Today sees the publication of a new paper by Michael P. Taylor, Mathew Wedel and myself in which we make a bold and controversial claim: based on data from living animals, we contend that the necks of sauropod dinosaurs – all sauropod dinosaurs – were most likely held habitually in erect poses, and not in horizontal or sub-horizontal poses (Taylor et al. 2009). This research should of course be significant if you’re interested in knowing what sauropods looked like when they were alive. However, it also impacts on hypotheses about sauropod behaviour, physiology and ecology [image below by the good Dr Mark Witton. Look for the in-joke].
Ideas on sauropod neck posture have varied a lot over the decades. At least some researchers have proposed that sauropod necks were relatively immobile, horizontal and beam-like (Martin 1987, Martin et al. 1997, 1998). This is thought by others (myself included) to be unlikely, given that there are various indications from anatomy that sauropod necks were reasonably flexible, and held in raised postures (read on). This alternative argument provides at least some sauropods with more erect neck postures in which the neck is shown projecting upwards at an angle of 45 or even 90° to the long axis of the back.
The ‘osteologically neutral pose’ hypothesis
Palaeontologists have generally assumed that one can get a good idea of an extinct animal’s neck posture by simply plugging the vertebrae and skull together, and arriving at a sort of ‘best fit’ where the zygapophyses (the articulating prongs and facets located on the neural arches of the vertebrae) are in substantial overlap. The resulting pose has been termed the osteologically neutral pose (ONP). Sauropod necks were reconstructed in ONP by Stevens & Parrish (1999, 2005a, b). Arguing that ‘with no known exception, the curvature characteristic of the axial skeleton of a given vertebrate arises, not from chronic flexion out of the neutral position, but from the morphology of the vertebrae in the undeflected state’ (Stevens & Parrish 2005a, p. 215), Stevens and Parrish reconstructed a variety of sauropods with near-horizontal necks, and they even concluded that some forms (like Dicraeosaurus) had down-sloping necks where the muzzle was positioned close to the ground [adjacent figure, from Stevens & Parrish (2005a), shows [top to bottom] the necks, in ‘neutral pose’, of Cetiosaurus oxoniensis, Euhelopus zdanskyi and Brachiosaurus brancai].
They haven’t been the only researchers to provide some sauropods with down-sloping necks: Wilson (2002) did likewise for Dicraeosaurus, and Nigersaurus has been shown in a similar pose (Sereno et al. 2007). Tracy Ford produced a series of ONP diagrams for sauropod necks, and showed Diplodocus and Apatosaurus with necks that bend sharply downwards in their anterior halves (Ford 1999). The idea that diplodocoids habitually walked around with their heads close to or at ground level seems rather unlikely, if not ridiculous: this would have made them easy targets for contemporary theropods, and it would have made them all but useless at detecting anything that was located more than about a metre in front of them. It also seems totally counter-intuitive that animals evolved such elongate necks purely to eat at ground level.
Stevens and Parrish are just two among many authors who have discussed sauropod neck posture. However, their work has been particularly influential because the diplodocoids featured in the TV series Walking With Dinosaurs were based entirely on the postures and ranges of movement that they reconstructed (Stevens & Parrish 1999). It should be noted that – while they depicted near-horizontal neck postures in sauropods – Stevens and Parrish did infer some flexibility within the sauropod neck, and hence did not limit sauropods to an immobile, beam-like neck posture. However, the ranges of motion they posited seem conservative relative to what might really have been possible. I’ll say no more on the subject of neck flexibility, as it’s an altogether different topic from the one we’re concerned with here.
Does ONP really tell you about the animal’s neck posture in life? No, forget it
As it happens, a reasonable amount of literature has been devoted to the subject of head and neck posture in living animals. By X-raying alert, unrestrained animals, Vidal et al. (1986) and Graf et al. (1992, 1995) showed that mammals, birds and lizards consistently do the same things with their heads and necks when in normal alert posture: the neck is strongly extended (that is, the cervico-dorsal junction is strongly bent such that the neck extends strongly ‘upwards’ relative to the dorsal vertebrae) while the head is strongly flexed relative to the neck (that is, the cranio-cervical joint is strongly bent such that the head is virtually at a right angle relative to the cervical vertebrae*). The ‘middle’ part of the neck is held relatively rigid, and the neck as a whole is held near-vertical. This is true even of animals that seem to have very short necks, like shrews, rodents and rabbits [the adjacent X-ray shows a rabbit: check out the strongly extended, vertical neck. From Vidal et al. (1986)].
* The terms ‘extension’ and ‘flexion’ are going to be used a lot here, so now is time to learn what they mean. When you curl your fingers and form a fist your fingers are undergoing flexion, and when you straighten your fingers, they are undergoing extension. It isn’t just fingers that can be flexed and extended, of course: you can flex and extend every joint in your body. If you look downwards, such that your chin is approaching your chest, you are flexing your cranio-cervical joint, and if you look upwards, such that your chin moves away from your chest and the back of your head approaches your shoulders, you are extending your cranio-cervical joint [the terms 'dorsiflexion' and 'ventriflexion', used by some authors, are redundant].
Given what we know about head and neck posture in living animals thanks to X-rays, we can test the claim that ONP really does characterise the posture in life. And the evidence is conclusive: when you plug vertebrae and skulls together in the ‘best fit’ method mentioned above, you never get the life posture. We did this with skeletons of modern animals (we figure a hare and a chicken in the paper): this is an incredibly basic thing to do, but it may or may not surprise you to learn that no-one has done it before (or, if they have done it, they’ve never published their results). In both extant mammals and extant birds, vertebrae articulated in ONP produce strongly flexed cervical columns, totally different from the strongly extended cervical columns of the living animals. Furthermore, when you manipulate dry bones alone, you just can’t get them to form the strongly extended poses present in living animals. As we say in the paper, ‘It is apparent that the soft-tissue of the neck (e.g., intervertebral cartilage) enables greater flexibility in the neck than the bones alone suggest’ (Taylor et al. 2009, p. 215).
All of this means that we’re severely limited in the kinds of inferences we can make about neck posture from bones alone. We cannot, alas, infer life posture by plugging vertebrae together: it just doesn’t work. We can, however, use the X-ray data from modern animals to make inferences about the neck poses of extinct animals.
A phylogenetic perspective
As discussed above, X-ray data shows that the necks of amniotes are extended at the cervico-dorsal junction, and flexed at the cranio-cervical joint. We looked at as many X-rays as we could (some unpublished, but most of them published in papers on neck posture or on the mechanics of respiration), and confirmed these observations in crocodilians, turtles and even in lissamphibians like salamanders. The neck is most strongly vertical in mammals and birds, but some turtles do a good job at maintaining a vertical neck, and monitor lizard and crocodilian necks are held elevated at between 20 and 40°. Even salamanders (which only have a single cervical vertebra) hold the neck elevated, and maintain a flexed cranio-cervical joint.
When we map these details on to a tetrapod cladogram, we have to conclude that extended cervico-dorsal junctions and flexed cranio-cervical joints are primitive for Amniota, and even for crown-group Tetrapoda (there’s currently more than one phylogenetic definition of Tetrapoda out there, so let’s not worry about the precise content of Tetrapoda right now). Near-vertical, strongly extended necks evolved at least three times within Amniota (Mammalia, Testudines and Aves).
The implications for sauropods
So: what does all of this mean for sauropods and other extinct tetrapods? It means that – in the absence of data to the contrary – we should assume that, when holding its head and neck in a normal, alert pose, a fossil amniote had an extended cervico-dorsal junction (and hence a ‘raised’ neck) and flexed cranio-cervical joint. Reconstructions which show flexed cervico-dorsal junctions and extended cranio-cervical joints – and this includes the Walking With Dinosaurs sauropods and Nigersaurus as reconstructed by Sereno et al. – are flatly at odds with these predictions and can be regarded as inconsistent with what we know about living animals.
It should be noted, of course, that the ‘alert’ posture I’m talking about here is by no means the only posture that the animal can adopt. As we note in the paper, the feeding posture adopted by an animal is by no means similar to the normal, alert posture it adopts at other times: look at horses and other grazing mammals, for example.
Some final thoughts
I should note that we discuss other stuff in our paper that isn’t covered here. Our conclusions rest on the assumption that sauropods were, while ‘extreme’, essentially similar to their living relatives in neck morphology. A few authors have proposed that sauropods were morphologically novel, and that they used unusual methods of neck support that made them very different from extant long-necked animals (such as prop-like cervical ribs that acted as ventral compression members, or turgid air-sacs that somehow provided neck support). We disagree with these alternative models, but this isn’t the place to smack them down properly: stay tuned.
On another matter, it’s recently been argued that the orientation of the semi-circular canals can provide reliable data on head posture. This is an excellent hypothesis, but it’s flawed. I’d like to explain this further, but I’ll have to direct you to the paper. And – speaking of the paper – the good news is that’s available free to all thanks to the good graces of those wonderful people at Acta Palaeontologica Polonica.
There are, of course, additional thoughts on the paper over at SV-POW! Those of you who have followed the story of fame and glory that is SV-POW! (that is, Sauropod Vertebra Picture of the Week) will know that this paper has special significance in that it’s the first time that the three SV-POWsketeers have appeared on the authorship of a paper together. May it be the first of many, or at least a few
Refs – –
Ford, T. L. 1999. How To Draw Dinosaurs, book 1. T. L. Ford (privately published).
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& Parrish, J. M. 2005b. Digital reconstructions of sauropod dinosaurs and implications for feeding. In Curry Rogers, K. A. & Wilson, J. A. (eds) The Sauropods: Evolution and Paleobiology. University of California Press (Berkeley, Los Angeles & London), pp. 178-200.
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