Necks lie, redux
September 1, 2011
In a recent post I showed photos of the trachea in a rhea, running not along the ventral surface of the neck but along the right side. I promised to show that this is not uncommon, that the trachea and esophagus of birds are usually free to slide around under the skin and are not constrained to like along the ventral midline of the neck, as they usually are in mammals. Here goes.
Here’s figure 5 from van der Leeuw et al. (2001): a lateral x-ray of a duck, reaching up just a bit with its head and neck, possibly to get a bite or just look around. Click through for the unlabeled version.
There’s a LOT of stuff going on in this image:
- As promised, the trachea (blue lines) is taking a very different path to the head than the vertebrae and skeletal muscles.
- As usual for tetrapods, the neck is extended
at the basein the caudal half and flexed at the headin the cranial half.
- The epaxial (dorsal) muscles at the base of the neck are not tied down to the vertebral column so they are free to bowstring across the U-bend at the base of the neck (black arrow)–this was the point of the figure in the original paper. Although the gross outline of the neck also deviates from the vertebral column on the ventral side near the head, this is caused by the trachea and gullet approaching the pharynx, not because the hypaxial muscles are bowstringed across the curve.
- As the post title intimates, this neck lies: the cervical vertebrae are significantly more extended than one would expect based on the external appearance of the neck alone. The red line shows the angle of the most strongly retroverted vertebra, which I measure at 48.5 degrees from vertical (41.5 degrees above horizontal)–slightly closer to horizontal than to vertical! We have seen this before, in most mammals and in a couple of small birds (see this post); here we see it even in a reasonably large, long-necked bird.
- Worse, the gross outline of the neck–what one can see from the outside–lines up with nothing on the inside: the trachea is less curved and the vertebral column is more curved.
Same points again, this time in a chicken in an alert posture (Vidal et al. 1986: fig. 7). Here the most strongly retroverted cervical is 36 degrees from vertical (54 degrees above horizontal).
What’s all this got to do with sauropods?
First, it shows that even in animals with long, slender necks, it’s not enough to show a photo or painting of an extant animal and make assertions about what the cervicals are doing (necks lie, again). It’s even less defensible to make the dual assertions that (a) the gross outline of the neck shows the path of the cervicals and (b) the cervicals are in ONP, all based on a photo or painting of a living animal. The first point can only be established by radiography, and the second by manipulation of the skeleton, either physically or digitally. It may seem like I’m tilting at windmills here, but we’ve seen these very assertions made in conference talks. As always, we’ll follow where the evidence leads, but not until we see some actual evidence.
Second, I am increasingly haunted by the idea that we are all waaay too influenced, even (maybe especially) subconsciously, by big mammals when we think about sauropods and their necks. Big mammals–like, say, horses and giraffes–have:
- only 7 cervical vertebrae;
- lots of big muscles that attach to the thorax and the head and cross the cervical column without attaching to it much or at all;
- presacral neural spines that max out, height-wise, over the shoulders, creating withers;
- alert neck postures that are elevated (like all tetrapods) but often short of vertical, with the vertebrae often held more-or-less straight through the middle section of the neck (camels are an obvious exception here).
In contrast, birds have:
- many cervical vertebrae, from a 12 or so up to 27 or 28;
- almost no muscles that span from thorax to skull;
- presacral neural spines that rise monotonically to the synsacrum (except–maybe–in Giraffatitan);
- alert neck postures that are S-shaped, with the craniocervical joint over or just slightly in front of the cervicodorsal junction.
Which group sauropods had more in common with is left as an exercise for the reader.
- van der Leeuw, A.H.J., Bout, R.G., and Zweers, G.A. 2001. Evolutionary morphology of the neck system in ratites, fowl, and waterfowl. Netherlands Journal of Zoology 51(2):243-262.
- Vidal, P.P., Graf, W., and Berthoz, A. 1986. The orientation of the cervical vertebral column in unrestrained awake animals. Experimental Brain Research 61: 549-559.