What heads tell us about necks
May 29, 2009
So far in our coverage of the new paper (Taylor et al. 2009) we’ve mostly focused on necks, following the discovery by Graf, Vidal, and others that when they are alert and unrestrained, extant tetrapods hold their necks extended and their heads flexed. (Although they turn up with distressing regularity, “ventroflexed” is redundant and “dorsiflexed” is an oxymoron; Darren lays down the law here.)
There’s more to the paper; about half of our argument is primarily about heads and only secondarily about necks, and has to do with semicircular canals (SCCs). SCCs are sense organs in the inner ear that determine the orientation and acceleration of the head. Hagfish have a single loop on each side, lampreys have two loops per side, and gnathostomes (jawed vertebrates, like us) have three per ear, all set at right angles to each other to capture position and movement information in all directions no matter how the head is oriented. There’s a brief overview of how the system works here, and here’s what SCCs actually look like (in this case in the theropod dinosaur Ceratosaurus, from Sanders and Smith 2005:fig. 5):
SCCs are relevant to posture and locomotion: animals that move rapidly tend to have big canals, especially big anterior canals, and the horizontal semicircular canals (HSCCs) are usually held more or less level as animals go about their business. It’s the “more or less” part that gets sticky, as we’ll see in a minute. SCCs and inner ear anatomy in general are areas of accelerating research in vertebrate paleontology, because the soft tissues that comprise them (the membranous labyrinth) are housed in dense bone (the bony labyrinth) which is often preserved and can be imaged non-invasively using CT. Even braincases that look pretty crappy from the outside can yield beautifully-preserved bony labyrinths, from which the dimensions of the membranous labyrinth can be measured and the acuity of the system can be estimated.
Where SCCs have really attracted attention in paleontology is the “more or less” horizontal orientation of the HSCCs in living animals. Some authors have argued that if you set the HSCCs level or close to level, you can figure out how the head was oriented in life.
Well, maybe. The problem is that there is a LOT of variation around level. In birds surveyed by Duijm (1951), HSCC orientation varied by 50 degrees among taxa, from 20 degrees below horizontal to 30 degrees above. Furthermore, in humans HSCC orientation varies by up to 20 degrees among individuals. Possibly humans are weirdly variable, but it seems at least equally likely that most critters are and we’ve only discovered that variation in humans because of the huge sample size.
However you slice it, those are darn big error bars around any given head posture. That doesn’t mean that HSCC orientations in dinosaurs and other extinct vertebrates are worthless for determining posture (they may also be a source of taxonomic information). Strictly speaking, it means that preserved HSCCs can get us in the 50-degree ballpark but can’t narrow things down any further. This is one of those areas in paleontology where we’re just going to have to live with a certain amount of uncertainty, at least for now.
We’re not done with heads, though. Once the HSCCs get us in that 50-degree range, we still have to figure out how the neck facilitated those postures. One thing that seems to hold across the board in sauropodomorphs is that when the HSCCs were in the -20 to +30 range around horizontal, the occipital condyles were pointed down. And that has major implications for the posture of the neck, as we’ll see in the following example.
Let’s start with this neatly abstract Apatosaurus skeleton, borrowed from Kent Stevens’s site here. Note that this version is from 2005 and Kent has updated his models considerably since then. I’m using this one because its elegant minimalism made it easy for me to play with, but it doesn’t represent Kent’s current thinking.
Here’s the same image with some lines drawn on to indicate the long axis of the skull, the orientation of the occipital condyle, and the angle of the anterior neck. In Apatosaurus and Diplodocus the occipital condyle is at right angles to the long axis of the skull. That means that if the cranio-cervical joint was held in “neutral pose”, the head would be at right angles to the anterior neck. Recall that extant tetrapods hold their heads flexed on their necks. This Apatosaurus has its head extended by 50 degrees. This is major extension–to see what it feels like, lean your head back until you’re looking straight up, and then lower your head until its almost halfway back to normal. Imagine walking around like that. In this pose the HSCCs are angled down, within the 50-degree ballpark but not level.
Just for the sake of argument, let’s set the HSCCs level and force the craniocervical joint into ONP. Now the head and first few cervicals are okay, but clearly this posture won’t work with the neck in the original pose. We’re going to have to move the neck up to meet the steep angle dictated by the HSCCs and the occipital condyle.
One option is to keep as much of the neck in the original pose as possible, and just elevate the vertebrae closest to the head. This is not so far off from how Apatosaurus has been depicted for more than a century. But it doesn’t agree with the data from extant tetrapods, in which the neck is extended at its base.
Here’s the partially Vidal-compliant version, with the cranio-cervical joint in ONP and the base of the neck extended. To be fully Vidal-compliant, the head would have to be flexed on the neck. In the diagram, that would have the effect of turning changing the angle between the long axis of the skull and the anterior cervicals from a right angle to an acute one. Since the orientation of the head is “fixed” by the semicircular canals (in this example), that means the neck would have to be even more steeply inclined.
One more for the road. Here the HSCCs are angled up by 20 degrees, which is in the upper part of the range but certainly not an extreme value for either birds or mammals; chances are you and your cat carry your HSCCs at about the same angle (intraspecific variation caveat applies!). Angling the HSCCs up moves the occipital condyle further down, which makes the neck steeper still.
You may look at that last picture and think it’s impossible or crazy, and I don’t blame you if you do. Remember that all I’ve shown you is two possibilities from within the 50-degree ballpark defined by the HSCCs. But even if we put the HSCCs at the very bottom of that range, the occipital condyle still points down at something like 25 degrees below horizontal, which means the anterior neck has to be angled up at 25 degrees just to keep the cranio-cervical joint in ONP; if the head is flexed on the neck, it has to be steeper.
The moral of the story is that, even within the broad range of postures allowed by the HSCCs, head posture still constrains neck posture to be elevated in most if not all sauropods. It will be VERY interesting to see how the skull of Brachytrachelopan is put together, when one comes to light.
- Duijm, M. 1951. On the head posture in birds and its relation to some anatomical features. II. Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen, Series C 54: 260–271.
- Sanders, R.K. and Smith, D.K. 2005. The endocranium of the theropod dinosaur Ceratosaurus studied with computed tomography. Acta Palaeontologica Polonica 50 (3): 601–616.
- Taylor, M.P., Wedel, M.J., and Naish, D. 2009. Head and neck posture in sauropod dinosaurs inferred from extant animals. Acta Palaeontologica Polonica 54 (2): 213–220.
Update (later the same day)
We’ve added scans of the print-edition coverage that we got in the UK’s national newspapers (and the London-only freebie Metro). Somehow, seeing it in an actual newspaper still feels more real than the same newspaper’s web-site. Scan are at the bottom of the paper’s home page.