I’ll summate briefly:
Sauropod neck posture has been debated fairly aggressively among those narrow-discipline sauropod specialists (and occassional avocation biologists) for about a century. Speculation has permitted some people to put sauropods into “snorkel” position deep under water, extending their necks far above their bodies in near-vertical postures, and it has also caused some people to think sauropods were incapable of holding their necks at any real posture above the horizontal, and even then just barely. Attempts to discover the “true” posture has engrossed some researchers, who sought what may be the “Holy Grail” of sauropod neck attitude, the “One True Posture.”
I think they’re all wrong.
Much of this debate has been covered before, and a recent paper (Sander et al., 2011) has sought to make some sense of it, while also proposing their own spin on it:
Sauropods probably employed different neck postures during different activities, like feeding, locomotion and standing at rest, so that reconstructions of neck postures can differ due to different approaches used for the reconstruction[.]
Note that this “spin” is not dismissive of any other theory. It encompasses them all. The problem arises when a theory attempts to dismiss the others, due to some internal property (extension of living animals’ necks, inability of necks to attain extreme postures wityhout exceeding some bony obstacle, etc.). Sander and the entire Sauropod Working Group, of which the quoted paper is the final review summary of, have investigated the posture of necks of more than one sauropod, and find that they tend to differ in attitude. Moreover, the authors (specifically Dzemski and Christian, 2005 & Christian and Dzemski, 2007) argue that different postures are attained at different activities, which just enforces the idea there is indeed no “one true posture.”
So why has the concept of OTP been so prevalent in reconstructing sauropod life strategies? If a sauropod can attain high levels of reach to feed, why must it maintain that posture bar none for any other thing? And why must we restore or reconstruct said posture at any interval? This has engrossed some people (Paul, 1998) who restore skeletal diagrams of sauropods in fast transit, yet alter the neck posture to the extreme (Brachiosaurus brancai, from the Tendaguru Formation of Tanzania, is shown consistently with a more-or-less “swan-like” neck posture [a posture said bird attains primarily when swimming, but not when feeding, flying, or even walking]) [n1]. Dzemski and Christian (2007) have even projected that not all sauropods attain similar neck postures during a single activity, such as feeding, due to varying constraints, regardless of the neck length.
So this isn’t really about extending a really long neck vertically, or preventing the neck from being extended outward. Taylor et al. (2009) argue instead that the neck should always be shown vertically elevated (around 45 degrees, really, but this is a mean, not an explicit value) because mammals and many other terrestrial amniotes hold their necks around this mean when in “alter posture.” Despite not valuing “alert posture” as a valid posture from which to evaluate neck posture in general, the authors generalize the neck system in contradiction of Stevens and Parrish’s ONP (osteological neutral pose), a system by which neck posture is almost always horizontal due to the apparent constraints of the centra and zygapophyses of the neck. Certainly, both arguments are beased on inherent assumptions which have been vastly generalized from a subset of animal behavior, without any firm constraint applied to the range of animal behavior.
Did you know most animals spend virtually all of their time either eating or sleeping? How much time did you think they spent in “alert” or “neutral” posture? Stevens and Parrish imply that feeding and sleeping can be better approximated through constraint to ONP, because the neck is closer to the horizontal in both of these, a finding Dzemski and Christian (2007) actually agree on … for some sauropods. “Alert posture,” presumably, would be the time spent when not eating or resting when an animal would be attentive and wary, essentially doing nothing but sitting, standing, looking, and listening. This is an argument for a “neutral” posture, and because it differs from ONP, it is a good refutation of said ONP, but its meaning in general or systematized posture (as Taylor et al. argue it is) is wrong.
Stevens and Parrish also systematize ONP due to a self-imposed constraint, that of articular neutrality, which would not be exceeded, but this is certainly false based on living animals ranges of movement, and is not found by Dzemski and Christian when assessing a variety of sauropods due to biomechanical investigation of strain and stress, which elevated the neck of Brachiosaurus from the horizontal, as well as Euhelopus (Brachiosaurus was taller) while Diplodocus was kept down.
Feeding strategy also plays a large portion in this. Tree-top foraging is a popular image for sauropods, and it goes well back into the mid 1900s, although popularized only later in (especially by Greg Paul who restored virtually all of his sauropods into elevated tree foragers). Dentition patterns, however, tell a different story, and constrain the diet to a narrower spectrum of plants. Even more problematic, the jaw design and neck length of some sauropods has produced what are likely vastly divergent ecological niches: dicraeosaurids had very short necks, but pencil-like dentition, and likely foraged low to the ground; rebbachisaurids, exemplified by Nigersaurus
tiguidiensis taqueti, were likely even lower-ground or surface foragers, despite the longer necks than dicraeosaurids; brachiosaurids and indeed probably also “mamenchisaurs” with their spoon-shaped teeth and long-necks were mid-to-high canopy browsers, and diplodocid sauropods with their pencil-shaped teeth but extremely long necks were low-to-mid browsers.
Mobility is not the issue, but statitc posture is. No sauropod kept in a single posture, and likely adopted various postures for feeding and travel, at which point it begs the question of why some recent authors concern themselves with this basic and poor assumption there there is or should be “one true posture?”
This isn’t rhetorical: I’d really like to know.
[n1] It should be noted that “swan-like” here refers to the extended S-shape often used for theropod dinosaur reconstructions, rather than the tight-S-shape that swans (and many other birds) often make.
Christian, A. & Dzemski, G. 2007. Reconstruction of the cervical skeleton posture of Brachiosaurus brancai Janensch, 1914 by an analysis of the intervertebral stress along the neck and a comparison with the results of different approaches. Fossil Record 10:38–49.
Dzemski, G. & Christian, A. 2007. Flexibility along the neck of the ostrich (Struthio camelus) and consequences for the reconstruction of dinosaurs with extreme neck length. Journal of Morphology 268, 707–714.
Paul, G. S. 1998. Terramegathermy and Cope’s rule in the land of titans. Modern Geology 23:179–217.
Sander, P. M., Christian, A., Clauss, M., Fechner, R., Gee, C. T., Griebler, E.-M., Gunga, H.-C., Hummel, J., Mallison, H., Perry, S. F., Preuschoft, H., Rauhut, O. W. M., Remes, K., Tütken, T., Wings, O. & Witzel, U. 2011. Biology of the sauropod dinosaurs: the evolution of gigantism. Biological Reviews 86:117-155.
Stevens, K. A. & Parrish, J. M. 1999. Neck posture and feeding habits of two Jurassic sauropods. Science 284:798–800.
Taylor, M. P., Wedel, M. J. & Naish, D. 2009. Head and neck posture in sauropod dinosaurs inferred from extant animals. Acta Palaeontologica Polonica 54(2):213–220.