture of the Week

Inspired by Mike’s recent post on the interior of Chondrosteosaurus from the Isle of Wight’s Wessex Formation, what could I do but weigh in yet again with one of my most-loved specimens: the beauty that is MIWG.7306 (aka ‘Angloposeidon’), a big brachiosaurid also from the Wessex Formation (Naish et al. 2004). As mentioned previously, it’s perhaps intuitively surprising that one of the most useful things about MIWG.7306 is that it’s broken in two, allowing us to see the broken faces of both halves of the vertebra. This allows us to see the internal structure of the specimen and, potentially, to work out, not just what the internal architecture was like, but also how pneumatic this beast was. To date Matt and I have done some preliminary work on this, but we have yet to publish anything, so what you’re getting here is a world first, never-before-seen by anyone outside.. well, me and Matt. In fact I can’t recall whether even Mike has seen this stuff: IT’S THAT SECRET.

Anyway, what we’re looking at here is the posterior broken face of the anterior half of MIWG.7306. The specimen was photographed lying on its side but I’ve reoriented the photo such that the dorsal surface is at the top, of course. The broken surface looks at first sight like a big mess, mostly obscured by sediment which has gotten preserved within the pneumatic spaces. However, it you look carefully you’ll see very dark (blackish) spars of bone scattered about the interior. These are the thin bony walls that surround the internal cavities, called camellae, that made up the vertebra’s interior. Much of the ventral part of the cross-section is taken up by the vertical median septum: much of the space lateral to the septum (on both its left and right sides) would have been occupied by large air sacs. I was hoping to use a fully labelled version of this photo but cleverly seem to have lost it (and don’t have time to knock up a new version). For the full details you’ll have to wait for the paper!

Ventral to the median septum, the vertebra is again wide when seen in cross-section, and in the photo here we’re looking at that ventral part, though this time we’re looking at the anterior face of the posterior half of the vertebra. What’s nice is that there are clearly at least four distinct camellae aligned along the centrum’s ventral edge, and they don’t appear to be equal in size (nor arranged symmetrically around the midline). The actual ventral floor of the centrum can also be clearly seen in cross-section, and it’s pretty thin. All of this is particularly interesting because we have comparative data from North American brachiosaurids (in particular from a specimen that Matt has scanned, BYU 12866) and from Sauroposeidon (Wedel et al. 2000a, b). ‘Angloposeidon’ is similar enough to both of these in its details to convince us that the same sort of thing is going on, but it’s also different in various subtle ways: the ventral floor of the centrum is not the same thickness in all of these animals for example.

In the article on Chondrosteosaurus, Mike noted that we can work out how much of a vertebra’s interior was occupied by air if we can calculate the bone : air space ratio. We did just this with MIWG.7306, using the photos shown here and several others. The result: MIWG.7306 approached Sauroposeidon in terms of its pneumaticity, being 80-90% air. This gives us some remarkable and significant data on the palaeobiology of this animal, as pneumaticity has all sorts of implications for the animal’s respiration, physiology and mass (Wedel 2003, 2005). And on that note, I shall say goodbye.

References

• Naish, D., 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.
  
• 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, K. (eds). The Sauropods: Evolution and Paleobiology. University of California Press (Berkeley), pp. 201-228.
• – ., Cifelli, R.L. & Sanders, R.K. 2000a. Sauroposeidon proteles, a new sauropod from the Early Cretaceous of Oklahoma. Journal of Vertebrate Paleontology 20, 109-114.
• – ., Cifelli, R.L. & Sanders, R.K. 2000b. Osteology, paleobiology, and relationships of the sauropod dinosaur Sauroposeidon. Acta Palaeontologica Polonica 45, 343-388.