Sauropod Feeding Strategies #2

A more thorough version of my last post is presented. There is some confusion over what the papers presented below actually say, some of them very technical. A summary, which may certainly be wrong in some cases, is required.

Stevens & Parrish [1] (including various subsequent works [e.g., [2,3]] backing the data up) argued that the orientation, shape and position of the centrum ball-and-socket and the zygapophyses produce a means of assessing movement envelopes. They tested this by adopting an arbitrary “stop” of 50% overlap of the zygapophyses while maintaining centrum/centrum contact and then moving vertebrae digitally. This did not text other bony stops, and visual examination of the material accord either additional stops (shallow gaps between zygapophyses prevent further slippage, while hyposphene/hypantrum articulations also prevent further movement). Using this argument, they applied their data to several sauropods, and argued that the “neutral” posture of sauropods was the moment of greatest overlap among all zygapophyses and centra. From this, they extrapolated limits based on various manipulations of the individual sets of vertebrae not exceeding the 50% benchmark. This produces an “envelope” of movement.

The dorsal extension of this envelope has interested researchers more than anything else, although no one has spent more time on the ventral and lateral extensions of this envelope, and until them, virtually no one did. And none have done since, either. It’s kinda strange. One would think that complete mobility would interest workers, as total mobility influences the feeding range of a sauropod. Vertical reach is important, but it belies the issues involving sauropod mobility by constantly addressing only one hypothesis of sauropod feeding strategies (treetop access). It ignores dental apparatus or even availability, and the energetics of the food resource system, as well as relative ecology, but more on this below.

Christian & Dzemski, in their first paper from the DFG Research Group on this subject [4], addressed biomechanically the stresses on the individual vertebrae of a proxies Brachiosaurus brancai cervical series, noting that it could reach above 60 degrees from the horizontal. Stevens adapted the hypothesis to assess range of movement in Brachiosaurus brancai, [2] finding a vertical range about 20 degrees above the horizontal (a bit lower than Taylor et al., [6] although see below). Unlike [6], they projected the neck would have a slight S-shape (swan-like) under even stresses, when extended. These authors do not assume that Brachiosaurus brancai would have assumed this posture at all times, however. As [5] remarks, the sauropod neck at rest (nearly horizontal or below that) is the most energetically conservative posture, and it is unlikely any researcher in modern animals would argue that a resting posture, which most terrestrial animals spend upwards of 50% of their time in, is in a pulled-back, rearing posture.

Researchers are arguing that “neutral” posture and “active” posture are valid standards with which to assess movement. While there is a driving need to find the baseline from which postures can be extrapolated, the truth is that most terrestrial amniotes adopts a range of postures throughout their daily lives, from resting, alert, feeding, and a host of other more specialized positions such as grooming, or where some postures are exapted into one another (e.g., some birds rest with their heads in a tight U-shape atop or under their wing, which corresponds to “grooming” in some mammals). One of the things I like about the DFG 533 guys is that their papers are not peppered with finding this ideal single “posture” with which to argue over. I think Stevens & Parrish’s “neutral” posture (which they argued was correspondent to an “active,” “walking” posture [1]) does not represent enough of a “neutral” posture to be the baseline; I would instead argue this baseline is “resting,” and [4] and [5] appear to use this when assessing energetics.

[6] extrapolate an “active” posture, but do not define the range of behaviors this entails, nor do they entail how much of their animals’ lives were spent in this posture to extrapolate that this was representative of a baseline stature to draw remarks from. On the other hand, they use a wide range of animals to draw parallels to sauropods, arguing that animals typically assume a posture in which the neck is about 30-45 degrees elevated (short-necked, long, moderate, whatever). Some of the animals they use (e.g., turtles) are clearly deviant on this regard, as most turtles necks, even at full extension, represent a U-shape or tight C-shape, as the basal cervicals are always ventrally declined from the dorsal series due to the presence of the rim of the shell; even in the few large or burrowing turtles which elevate the anterior aperture of the shell, the neck is initially declined in all but the most elevated, browsing behavior, or during sex. Are these normative behavior that we should draw conclusions from? No, but its not all of the taxa we can look at.

Work in [6] corresponds generally with [4] and [1] in arguing that the sauropod neck was generally straight, bent upwards at the dorsal/cervical junction, and gradually declined in the anteriormost 3-5 cervicals leading to the skull. Gradualism of the cervical column’s curvature is greater in [1] and in [4] than it is in [6]. Unlike all of the other works, [4,5] argue from a point where direct biomechanical models of the vertebrae are used to assess limits; in this, they exceed those of [1,2,3] and in [6], where physical models or habitual postures are used. I would expect further data to be used to assess sauropod cervical postures, including FEA analysis, to explain individual vertebral loading, but this would likely copy the DFG 533 work.

The ecology of sauropod feeding, however, tells us that vertical reach is but ONE aspect of diet. It seems odd when some researchers argue that sauropod necks at a horizontal is “mainstream,” following primarily [1], I assume, when such popular artists as Greg Paul have maintained a guilt-free upwards-reach in art in the last decade during this “mainstream” period, and we are all very familiar with the dominance of Greg’s work in the mindset of paleontological reconstruction. If this argument were not enough, Greg is the most emulated artist of the paleontological world following John Sibbick and Chas Knight (historically), while he is THE most emulated worker in current art. With elevated and vertical-necked sauropods and a strong maintenance of the swan-necked Humboldt brachiosaur, I find it difficult to argue horizontality in the neck is “mainstream,” especially given that few sauropods were described in the last decade where the neck did not follow older models (omeisaurs are conventionally restored in China with horizontal necks, and this follows the mounts, maintaining a “longest sauropod” meme) that have nothing to do with [1].

We argue whether they could do it, and produce mathematical models providing they couldn’t (very few of these), versus mathematical models showing how far they could, backed up by extant observations and biomechanical stress analyses. Before this current debate assessing the range of postures in sauropod necks, the old mode of the debate was about the ecology and energetics of sauropods: Could their environment sustain them? And what parts of their environment could not? This is more difficult to tell, because we do not know enough of it. What we do know is that sauropods are known to travel in groups (some of them, anyways, and some of the time), and that they could reach up (some sauropods to different heights, which contradicts a broad “all sauropod necks at 45 degrees” argument), providing them a range of provender. [5] tells us that the energetics of reaching up to ~30 degrees for Euhelopus zdanskyi could be maintained with a regular feeding regimen, although data on how much calories it would need to process in a given minute is known, how many calories it could consume is less well-known. What I think is clear is that different sauropods held their necks in different postures for similar activities, based on massive differences in cervical architecture and size. Some workers have even purported the neck posture of the neck can be assessed through position and orientation of the lateral semicircular canal [7], an argument that has been contested and tested for about 2 decades (e.g., see [8,9,10]), but which comes into conflict when the orientation of the canals differs among general head posture by as much as 15 degrees, but generally no more [10]. This is important, because sometimes the orientation of the brainstem (and consequently the relationship of the head to the cervical column), can differ strongly when the argument of the cervical column is mandated. This is more relevant with a sauropod like Nigersaurus taqueti [11], where the cervical column attached to the skull at 90 degrees from the lateral semicircular canal’s orientation [12]; even accepting the 15 degree variation that has been accepted by most researchers [10], this variation forces the cervical column down nearly horizontally, which corresponds with the grazer-like dental arcade that has been described by Sereno & Wilson [13]. Further assessment is required when some sauropods have strongly divergent cervical anatomy, such as Brachytrachelopan mesai [14] and Dicraeosaurus hansemanni [15], precluding a neck that would not easily attain great heights, but is composed of craniocaudally short and dorsoventrally deep vertebrae with neural spines that provide a further “stop” to cervical flexure. These neck and head differences invoke deep ecological variations which cannot be categorized with the same taxa addressed before.

And this is without addressing dental variation among sauropods, some of which seem clearly suited to grazing rather than browsing [13]. Modern grazing animals such as cattle spend virtually all of their time feeding, walking to find more food, resting, etc., and most of this is done with the head held downwards and the neck horizontal or nearly so. Potential for vertical reach does not, in any way, mean the neck was always held in such an upright posture, but despite this, some workers and artists persist in arguing that the neck was held upright, full stop. This is almost certainly both incorrect for some taxa, but flat wrong in general.

[1] Stevens, K. A. & Parrish, J. M. 1999. Neck posture and feeding habits of two Jurassic sauropod dinosaurs. Science 284:798-800.
[2] Stevens, K. A. & Parrish, J. M. 2005. Neck posture, dentition and feeding strategies in Jurassic sauropod dinosaurs. pg. 212-232 in Tidwell & Carpenter (eds.) Thunderlizards: the Sauropodomorph Dinosaurs. (Indiana University Press, Bloomington.)
[3] Stevens, K. A. & Parrish, M. J. 2005. Digital reconstructions of sauropod dinosaurs and implications for feeding. pg. 178–200 in Wilson & Curry-Rogers (eds.) The Sauropods: Evolution and Paleobiology. (University of California Press, Berkeley.)
[4] 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 10:37–48.
[5] Christian, A. 2010. Some sauropods raised their necks — evidence for high browsing in Euhelopus zdanskyi. Biology Letters (published online, June 2, 2010), doi: 10.1098/rsbl.2010.0359.
[6] 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.
[7] Witmer, L. M., Chatterjee, S., Franzosa, J. & Rowe, T. 2003. Neuroanatomy of flying reptiles and implications for flight, posture and behaviour. Nature 425:950-953.
[8] de Beer, G. R. 1947. How animals hold their heads. Proceedings of the Linnaean Society of London, B 159:125–139.
[9] Duijm, M. 1951. On the head posture of some birds and its relation to some anatomical features. Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen C 54, 202–211, 260–271.
[10] Spoor, F. & Zonneveld, F. 1998. Comparative review of the human bony labyrinth. Yearbook of Physical Anthropology 41:211–251.
[11] Sereno, P.C., Beck, A.L., Dutheil, D.B., Larsson, H.C.E., Lyon, G.H., Moussa, B., Sadleir, R.W., Sidor, C.A., Varricchio, D.J., Wilson, G.P. & Wilson, J.A. 1999. Cretaceous sauropods from the Sahara and the uneven rate of skeletal evolution among dinosaurs. Science 286(5443):1342-1347.
[12] Sereno, P.C., Wilson, J.A., Witmer, L.M., Whitlock, J.A., Maga, A, Ide, O. & Rowe, T. A. 2007. Structural extremes in a Cretaceous dinosaur. PLoS ONE 2(11): e1230.
[13] Sereno, P. C. & Wilson, J. A. 2005. Structure and Evolution of a Sauropod Tooth Battery. pg. 157-177 in Curry-Rogers & Wilson (eds.) The Sauropods: Evolution and Paleobiology. (University of California Press, Berkeley.)
[14] Rauhut, O.W.M., Remes, K., Fechner, R., Cladera, G. & Puerta, P. 2005. Discovery of a short-necked sauropod dinosaur from the Late Jurassic period of Patagonia. Nature 435:670-672.
[15] Janensch, W. 1914. Übersicht über die Wirbeltierfauna der Tendaguruschichten, nebst einer kurzen Charakterisierung der neu aufgeführten Arten von Sauropoden [Overview of the vertebrate animal fauna the Tendaguru layers, along with a short analysis of the specified types of Sauropoda]. Archiv für Biontologie 3(1):81-110.

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