Your coccyx is contemptible
April 8, 2008
What you’re looking at here is the first caudal vertebra (i.e. the first tail bone) of Apatosaurus ajax, the newish specimen NSMT-PV 20375 described by Upchurch et al. (2005). The drawings are all from the plates at the end of that lavishly illustrated paper: all I’ve done is composite them. The top row, from left to right, shows the vertebra in anterior, left lateral, posterior and right lateral views. Below the left lateral view is a dorsal view, with the front pointing to the left (as in the left lateral view).
Oddly, the size of this vertebra doesn’t seem to be stated in the paper, but two lines of evidence suggest that it’s about 65 cm in total height. First, measuring the caudal on the skeletal reconstruction that is the frontispiece, and comparing with that figure’s 1m scale-bar, yields a height of 64 cm; second, the neural spine’s height (measured from the ventral margin of the poztzygapophyses) is given in Table 9 as 392 mm, and that extrapolates, using the posterior view figure, to a total height of 665 mm. So about 65 cm, then.
The caudal vertebrae of diplodocids such as Apatosaurus, Diplodocus and Barosaurus are unusually complex for sauropods, having been somewhat “dorsalised”, i.e. taking on some of the complex morphology of posterior dorsals rather than being the rather dull round-centrum-with-a-flat-spine-on-top affairs you get hanging off the rear end of brachiosauruids. You’ll notice that the lateral processes, or “caudal ribs”, take the form of tall, broad plates, so that the middle part of the vertebra is trapezoidal in anterior view. This is as different as can be from the boring, stick-like caudal ribs of Brachiosaurus. (What actually are caudal ribs? So far as I can tell, amazingly, no-one really knows. They might be homologous with the diapophysis of dorsal vertebrae, or with the parapophysis, or perhaps both of them fused, or one or both fused with an actual rib.)
Oh, yes: also in the picture is your coccyx, that is, the four or five bones that make up your vestigial tail. It is, needless to say, contemptible. It’s surprisingly hard to find a reference for how big it should be, but by cross-scaling from illustrations of whole human skeleton and sacra, I’ve come up with a figure of about 2.5 cm, and that’s what I’ve used here. If you want to compare your tail with Apatosaurus‘s, remember that Apato had about eighty caudals: they diminish in size posteriorly, of course, but they do stay about the same anteroposterior length for much of the tail. In fact, diplodocids have tremendous tails, something like half the entire length of the entire animal. One of my long-standing bugbears is that the biomechanics of sauropod tails gets almost no attention (except for speculations about whip-cracking) compared with the love and care lavished on their necks. One day, one of us might do something about that.
That concludes our short but humiliating series of abuse directed at your frail human body. I’ll have to come up with something else next time it’s my turn. Hope you’ve enjoyed the ride.
- Upchurch, Paul, Yukimitsu Tomida, and Paul M. Barrett. 2005. A new specimen of Apatosaurus ajax (Sauropoda: Diplodocidae) from the Morrison Formation (Upper Jurassic) of Wyoming, USA. National Science Museum Monographs No.26. Tokyo.