Look ma, no ventral bracing!
April 29, 2008
So, you’ll all recall the previous post where we looked at the absurdly broad neck base of the Upper Jurassic macronarian Camarasaurus. This time round we’re playing the same game, but looking up at the neck base of the diplodocoid Diplodocus, and again it is of course the Natural History Museum’s (London) mount of the Carnegie cast of D. carnegii (image © NHM). Note how elongate and narrow the centra are: the length-to-width ratio of sauropod cervical vertebrae has proved to be a useful character, and was formalised as the ‘EI’, or elongation index, by Paul Upchurch (1998). However, EI has also been used as centrum length-to-height by other sauropod workers (Wilson & Sereno 1998, Wedel et al. 2000), and this is the version we’re using here. The EI of Diplodocus is reasonable, ranging from 3.1 to 4.7 between C2 and C6, and with a maximum of 4.9 in C7, but it is exceeded by that of brachiosaurs (where it can be over 5.0 and even over 6.0), and – among diplodocoids – by those of Barosaurus, Australodocus and Supersaurus (where the EI ranges to an incredible 7.5). Erketu and a few other particularly long-necked taxa also have particularly high EIs. Note that the ventral surfaces of the centra are shallowly concave and lack any sort of midline ridge – these details are variable among sauropods, with some having flattened bases and some having low midline keels.
Finally for now, note also that the cervical ribs are not making contact with one another: a very basic observation which makes a mockery of the idea that the ribs somehow propped up the neck from underneath. Some sauropod workers have actually proposed this (Martin et al. 1998), in part because (I think) there seems to be an assumption among some scientists that bizarre products of evolution – like the incredible necks of sauropods – must have operated in bizarre and novel ways. That might sound like a reasonable position, but it is in fact countered by the evidence: so far as we can tell, sauropod necks worked much like those of other saurischian dinosaurs, and they were not off-the-scale whacky bizarre in terms of morphological innovation. More on this topic in the future, oh yes.
- Martin, J., Martin-Rolland, V. & Frey, E. 1998. Not cranes or masts, but beams: the biomechanics of sauropod necks. Oryctos 1, 113-120.
- Upchurch, P. 1998. The phylogenetic relationships of sauropod dinosaurs. Zoological Journal of the Linnean Society 124, 43-103.
- Wedel, M. J., Cifelli, R. L. & Sanders, R. K. 2000. Osteology, paleobiology, and relationships of the sauropod dinosaur Sauroposeidon. Acta Palaeontologica Polonica 45, 343-388.
- Wilson, J. A. & Sereno, P. C. 1998. Early evolution and higher-level phylogeny of sauropod dinosaurs. Society of Vertebrate Paleontology Memoir 5, 68 pp.